ADU Plumbing and Utilities in California: Sewer, Water, Pressure, Water Heaters, Gas and Fees

An ADU lives or dies underground. The sewer lateral, the slope in the trench, the size of the water service, and one state statute — Government Code section 66311.5 — decide whether plumbing is a routine line item or a five-figure surprise. This guide works through all of it in build order.

Two different rulebooks govern ADU plumbing in California, and confusing them is the most common planning mistake. State ADU law (Gov. Code §§66310–66342) decides whether a utility may require a new connection and what it may charge. The California Plumbing Code (CPC) decides how the pipes themselves must be sized, sloped, vented, and tested. A city cannot use the plumbing code to defeat the fee protections in section 66311.5, and section 66311.5 does not waive a single CPC slope, vent, or pressure rule. This page is the Plumbing and Utilities chapter of our California Accessory Dwelling Unit Handbook.

Sewer: The Lateral You Already Own

Your existing sewer lateral (Topic 201)

The building sewer begins two feet outside the building under the CPC, and the junction between the building drain and the building sewer is where a two-way cleanout, extended to grade, belongs. Nothing in the CPC bans reusing an existing lateral for an ADU. Reuse turns on three checks: the pipe is large enough under CPC Table 703.2 for the combined drainage fixture units, it is in sound condition, and the sewer agency accepts the connection. Pitfall: assuming the lateral is fine because the house drains fine today — an ADU adds a full bathroom and kitchen to a pipe that may be 60-year-old clay with root intrusion at every joint.

Sewer camera inspection (Topic 202)

Here is a verified negative worth knowing: no California statute, the CPC, or the HCD Handbook requires a sewer camera inspection for an ADU. The CPC requires new work to pass a water or air test (5 psi air test), not a camera pass. Some cities and sewer districts impose private-sewer-lateral ordinances triggered by a sale, a large remodel, or a meter upsizing — those are local rules, checked at intake. We camera laterals before design anyway, as practice: a $300–$500 inspection (typical market range, not a code figure) finds the offset joint or belly that would otherwise surface as a change order after the slab is poured.

Sewer pipe slope (Topic 203)

California follows the UPC-based CPC, not the IPC — the slope numbers most websites quote are the wrong code. CPC section 708.1: horizontal drainage runs at a uniform slope of 1/4 inch per foot (about 2%). Only where that is impractical, and only with the building official’s approval first, may pipe 4 inches or larger run at 1/8 inch per foot (about 1%). Too flat and solids settle; too steep is not the fix homeowners imagine either — the code answer is uniform slope at the specified grade, verified before backfill. Slope is why sewer depth at the ADU and distance to the connection point drive cost more than fixture count does.

Connection location, separate connections, and capacity (Topics 204–206)

The CPC governs the on-lot work — pipe separations from water lines, building-sewer construction, and cleanouts. Whether the ADU ties into the existing lateral or needs a new tap to the street main is the sewer agency’s call, not the plumbing code’s. State law then limits what that call can cost you:

On capacity, the math is fixture units. CPC Table 702.1 counts a private toilet (1.6 gpf tank) at 3 DFU, a lavatory at 1, a tub or shower at 2, a kitchen sink at 2, and a clothes washer at 3 — a typical one-bath ADU totals roughly 10–13 DFU before the house’s load is added (an illustration, not a substitute for the project count). CPC Table 703.2 then caps horizontal pipe: a 3-inch line carries 35 DFU maximum, a 4-inch line 216. Most sound 4-inch laterals have ample capacity; condition, not capacity, is usually the fight.

Water: Service, Meters, and Pressure

Water service sizing (Topic 207)

Water sizing runs the opposite direction from drainage: CPC section 610.0 and Tables 610.3–610.4 convert water supply fixture units (WSFU) through street pressure and developed pipe length into a meter and service size. Private-use WSFU values include 4.0 for a tub, 4.0 for a clothes washer, 2.5 for a flush-tank toilet, 2.0 per shower head, and 1.5 each for a kitchen sink and dishwasher. The minimum building supply is 3/4 inch. Pitfall: sizing off fixture count instead of the table — a long run from the meter at modest street pressure needs a larger service than the same ADU ten feet from the main.

Meters: required, separate, or submeter? (Topics 208–209)

The CPC does not size or require water meters — your water utility’s tariff and standards do. State ADU law mirrors the sewer split: for a §66323(a)(1) conversion or JADU, the agency may not require a new or separate direct connection (§66311.5(d)); for other ADUs it may, at a proportionate charge (§66311.5(e)). Section 66311.5 regulates a connection directly to the utility — a private submeter behind the house meter is a different device entirely: neither required nor barred by that statute, and the practical tool when a landlord wants usage-based billing without a second utility account (see Shared Billing below).

Water pressure and pressure regulators (Topics 210–211)

CPC section 608.2 sets the ceiling: 80 psi static, maximum. Where static pressure exceeds 80 psi, an approved pressure regulator preceded by a strainer (strainer not required at 1-1/2 inches and larger), complying with ASSE 1003 or AWWA C530, must reduce pressure to 80 psi or less; an expansion tank belongs downstream, and section 608.3 independently requires expansion control on any closed system created by a check valve, backflow preventer, or regulator. At the other end, section 608.1 expects at least 15 psi residual at the most remote fixture. Hillside and valley neighborhoods in the same city can sit on opposite sides of both numbers — measure at the hose bibb with a gauge before design, not after the shower disappoints.

Hot Water: Tank, Tankless, or Heat Pump

The system choice (Topic 212)

No code requires an ADU to have its own water heater — sharing the house’s heater is legal where capacity and the energy code allow — but a separate heater is the usual rental answer. Any storage heater install carries the same CPC package: permit (§503.0), temperature-and-pressure relief with an unobstructed discharge terminating 6–24 inches above grade (§§504.6, 608.5), seismic strapping in the upper and lower thirds (§507.2), and ignition sources at least 18 inches above a garage floor unless the unit is flammable-vapor-ignition-resistant (§507.13). The energy code adds pipe insulation — all domestic hot-water piping insulated, 1 inch thick (R-7.7) on 3/4-inch to under-1-inch piping — and prescriptive recirculation means demand recirculation with manual controls, not a timer loop heating pipe all day.

Tankless (Topic 213)

Tankless units are water heaters under CPC Chapter 5 (permit, relief, venting, and Chapter 12 gas sizing all apply; the storage-tank strapping rule does not). The 2025 Energy Code changed the planning math: the old prescriptive exception that let a gas tankless heater (up to 200,000 Btu/h) satisfy the water-heating budget in several climate zones was removed — for new construction and additions, gas tankless now fits through the performance path, and a new detached ADU using gas appliances also triggers the electric-ready provisions discussed below. Small all-electric units remain prescriptive in one narrow lane: an ADU of 500 square feet or less may use point-of-use electric water heating under the cited exception and reference appendix limits on run length.

Heat pump water heaters (Topic 214)

A new detached ADU is a newly constructed building under the 2025 Energy Code, section 150.1(c)8. The prescriptive water-heating options are a single 240-volt heat pump water heater, a NEEA Tier 3 or better 240-volt heat pump water heater, or solar water heating with electric backup at a solar savings fraction of at least 0.7. Two exceptions matter for small units: 500 square feet or less may use point-of-use electric, and an ADU with one bedroom or fewer may use a 120-volt heat pump water heater. Conversions are additions, judged under section 150.2(a) instead — and a like-for-like heater replacement in an existing unit is an alteration, where gas replacement remains expressly allowed. Pitfall: quoting “heat pump water heater required in every ADU” — it is one prescriptive option for new detached construction, not a universal mandate.

Drains, Vents, and Cleanouts Inside the Unit

Kitchen drain and vent (Topic 215)

A kitchen sink runs on a 1-1/2-inch trap and trap arm at 2.0 DFU (a disposer and a dishwasher plumbed through it do not add fixture units), with a 2-inch minimum drain below. Trap-arm geometry is where UPC and IPC part company — California’s Table 1002.2 allows 42 inches maximum for a 1-1/2-inch trap arm and 60 inches for 2-inch, with a 2-to-4-inch trap seal. An island sink cannot reach a wall vent, so the CPC answers with the section 909.1 island fixture vent (loop vent): the vent rises to at least the drainboard height, returns down through a wye with a foot vent sloping back at 1/4 inch per foot, and carries an accessible cleanout — and that island drain serves no other fixtures. A dishwasher needs a proper air gap, not just a high loop. One caution: air admittance valves are widely sold in California but are not a CPC entitlement — treat them as an alternate-method request to your building official, never a plan assumption.

Bathroom drainage and wet venting (Topic 216)

Bathroom DFUs: toilet 3, lavatory 1, tub 2, shower 2 (2-inch shower trap). Wet venting is allowed but narrow — CPC section 908.2 permits it for a bathroom group only, with the lavatory(s) forming the wet vent, the toilet discharging last and downstream, sized 2 inches up to 4 DFU and 3 inches at 5 DFU and above (§908.2.2). Where the ADU sits below the sewer’s grade — a downhill detached unit or a basement bath — the answer is a sewage ejector under section 710.3: at least 20 gpm capacity, a discharge of 2 inches or more with backwater and gate valves (a single dwelling’s pump must pass a 1-1/2-inch ball), in a gas-tight, vented pit. An ejector is a maintenance commitment for the life of the unit, which is why gravity is designed first wherever the slope allows.

Cleanouts (Topic 217)

Cleanouts are the cheapest insurance in the whole system. CPC section 707.4 requires one at the upper terminal of each horizontal drain, one for each 100 feet of run, and one for each aggregate change of direction over 135 degrees; the two-way cleanout outside the building at the drain/sewer junction, extended to grade, is the access point every future camera and cable job will use. Clearance in front of a cleanout is 12 inches for pipe 2 inches and smaller, 18 inches above that (§707.9). Burying or paving over the junction cleanout in a conversion is a classic self-inflicted wound.

Backflow prevention (Topic 218)

Backflow rules protect the public main from your property, and they are set by your water purveyor’s hazard assessment, not by a single statewide device list: the device must match the hazard (air gap above reduced-pressure assembly above double-check), and assemblies are tested at installation and annually. Note for anyone reading older guidance: the former Title 17 backflow regulations were repealed; current authority is the State Water Resources Control Board’s Cross-Connection Control Policy Handbook, effective July 1, 2024. Most single-family ADUs on a standard service need nothing beyond what the purveyor already requires at the meter — irrigation systems, fire sprinklers, and auxiliary water supplies are what usually trigger an assembly.

Gas, All-Electric, and Getting Utilities to the Unit

Gas service options (Topic 219)

Where gas is used, CPC Chapter 12 sizes the whole system by the longest-length method: total the connected Btu load, convert to cubic feet per hour using the local heating value (about 1,100 Btu per cubic foot in SoCalGas territory), and size every segment for the standard low-pressure basis of 0.5 inch water-column pressure drop. Adding an ADU’s furnace, water heater, range, and dryer to a houseline sized for the house alone is a calculation, not a guess — undersized gas is the rough-in failure that shows up at the first cold shower. Utility practice in Southern California: the houseline passes city inspection before the meter is set, and for service applications after June 30, 2023, SoCalGas charges actual project costs with no allowances (a CPUC-driven change). Size the load early; a meter upsizing discovered late re-sequences the whole job.

All-electric ADUs (Topic 220)

The California Energy Commission says it plainly: the 2025 Energy Code does not mandate specific technologies or fuel types. What it does require of a new ADU that installs gas appliances is readiness for the electric future — section 150.0(n) electric-ready provisions (dedicated circuits and panel space for future electric water heating, cooking, and drying), with battery-storage readiness under section 150.0(s) subject to a small-panel exception. Conversions and additions sit under section 150.2(a), where the CEC’s own manual imposes no electric-ready requirement — so a garage conversion may go gas, all-electric, or mixed on the merits, while a new detached unit that chooses gas must also wire for electric. Local reach codes vary city by city and have been in legal flux since the Berkeley litigation; confirm your city’s current ordinance at intake rather than relying on any statewide claim.

Trenching routes (Topic 221)

Every detached ADU digs. California’s dig law, Government Code section 4216, runs through DigAlert in Southern California: notify at least 2 working days before digging (the notification day does not count) and no more than 14 calendar days ahead; the ticket is valid 28 days; and inside the tolerance zone — 24 inches on either side of the marks — you locate lines by hand before any power digging. Route design itself balances the shortest code-compliant path against roots, hardscape you intend to keep, and the CPC’s same-trench water/sewer rules (section 609.2: approved materials, with the water line above the sewer under the stated separation scheme — not the 5-foot rule quoted from other codes). Photograph every open trench before backfill; that photo set is the as-built your future self will pay for otherwise.

Utility easements (Topic 222)

Another verified negative: state ADU law contains no easement rule and no easement override. Sections 66310–66342 never mention easements, and a ministerial ADU approval does not authorize building inside one. General property law governs — an easement is a right-of-way interest under Civil Code section 801, and its extent is fixed by the terms of the recorded grant (section 806 — see our property eligibility guide, which works the full easement sequence). For plumbing specifically: trenching a new lateral through a utility easement is exactly what the easement is usually for; placing the ADU’s slab over one is the violation. Plot easements on the site plan before the trench route is drawn.

Paying for Water: Billing and Fees

Shared billing and submeters (Topic 223)

Start with the connection question from section 66311.5 above — it decides whether a second utility account can even be required. Behind one meter, three legal lanes govern how costs reach a tenant. For gas and electric, Civil Code section 1940.9 requires disclosure and a written agreement when a tenant’s meter also serves an area outside their unit, with reimbursement or customer-of-record remedies. For water, Water Code section 537.1 and Civil Code sections 1954.201–1954.219 set the submeter billing rules for covered multi-unit properties: bill at the purveyor’s rate with no markup, disclose before the lease, and itemize — ratio billing (RUBS) for water is left to local law, so check your city before promising a split formula. Whether section 537.1 reaches a single-family-lot ADU is not settled on the sources — confirm with your purveyor at intake. The design-stage decision that avoids most of this: separate what is cheap to separate during construction (a water submeter, separate electric panel), and put whatever remains shared in the lease in plain numbers.

Water and sewer district fees (Topic 224)

The fee stack, in the order section 66311.5 builds it: an ADU is not a new residential use for connection or capacity charges (§66311.5(b), new-house exception aside); no impact fee at all for an ADU under 750 square feet of internal livable space, and only a proportionate fee above it (§66311.5(c)(1)); conversions under §66323(a)(1) pay no connection or capacity charge (§66311.5(d)); and any charge on another ADU must be proportionate by square footage or fixture units and capped at the reasonable cost of service (§§66311.5(e), 66013). Ongoing monthly rates are a separate legal lane (Proposition 218) — the statute limits what connecting costs, not what water costs. Get the district’s fee letter in writing before permit issuance; a verbal estimate at the counter is not a number to underwrite a loan with.

Leak detection (Topic 225)

Final verified negative: no California code — CPC, CRC, CALGreen, or the Energy Code — requires electronic leak detection or an automatic shutoff valve in a single-family ADU. What the code does require around water damage is mechanical: a water-heater pan at least 1-1/2 inches deep with a drain of at least 3/4 inch where a leak could damage the building (CPC §507.5), T&P relief discharge (§§608.3, 608.5), and seismic strapping (Health & Safety Code §19211). Smart leak detection with automatic shutoff is an insurance-and-peace-of-mind choice — genuinely valuable over a finished garage conversion’s new floor, and increasingly requested by insurers — but anyone who tells you the city requires it is selling, not citing.

The Pre-Design Plumbing Checklist

1. Camera the lateral and get its size, material, depth, and condition on video before design money is spent.
2. Run the DFU count for house + ADU against CPC Table 703.2, and the WSFU/pressure math for the water service.
3. Classify the ADU under §66311.5 first — conversion or new build decides the connection and fee answers before anyone quotes them.
4. Gauge the pressure (static and residual) and plan the regulator/expansion tank if you are over 80 psi.
5. Pick the water heater against the right energy-code section — §150.1(c)8 for new detached, §150.2(a) for conversions — and the gas load calculation if any gas appliance stays.
6. Call DigAlert before any trench, plot easements on the plan, and photograph open trenches before backfill.

Run in that order, plumbing stops being the surprise line in the budget. When you want those checks run on your own property, our ADU Feasibility Report covers utility capacity before design — or start with a free consultation and bring us the address.

Sources (verified November 2026): California Plumbing Code (IAPMO) sections 503.0, 507.2, 507.5, 507.13, 504.6, 608.1–608.3, 608.5, 609.2, 610.0 and Tables 610.3–610.4, 702.1, 703.2, 707.4, 707.9, 708.1, 710.3, 908.2, 909.1, 1002.2, and Chapter 12; Government Code sections 66311.5, 66013, and 4216 (DigAlert); Civil Code sections 801, 806, 1940.9, and 1954.201–1954.219; Water Code section 537.1; Health and Safety Code section 19211; California Energy Code (2025) sections 150.0(j), 150.0(n), 150.0(s), 150.1(c)8, and 150.2(a) with CEC ADU guidance; State Water Resources Control Board Cross-Connection Control Policy Handbook (eff. July 1, 2024); HCD Accessory Dwelling Unit Handbook, March 2026 edition (utility-fee framework). Where this guide describes inspection, trenching, or billing practice, it is labeled as practice in the text; utility-specific requirements are confirmed with the serving agency. This page is part of the California Accessory Dwelling Unit Handbook on this site (Plumbing and Utilities, Questions 401–450). Related: ADU FAQs · ADU Permit Process · Property Eligibility · ADU Feasibility Report