ADU Construction and Foundations in California: Soils, Foundations, Framing, Waterproofing, Insulation and Finishing the Build

An ADU stands on decisions most homeowners never see: what the soil can carry, which foundation the ground actually calls for, how the frame resists an earthquake, and whether the wall assembly sheds water or traps it. California ADU law says almost nothing about any of this — the building code does, and this guide walks it in build order.

Start with a verified negative, because it corrects the most common assumption in this space: state ADU law and the HCD ADU Handbook contain no foundation, soil, or concrete rules at all. What they say is that an ADU must comply with the building code and health and safety requirements for dwellings — which means the California Residential Code (Title 24, Part 2.5, the prescriptive path most ADUs use) and the California Building Code (Title 24, Part 2, the engineered path) write every rule on this page. The two paths sometimes give different numbers for the same thing, and this guide labels which path each figure belongs to. Edition matters too: permit applications before January 1, 2026 are reviewed under the 2022 codes; applications on or after that date fall under the 2025 codes, and a few citations moved between editions — we flag each one where it matters. This page is the Construction and Foundations chapter of our California Accessory Dwelling Unit Handbook.

Start With the Soil

Soil bearing capacity (Topic 176)

Every foundation design starts from one number: how much load the soil under the footprint can carry, in pounds per square foot. The code supplies presumptive values by soil class (CBC Table 1806.2; the same values appear in CRC Table R401.4.1):

Two traps hide in that table. First, 1,500 psf is not “California’s bearing capacity” — it is only the clay row, and the CRC footing tables are keyed to these values, so weaker soil means a wider footing for the same building. Second, mud, organic soils, peat, and unprepared fill get no value at all — a design may not exceed the table unless substantiating data are submitted and approved, and where the building official determines soils under 1,500 psf are likely, a soils investigation is mandatory. Pitfall: assuming a neighbor’s soil result, or the main house’s, transfers to the ADU footprint — fill history can change across a single backyard.

Geotechnical exploration — the soils report (Topic 177)

A soils report is not a universal ADU requirement; it is a trigger-based one. Under CBC section 1803, a geotechnical investigation is required where the investigated conditions are present — questionable or expansive soil, deep fill, deep foundations, groundwater questions, and the seismic conditions below — and the building official decides for a CRC project under section R401.4 where available data indicate expansive, compressible, shifting, or other questionable soils. The report itself has required content (section 1803.6): boring or test-pit logs, a soil profile, the water-table elevation, foundation recommendations with a bearing capacity, expected settlement, and expansive-soil provisions where they apply. How many borings? The code deliberately does not say — the registered design professional determines the number and types of borings and test pits (section 1803.3.1), so any source quoting you a fixed boring count as a “code requirement” is inventing it. The building official may also waive the investigation where satisfactory data from adjacent areas show none of the investigated conditions exist (section 1803.2 exception) — a waiver is the official’s decision on data, never an applicant’s entitlement. Pitfall: drawing footings to presumptive values, then learning at plan check that the city requires a report whose recommendations change the footing size, depth, or type.

Expansive soils (Topic 178)

“Expansive” is a tested legal classification, not a neighborhood rumor. CBC section 1803.5.3 and CRC section R403.1.8.1 apply the same four-part test: Plasticity Index of 15 or more, more than 10 percent passing the No. 200 sieve, more than 10 percent of particles smaller than 5 micrometers, and an Expansion Index greater than 20 under ASTM D4829. Watch the labeling trap: ASTM calls an Expansion Index of 21–50 merely “Low” expansion potential, but anything over 20 is already legally expansive under the code test — “low” does not mean “exempt.” Expansive soil is a design condition, not a ban: CBC section 1808.6 offers four compliance routes — foundations designed to resist differential volume change within the active zone, slabs designed by the WRI/CRSI or PTI DC 10.5 methods (the post-tensioned route is one option, not a mandate), removal of the expansive soil to the depth of constant moisture and replacement with non-expansive fill, or building-official-approved stabilization. On the prescriptive CRC path, excavated expansive soil may not be re-used as fill within the active zone. Pitfall: pouring a standard prescriptive slab on untested clay in a known expansive-soil area — the CRC path expressly exits to engineered design here.

Choosing and Building the Foundation

Foundation type selection (Topic 179)

There is no code ranking of foundation types and no rule that one type is “best” for an ADU — the soil classification, the slope, and the seismic category make the choice, in that order. Presumptive-value soil on a flat lot supports the prescriptive path: continuous concrete or masonry footings under all exterior walls, bearing on undisturbed natural soil or engineered fill (CRC section R403.1; CBC section 1809 sets a 12-inch minimum footing width and 12-inch minimum depth below undisturbed ground for shallow foundations). Expansive soil routes you to section 1808.6 design; slopes steeper than 1 vertical to 3 horizontal trigger the hillside geometry below; deep foundations are always engineered on a geotechnical investigation (CBC section 1810) — there is no prescriptive pier depth anywhere in the code. And a useful verified negative: California does not require engineered foundations for every ADU. The CRC prescriptive path exists precisely for conventional lots; engineering is triggered by conditions, not by the letters “ADU.”

Slab-on-grade (Topic 180)

The code minimum slab is thinner than most people are told: 3½ inches, under both CRC section R506.1 and CBC section 1907.1. Four inches is common practice and often specified — but publish it as practice, not as the code floor. Site preparation is code, though (CRC section R506.2): vegetation, topsoil, and foreign material come out; fill is limited to 24 inches of clean sand or gravel or 8 inches of earth; and a 4-inch base course of clean graded material goes under a below-grade slab. The detail that fails inspections is the vapor retarder, because the two code paths genuinely diverge: on the CRC path, the verified 2022 CRC rule (section R506.2.3) is a 10-mil vapor retarder conforming to ASTM E1745 Class A, joints lapped at least 6 inches, placed between the slab and the base course; on the CBC path the figure is 6-mil polyethylene. A living-space ADU slab qualifies for none of the CRC exceptions (garages, small unheated storage, flatwork). Where slab reinforcement is provided, it must be supported so it stays in the center or upper third of the slab (section R506.2.4) — the “mesh in the dirt” pour is the classic failure. Note that section R506 sets no control-joint spacing; joint layout comes from the design, not from a CRC number.

Raised foundations and crawl spaces (Topic 181)

A raised floor replaces the slab questions with crawl-space questions. Foundation walls carry the reinforcement tables of CRC section R404.1, and in Seismic Design Category D the stem wall gets the seismic bar pattern covered under reinforcement below. The wall must extend at least 4 inches above finished grade where masonry veneer is used, 6 inches in other cases (section R404.1.6), and you may not backfill against it until it has adequate strength and is braced or anchored (section R404.1.7). A vented crawl space needs openings totaling at least 1 square foot per 150 square feet of under-floor area (section R408.2), reducible to 1 per 1,500 only where the ground carries a Class I vapor retarder; the unvented alternative (section R408.3) trades the vents for a sealed ground cover plus mechanical exhaust, conditioned supply air, a plenum, or dehumidification. Access is sized, not improvised: 18 by 24 inches through the floor, 16 by 24 inches through a perimeter wall (section R408.4), and the under-floor grade must be cleared of vegetation, organic material, form wood, and construction debris before occupancy (section R408.5). Below-grade enclosed space triggers foundation drains (section R405.1), and finished grade must fall away from the foundation at least 6 inches in the first 10 feet (section R401.3). One more verified negative: the CRC sets no minimum crawl-space height — what it regulates is the clearance that protects the wood (next sections), not a number of inches of headroom.

Pier and grade beam foundations (Topic 182)

“Pier” means three different systems, and confusing them is expensive. The CRC prescriptive pier-and-curtain-wall path (section R404.1.5.3) serves light-frame buildings of two stories or less, with all load-bearing walls on continuous footings integral with the exterior footings, curtain walls no more than 4 feet high, and limited unbalanced fill — in SDC D it adds horizontal joint reinforcement and a No. 4 vertical bar at 48 inches on center, grouted. Isolated masonry piers under girders have their own limits in section R404.1.9, including solid caps at least 4 inches thick that double as part of the termite strategy. An engineered drilled-pier and grade-beam system is a different animal entirely: designed under CBC sections 1810 and 1808 on a geotechnical investigation, with no prescriptive depths, diameters, or spacings in either code. Code-path honesty matters here: the CBC (section 1809.10) prohibits pier-and-curtain foundations in SDC D, E, and F, while the CRC path permits them in SDC D with the reinforcement above — which book your permit is under decides which sentence governs your project.

Hillside foundations (Topic 183)

The hillside rules switch on at a defined slope: steeper than 1 vertical to 3 horizontal (33.3 percent). Above that trigger, CBC section 1808.7 and CRC section R403.1.7 impose geometry on top of the zoning setbacks: clearance from an ascending slope is the smaller of half the slope height or 15 feet; footing setback from a descending slope is the smaller of one-third the slope height or 40 feet; and the top of the exterior foundation must sit at least 12 inches plus 2 percent above the adjacent street gutter or drainage discharge point. Alternate setbacks are possible, but only as approved on the basis of a geotechnical investigation — hillside relief runs through the soils report. On ordinary sloped lots the everyday detail is the stepped footing: footing tops level, bottoms sloping no steeper than 1 vertical to 10 horizontal, stepping where the grade changes faster (CRC section R403.1.5). Two hedges belong in print: 33.3 percent is a setback-rule trigger, not a building prohibition, and hillside does not automatically mean caissons — stepped spread footings are expressly contemplated. Where a city has adopted CBC Appendix J, its grading geometry (cut and fill slopes generally no steeper than 1 vertical to 2 horizontal without geotechnical justification) applies; Appendix J is not statewide law, and the city grading ordinance is the authority to check.

Retaining walls (Topic 184)

This topic carries the batch’s most-copied miscitation, so here are the three rules kept separate, the way the codes keep them. The design trigger is CRC section R404.4: a retaining wall not laterally supported at the top that retains more than 48 inches of unbalanced fill — or a wall over 24 inches high that also resists lateral loads beyond soil, such as a surcharge — must be designed in accordance with accepted engineering practice. The permit exemption is CBC section 105.2: no building permit is required for a retaining wall not over 4 feet measured from the bottom of the footing to the top of the wall, unless it supports a surcharge — and exemption from a permit is not exemption from zoning, grading, or drainage rules, and many cities set the line lower by local ordinance. The design criteria, once design is required, are CBC section 1807.2: stability against overturning, sliding, excessive foundation pressure, and water uplift, with a safety factor of 1.5 against sliding and overturning (1.1 where earthquake loads are included); in SDC D–F, walls supporting more than 6 feet of backfill also take dynamic seismic earth pressure from the geotechnical investigation. Measure height the way the exemption does — footing bottom to wall top, not exposed face — because a wall showing 3 feet of face on a deep footing can already be a 4-foot wall. Pitfall: two “exempt” tiered walls stacked close together function as one tall wall, and the upper one surcharges the lower; treat tiering as a question for the building official, not a loophole.

Digging and Dirt

Excavation depth (Topic 185)

The baseline dig is set by the footing rule: exterior footings extend at least 12 inches below the undisturbed ground surface (CRC section R403.1.4; CBC section 1809.4) — measured from undisturbed grade, which is where re-graded lots quietly fail. Digging next to the existing house has its own rule: excavation may not reduce the vertical or lateral support of an existing foundation without underpinning or other protection (CBC section 1804.1), and on granular soils the line drawn from the bottom edge of one footing to a lower footing may not exceed 30 degrees (section 1809.6) — the geometry that governs a sewer trench running deeper than the house footing beside the house. Two separate rulebooks meet in the trench, and this guide keeps them in separate lanes: worker protection is Cal/OSHA (Title 8), binding the employer during construction — a protective system for excavations, with the under-5-foot exception available only where a competent person’s examination shows no indication of a potential cave-in, a Cal/OSHA permit and shoring plans required before a person descends into a trench or excavation 5 feet or deeper, and a default simple slope of 1½ horizontal to 1 vertical. And before any digging: the 811/DigAlert notification at least two working days ahead (Government Code section 4216), covered in our plumbing and utilities guide.

Import and export soil (Topic 186)

Here is a verified negative to plan around: no California statute or building-code section sets quantities, fees, haul routes, or disposal rules for moving soil on an ADU project. What governs is the grading permit (your city’s grading ordinance; CBC Appendix J where adopted requires the application to state estimated quantities of excavation and fill), plus the quality rules for the dirt itself. Backfill must be free of organic material, construction debris, cobbles, and boulders, placed in lifts and compacted without damaging the foundation or its waterproofing (CBC section 1804.3). Compacted fill supporting shallow foundations follows the approved geotechnical report; the narrow exception is fill 12 inches deep or less verified at at least 90 percent of maximum dry density under ASTM D1557 by special inspection (section 1804.6), and fill more than 12 inches deep under a foundation pulls the geotechnical investigation back in (section 1803.5.8). A footing may bear on engineered fill — fill placed and tested under that regime — not on dirt that was merely driven over. The most common ADU import/export driver is code-grounded: expansive soil removed under section 1808.6.3 is replaced with non-expansive fill, and the excavated expansive soil cannot go back into the active zone. Truck counts are estimating practice (excavated soil swells loose compared with its in-place volume), not code — get the quantities from the grading application, not from a rule of thumb.

Termites, Decay, and the Steel in the Concrete

Termite protection (Topic 187)

The code does not require “a termite pre-treatment” or a termite warranty — it requires one or more of six lawful control methods (CRC section R318.1 in the 2022 CRC; the 2025 CRC renumbered termite protection to section R305 and decay protection to section R304, so cite by your permit’s edition): (1) chemical termiticide treatment, applied strictly per the manufacturer’s label; (2) an installed and maintained termite-baiting system; (3) pressure-preservative-treated wood; (4) naturally durable, termite-resistant wood; (5) physical barriers; and (6) cold-formed steel framing. A metal termite shield on top of the foundation wall counts only in combination with another method — a shield alone is a failed plan note. Soil pre-treatment before the pour is how the chemical method is usually delivered, performed by a licensed structural pest control operator; the code requires the method and label compliance, not the word “pre-treat.” Decay protection is the partner rule (CRC section R317.1): naturally durable or preservative-treated wood where joists sit within 18 inches of exposed ground in a crawl space, girders within 12 inches, columns within 8 inches, framing on exterior foundation walls within 8 inches of the ground, and exterior siding, sheathing, and framing within 6 inches of the ground (or 2 inches above concrete surfaces) — with field treatment of every cut end in treated wood and corrosion-resistant fasteners in contact with it.

Concrete reinforcement (Topic 188)

In Seismic Design Category D — most of Southern California — “plain concrete” does not mean “no steel.” The prescriptive stem-wall pattern (CRC section R403.1.3) is specific: at least one No. 4 vertical bar at no more than 4 feet on center, hooked, extending to the bottom of the footing and at least 14 inches up into the stem wall; one No. 4 horizontal bar within 12 inches of the top of the stem wall; and one No. 4 horizontal 3 to 4 inches from the bottom of the footing. A monolithic slab with a turned-down footing takes one No. 4 at the top and one at the bottom, or the section’s middle-third alternative. Minimum concrete strength is 2,500 psi for typical CRC foundations (CRC Table R402.2; the CBC’s foundation table matches for light-frame buildings of two stories or less in SDC D–F and rises to 3,000 psi for other buildings there). Cover is the field detail that gets shorted: 3 inches where concrete is cast against earth, less only where the section’s formed-and-exposed figures apply — bars resting on the trench bottom without chairs or dobies are not at 3 inches. And the substitution to refuse: fibermesh does not replace rebar. Fibers appear in the CRC only as a nonstructural material allowance; nothing in sections R403, R404, or R506 credits fiber as the required reinforcement in footings, stem walls, or foundation walls, and welded wire mesh is not mandated in every slab either — slab reinforcement is required where the design requires it, as on expansive soils.

Anchor bolts (Topic 189)

The frame ties to the foundation at the sill plate, and the prescriptive rule is exact (CRC section R403.1.6): anchor bolts at least ½ inch in diameter, spaced no more than 6 feet on center, embedded at least 7 inches into concrete or grouted masonry, located in the middle third of the plate, with a nut and washer tightened on each bolt, at least two bolts per plate section, and an end bolt more than 7 bolt diameters (3½ inches for a ½-inch bolt) and no more than 12 inches from each end. Buildings over two stories tighten the spacing to 4 feet. In SDC D0, D1, and D2 there is a second half to the rule that is the inspection on most SoCal ADUs: steel plate washers at least 0.229 inch thick by 3 inches by 3 inches on every anchor bolt along the full length of every required braced wall line (sections R403.1.6.1 and R602.11.1) — a thin hardware-store square washer is the classic failed item, and ordinary cut washers are permitted only on wall lines without braced panels. Approved anchor straps may substitute, but their spacing comes from the strap’s own approval as an equivalent to the bolt rule, not from the 6-foot figure. On the CBC path the same prescriptive numbers live at sections 2308.3.1–2308.3.2; CBC section 1905 is the engineered anchorage route through ACI 318, not the source of the prescriptive dimensions.

The Frame: Lumber, Bracing, Earthquakes, and the Roof

Framing lumber (Topic 192)

Framing starts with a stamp. Lumber must carry the grade mark of an approved grading or inspection agency (CRC section R602.2), because every span table in the framing chapters assumes a species and grade — the same 2×8 is a different structural member in Douglas fir–larch No. 2 than in a lower grade, so a lumberyard substitution is a design change, not a shopping choice. The nailing is likewise prescribed, in Table R602.3(1): the row that ties framing to the lateral system is the braced-wall-panel sole plate, at 3-16d nails at 16 inches on center, against the lighter ordinary sole-plate schedule. Stud walls are capped with a double top plate, overlapped at corners and intersections with bearing partitions (section R602.3.2); a single top plate is allowed only with the section’s metal ties and stud-alignment conditions attached — copy that omits the conditions misstates the rule. Headers are sized and supported from section R602.7 and its span tables, jack-stud counts included (an older citation to section R602.5 for headers still circulates; it is not the header section in the current code). And wet framing may not be enclosed: the enforceable pre-enclosure limit is 19 percent moisture content, and its home is CALGreen section 4.505.3 — not the decay-protection section, another mis-citation in wide circulation.

Shear walls and wall bracing (Topic 190)

Terminology first, because the numbers follow the vocabulary: the CRC prescriptive system is wall bracing — braced wall lines and braced wall panels under section R602.10 — while “shear wall” with rated capacities in pounds per foot belongs to engineered design. The amount of bracing comes from Table R602.10.3(1) for wind and Table R602.10.3(3) for seismic, and the greater of the two governs; the seismic table is built on Soil Class D, a 10-foot wall height, and stated dead loads, and those assumptions travel with any number taken from it. Here is the correction this section exists to print: the current tables specify a minimum total length, in feet, of braced panels per braced wall line — not percentages. The familiar “45 percent of the wall line” tables are the pre-2012 format still circulating in tutorials. In SDC D, braced wall lines may be spaced no more than 25 feet apart in either direction (with a codified exception up to 35 feet for a single room of 900 square feet or less), the standard wood-structural-panel panel minimum is 48 inches, and methods fall away as the category rises — let-in bracing and the gypsum-family methods are simply not permitted in SDC D2 cells of the table. Hold-downs follow the same discipline: ordinary prescriptive panels carry no universal hold-down rating — hold-downs are prescribed inside specific systems (the portal frame with hold-downs, the CBC alternate braced wall) and, on engineered jobs, come off the shear-wall schedule as a design output. The uplift path is codified separately at section R602.3.5: where net uplift at the top of a wall exceeds 100 pounds per linear foot, approved connectors must carry a continuous path down to the foundation.

Seismic design (Topic 191)

There is no single seismic category for Southern California, and any source that assigns one — “all of SoCal is D2” — is publishing a map it never read. The category is determined per site: on the CBC path under section 1613.2 or ASCE 7, from the mapped spectral accelerations, the site class (A through F, with Site Class D as the default where soil properties are not known in sufficient detail), and the risk category — a dwelling is Risk Category II; on the CRC path under section R301.2.2, which splits the high end into D0, D1, and D2. Most Southern California ADU sites do determine out to those high-seismic categories, which is why the plate washers, the seismic bracing table, and the investigation scope on this page matter — and a near-fault or soft-soil site can push a design out of the prescriptive CRC entirely, as can the irregular-building conditions listed in section R301.2.2.6. In SDC C through F the geotechnical investigation must evaluate four hazards — slope instability, liquefaction, total and differential settlement, and surface displacement (CBC section 1803.5.11) — and in SDC D through F it adds the liquefaction and soil-strength-loss assessment at maximum considered earthquake motions, its consequences, and mitigation (section 1803.5.12). Keep three different things from merging in your head: the Seismic Design Category (a building-requirement level), the site class (a soil description), and the state’s mapped Seismic Hazard Zones for liquefaction and landslides under the Seismic Hazards Mapping Act, administered by the California Geological Survey with evaluation guidelines in Special Publication 117A — a different map, checked separately for your parcel, with the investigation requirement applied through your local building official.

Roof framing (Topic 193)

Prescriptive roof framing is span-table law (CRC section R802): joist and rafter size, species and grade, spacing, and span are read from the tables for the actual load case — including the covering’s weight, where a tile roof is the 20-psf dead-load case, not the 10-psf light-covering case — and spans are measured on the horizontal projection, not along the slope. The structural idea a homeowner should hold onto is the tie: every rafter pair must be tied across the building by ceiling joists at the plate or by rafter ties in the bottom third of the rafter height; where the ties would sit above the bottom third — the vaulted-ceiling case — the ridge stops being a board and must be designed as a beam. Collar ties are a different device: minimum 1×4 in the upper third, spaced no more than 4 feet on center, resisting ridge separation under uplift — they do not keep walls from spreading, and no prescriptive path lets them substitute for rafter ties. Uplift connections follow section R802.11: the toenail schedule suffices only at or below the section’s 200-pound-per-rafter threshold and its stated wind conditions; beyond it, approved connectors rated for the table value are required at each rafter or truss, and the path continues down the wall under section R602.3.5. Trusses are engineered products under section R802.10 — designed, braced, and connected per the truss documents, and never field-cut.

Keeping Water Out

Waterproofing details (Topic 194)

Dampproofing and waterproofing are different systems with different triggers, and the code does not use the words interchangeably. The default for foundation walls that retain earth and enclose space below grade is dampproofing (CRC section R406.1), from finished grade down to the higher of the top of the footing or 6 inches below the top of the basement floor — on masonry, at least a 3/8-inch portland cement parging under the coating. Waterproofing — a listed membrane system — is mandatory only where a high water table or other severe soil-water conditions are known to exist (section R406.2), a finding that in practice comes from the geotechnical report or the building official; on the CBC path the same split runs through section 1805 by whether hydrostatic pressure will occur. Now the ADU reality check: most detached ADUs are slab-on-grade at or above surrounding grade, with no below-grade wall at all — section R406 is simply not triggered, and blanket “waterproof every ADU foundation” advice is wrong. What does apply to that slab is the vapor retarder in the slab section above and the drainage rules: grade falling away from the foundation, and foundation drains where below-grade space exists. Waterproofing a wall while omitting the drainage that relieves the pressure is half an assembly.

Weather-resistant barriers (Topic 195)

Behind every cladding sits a drainage plane. CRC section R703.2 requires not fewer than one layer of water-resistive barrier over the studs or sheathing of all exterior walls — No. 15 asphalt felt complying with ASTM D226 Type I, or another approved barrier — installed horizontally with the upper layer lapped over the lower by at least 2 inches, joints lapped at least 6 inches, the barrier overlapping required flashings by at least 2 inches so it sheds onto them, and continuous to the top of the wall. The system rule behind it (section R703.1.1) is that the envelope must prevent water accumulating inside the wall by combining the barrier with a means of draining penetrating water back out. Two corrections belong here. First, an ADU is a dwelling — it cannot borrow the detached-accessory-building exception to skip the barrier. Second, the much-shared claim that the new code cycle requires all housewrap laps to be taped and sealed is not in the 2024-cycle text: sealed laps are required where the manufacturer’s instructions require them, or where the barrier is also serving as the project’s air barrier. Stucco walls are the two-layer case, next topic.

Window flashing (Topic 197)

Flashing is applied shingle-fashion — every layer shedding onto the next — from approved corrosion-resistant materials (CRC section R703.4): self-adhered membranes per AAMA 711, fluid-applied membranes per AAMA 714, mechanically attached flexible flashings per AAMA 712. At window and door openings the flashing must extend to the surface of the exterior wall finish or to the water-resistive barrier for subsequent drainage, and the rough-opening gap is air-sealed on the interior side. The compliance hierarchy matters more than any single detail: install per the fenestration manufacturer’s instructions first (or the flashing manufacturer’s, where the window maker is silent); where no instructions are provided, pan flashing is required at the sill, sealed or sloped to direct water out to the finish surface or the barrier, with protection at the head and sides; a registered design professional’s detail or another approved method are the remaining paths. The code requires the pan to drain out — it sets no slope in degrees, and any degree figure quoted as a code requirement is trade practice wearing a costume. The diverter flashing where a roof eave dies into a sidewall — the kickout — lives in the roof-flashing provisions (section R903.2 family, metal flashing not less than 0.019 inch thick) together with section R703.4’s wall-and-roof intersection item; citing it to the window subsection is the tell that a page was never checked. Pitfall: head flashing buried behind the barrier above it, which turns the drainage plane into a funnel aimed at the opening.

Exterior cladding (Topic 198)

Cladding is the visible layer of the same water-management system: an approved material, installed per its section of CRC Chapter 7 over the barrier, with flashing, and a drainage exit at the bottom. The Southern California production case is stucco — portland cement plaster under section R703.7 in the current code cycle (older California plan sets citing section R703.6 for stucco are using the previous numbering; in the current code that section covers wood shakes and shingles) — applied per ASTM C926 over lathing per ASTM C1063, in three coats over metal lath (two coats over masonry, concrete, or other listed backing). Behind plaster over wood-based sheathing go two independent layers of vapor-permeable barrier, each at least 60-minute Grade D performance — the most-missed cladding provision in California residential work is stucco over a single layer of housewrap. The drainage exit is the weep screed (section R703.7.2.1): corrosion-resistant, at least 0.019 inch (No. 26 gage) with a minimum 3½-inch attachment flange, at or below the foundation plate line, at least 4 inches above the earth or 2 inches above paved areas, with the barrier lapping the flange and the lath terminating on it — a screed buried by landscaping or sealed over by plaster has closed the drain it exists to open. Vinyl siding (section R703.11) and fiber-cement are installed per their manufacturer’s instructions over the same barrier-and-flashing system; clearances from Topic 187’s decay rules apply to all of them. One zone-triggered overlay: parcels inside a mapped Fire Hazard Severity Zone or designated wildland-urban interface fire area take the exterior-covering rules of CRC Chapter R337 (section R337.7: noncombustible, ignition-resistant, heavy timber, or a listed assembly) — most flatland urban lots are outside those zones, foothill and interface lots are the case, and the map check comes before the material list.

Insulation: The Other Half of “Installed”

Insulation installation (Topic 196)

Insulation answers to the Energy Code, and classification decides the rulebook before any number does: a new detached ADU is a newly constructed building (section 150.1), an attached ADU or a conversion of unconditioned space is an addition (section 150.2(a)), and a conversion of already-conditioned space is an alteration. For new construction the prescriptive standard is an assembly U-factor, not a bare R-value (Table 150.1-A): wood-framed walls at U-0.048 in climate zones 1–5 and 8–16 — typically 2×6 framing with R-21 cavity insulation plus R-5 continuous insulation, the continuous layer being what defeats framing thermal bridging — and U-0.065 in coastal zones 6 and 7. Mandatory wall minimums moved in the 2025 cycle: section 150.0(c) now reads R-15 in 2×4 walls and R-21 in 2×6 walls — the R-13 and R-20 figures still printed everywhere are the superseded cycle. Ceilings under the 2025 prescriptive options run R-38 in zones 1 and 8–16 and R-30 in zones 2–7 for the vented-attic case with ducts in conditioned space, R-38 in all zones for cathedral ceilings, and raised floors at R-19-class (U-0.037) — no California prescriptive case verified for this guide exceeds R-38, so “R-49 required” copy is imported national content. Your zone is found by address in the CEC climate-zone lookup, never assigned by city name. For additions and conversions, section 150.2(a) carries the rule our garage conversion work runs on: where existing siding is not removed, cavity insulation of R-15 in 2×4 or R-21 in 2×6 framing, with no continuous-insulation requirement. Installation quality is itself regulated where Quality Insulation Installation applies — all prescriptive new construction, and additions over 700 square feet; additions of 700 square feet or less are prescriptively exempt from QII (section 150.2(a)1Bv), unless QII was modeled for a compliance credit, in which case an ECC rater verifies it anyway. Where QII applies, Reference Appendix RA3.5 is the standard and the four named failure modes are the whole checklist: a missing or broken air barrier, insulation not in contact with the air barrier, voids and gaps, and compression — batts split around wiring and plumbing, cut square around boxes, never stuffed. A conversion’s QII exemption waives only the verification layer, not the insulation itself. The electrical half of the envelope — recessed lighting and ventilation — lives in our electrical and HVAC guide.

Running the Job, and Finishing It

Construction staging (Topic 199)

Verified negative, stated plainly so nobody has to hunt for a rule that does not exist: neither the CBC nor the CRC contains any construction-staging standard for a backyard ADU — nothing on stockpile locations, delivery sequencing, temporary access routes, or staging dimensions. This section is therefore practice, labeled as practice. What actually governs the edges: if any part of the operation — a dumpster, a crane pick, a concrete pump, a material drop — occupies the street, sidewalk, or parkway, that is a local encroachment permit from the city or county public works department, triggered by use of the public right-of-way, not by project size; worker safety on site is Cal/OSHA, binding the contractor as employer and kept in a separate lane from the building permit; and stockpiles and spoils may not regrade the lot so it drains against the foundation or the neighbor’s. The practice itself is unglamorous and decisive on small occupied lots: stage materials on the lot and off the right-of-way, sequence deliveries to the build (the framing package lands after the foundation is signed off, not before), keep inspection access and the occupants’ egress clear, protect the existing house’s utilities, and settle neighbor access by agreement in advance — there is no code shortcut for any of it. How staging is handled is one of the clearest tells of how the rest of the job will be run; our process page shows the sequence we hold to.

The final punch list (Topic 200)

“Punch list” is not a code term either: no code defines one, sets a procedure for one, or fixes a number of days to clear one. The punch list is contract practice — a final walk-through, a written list of incomplete or deficient items, completion, then final payment — and the law supplies the frame around it in three places. The occupancy gate: a building may not be used or occupied until the building official has issued a certificate of occupancy (CBC section 111.1; CRC section R110) — with the California practice note that many jurisdictions close an ADU permit by final-inspection sign-off, the signed inspection record or certificate of completion functioning as the occupancy document; either way, occupancy follows the city’s final sign-off, cosmetic punch items do not soften that, and a certificate is expressly not an approval of any code violation. The contract statute: Business and Professions Code section 7159 governs the home improvement contract itself — written, signed, and a copy in the buyer’s hands before work starts — and section 7159(c)(4) puts the lien-release promise inside every contract: on satisfactory payment for any portion of the work, the contractor must, before any further payment is made, furnish a full and unconditional release for that portion. Extra work becomes part of the contract only by a written change order signed by both parties before the work begins — the rule that decides most punch-list payment disputes. The releases: California’s lien waivers are statutory forms, and the four are not interchangeable — Civil Code section 8132 (conditional release on progress payment), section 8134 (unconditional on progress), section 8136 (conditional on final payment), and section 8138 (unconditional on final payment, the project-closing release, signed only after funds have cleared). The close-out sequence that follows from the three frames: punch walk documented in writing against the contract and signed change orders → items cleared → final inspection and occupancy sign-off on file → final payment exchanged against the conditional final release, with unconditional final releases collected from the contractor and every claimant once payment clears. One number this guide deliberately does not print: no retention percentage for private residential work is stated here, because we have not verified one from the statutory text — holdback on an ADU is set by your section 7159 contract, so agree the amount and its release conditions in writing before work starts.

The Order We Build In

1. Soil before drawings: the bearing class, the expansive-soil test, and the geotechnical triggers decide the foundation before the floor plan is priced.
2. Foundation matched to the ground: prescriptive spread footings where the lot allows them; section 1808.6 design on expansive soil; hillside geometry and retaining-wall triggers checked against the survey, not the zoning map alone.
3. Protection cast in: termite method on the plan note, stem-wall steel and cover verified before the pour, anchor bolts laid out with their plate washers.
4. Frame as a system: grade stamps matched to the plans, bracing read from the current tables in feet, ties in the bottom third, uplift path continuous to the foundation.
5. Envelope as one assembly: dampproofing or waterproofing by the actual trigger, barrier lapped shingle-fashion over flashing, stucco on two layers with an open weep screed, insulation installed to RA3.5 where QII applies.
6. Close-out on paper: final sign-off before occupancy, punch items against the contract, releases in the statutory forms.

Run in that order and the structure stops being the surprise. When you want the ground truth on your own lot before design, our ADU Feasibility Report covers soils and foundation conditions up front — or start with a free consultation and bring us the address.

Sources (verified November 2026): California Residential Code (Title 24, Part 2.5) sections R317, R318 (2025 CRC: R304, R305), R401–R408, R403.1, R404.1, R405.1, R406, R506, R602.2, R602.3, R602.7, R602.10, R602.11.1, R703, R802, R903.2, and Chapter R337; California Building Code (Title 24, Part 2) sections 105.2, 111, 1613, 1803–1810, 1804, 1805, 1806, 1807, 1808, 1809, 1907, 2308, and Appendix J (where locally adopted); ASTM D4829 (Expansion Index) and D1557; California Energy Code (Title 24, Part 6, 2025) sections 150.0(a), 150.0(c), 150.1 and Table 150.1-A, and 150.2(a), with the CEC’s 2025 Energy Code ADU FAQs and Reference Appendix RA3.5; CALGreen section 4.505.3; Cal/OSHA Title 8 excavation provisions; Government Code section 4216; Business and Professions Code section 7159; Civil Code sections 8132–8138; Seismic Hazards Mapping Act and CGS Special Publication 117A; HCD Accessory Dwelling Unit Handbook, March 2026 edition (building-code compliance framing). Where this guide describes construction practice, it is labeled as practice in the text; edition-specific citations are labeled by edition, and local grading, encroachment, and permit-threshold ordinances are confirmed with the project’s city or county. This page is part of the California Accessory Dwelling Unit Handbook on this site (Construction and Foundations, Questions 351–400). Related: Electrical & HVAC · Plumbing & Utilities · ADU Permit Process · Property Eligibility · ADU FAQs