Types of ADUs in California: Basement & Attic Conversions, Prefab, Modular, Accessible ADUs and More

Detached, attached, and garage conversions get most of the attention — but California ADU law reaches much further: basements, attics, and spare rooms inside the house, units built in factories instead of on site, accessible designs, ADU-and-JADU combinations, and what happens when a disaster destroys a home. This guide covers every remaining type, and the two questions that classify any of them.

Every ADU is sorted by two separate questions. First: where does the space come from? — converted from space that already exists, or built new. That is the question state ADU law actually asks, and it decides your approval pathway, your setbacks, and your size limits. Second: how is it built? — framed on your lot, built in a factory, or manufactured to the federal HUD code. That question is answered by a different body of law entirely (the Health and Safety Code), and it decides which building code applies and who inspects what. Confusing the two questions is where most “prefab ADU” marketing goes wrong. This page is the Types chapter of our California Accessory Dwelling Unit Handbook; the headline types have their own pages — detached, attached, garage conversion, and second-story ADUs.

The Types at a Glance

Basement Conversions: The Space Under the House

A basement conversion creates the ADU inside space that already exists below the primary home, and state law recognizes it directly. On a single-family lot, the conversion pathway in Government Code section 66323(a)(1) covers an ADU created “within the proposed or existing space of a single-family dwelling” — and the California Department of Housing and Community Development (HCD) confirms in its ADU Handbook that an attached ADU can be built in a basement, including a newly constructed one under section 66314(d)(3). On a multifamily lot the recognition is explicit: section 66323(a)(3)(A) lists basements among the non-livable spaces (with storage rooms, boiler rooms, passageways, attics, and garages) that may be converted to ADUs. Because a conversion sits inside an existing structure, conversions are not subject to setback requirements, and their height is limited to the dimensions of the original structure — but they must meet all building, health and safety, and fire standards for dwellings, like every ADU.

Three physical checks decide whether a basement works:

• Ceiling height. The California Residential Code (CRC) generally requires 7 feet of ceiling height in habitable rooms (section R305.1), and its basement provisions can allow 6 feet 8 inches in a qualifying basement of a dwelling unit. Measure the finished height — floor to ceiling after the new floor and ceiling finishes go in, accounting for beams and ducts — before you fall in love with the layout. If the slab has to be lowered to win headroom, that is a structural and waterproofing project, not a cosmetic one.
• Escape and rescue openings. A basement ADU’s sleeping rooms need an emergency escape and rescue opening under CRC section R310 — a window or door a person can climb through and rescuers can enter, with a minimum net clear opening of 5.7 square feet (5.0 square feet for a grade-floor opening), a sill no more than 44 inches above the floor, and minimum clear dimensions of 24 inches high by 20 inches wide. In a basement that usually means a window well sized and drained so the opening can actually be used. Light and natural ventilation for habitable rooms are checked at the same time.
• Water. Below-grade living space lives or dies on moisture: exterior waterproofing or a drainage system, a vapor barrier under any new floor finish, Radon-resistant construction where the local code requires it, and plumbing that accounts for below-grade drainage (a sewage ejector where the sewer line sits above the basement floor). None of that is ADU-specific law — it is the ordinary building standard for turning below-grade space into a dwelling, and it is where basement budgets are won or lost.

What to verify: finished ceiling height, the escape-opening design, and how the existing drainage and sewer elevations work — all before design money. The pitfall: pricing a basement conversion like a garage conversion. The permit pathway is similar; the below-grade physics are not.

Attic Conversions: The Space Over the House

An attic conversion uses the same conversion pathways as a basement: within the existing dwelling under section 66323(a)(1) on a single-family lot, and on a multifamily lot section 66323(a)(3)(A) names attics expressly alongside basements. The permit category is easy; the geometry is the project.

• Headroom is measured by the code’s counting rules, not by standing in the middle. Under CRC section R305.1, habitable rooms generally need a 7-foot ceiling; for sloped ceilings, at least half of the room’s required floor area must meet that height, floor area under a ceiling lower than 5 feet does not count as habitable area, and a habitable attic needs a qualifying area — at least 70 square feet where the ceiling is at least 5 feet high. A wide attic with a low ridge can produce a much smaller legal bedroom than the floor plan suggests.
• Access. The unit needs real stairs, not a pull-down ladder: the CRC stair rules require 6 feet 8 inches of headroom over the stair run (section R311.7.2). One expansion is written into the conversion pathway: up to 150 square feet beyond an existing accessory structure’s dimensions, solely to accommodate ingress and egress (section 66323(a)(1)(A)) — HCD’s own example is a stairwell added for a second-story conversion. For an attic inside the house itself, whether a stair enclosure or dormer addition fits that allowance is a plan-check question to settle with the building department before design, not an assumption to build on.
• Structure. An attic framed for storage was not framed for a home. Residential design loads for living and sleeping space (30–40 pounds per square foot under ASCE 7) are higher than attic storage loads, so the floor joists, bearing, and load path down to the foundation get verified — and commonly upgraded — under a structural engineer’s review. Get that answer before the design is drawn, not at plan check.
• Heat and energy. The roof assembly becomes the unit’s ceiling: insulation, ventilation, and the Energy Code compliance for the conversion are resolved in the permit set (conversions are generally handled under the Energy Code’s rules for changes to existing buildings — your designer confirms the exact path at plan check).

What to verify: ridge height and the 5-foot/7-foot floor-area count, a stair location that fits, and the joist capacity. The pitfall: counting the whole attic footprint as the ADU’s size — the legal unit is only the part you can stand in, heat, and escape from.

Interior Conversions — and the JADU Fork

Carving a unit out of the home’s existing interior — a primary-suite wing, a little-used family room level, part of a large floor plan — is a conversion under section 66323(a)(1): one ADU per lot within the converted space of the dwelling, with its own exterior access (section 66323(a)(1)(B)). The 150-square-foot ingress-and-egress expansion in (a)(1)(A) is written for conversions of accessory structures — it is not part of an in-house conversion’s allowances. It is often the lowest-impact way to add a unit because the envelope, roof, and utilities already exist.

It is also where homeowners meet the one fork that changes everything downstream:

• An interior ADU is a complete, independent unit — full kitchen, its own bathroom, no size cap from the conversion pathway itself other than the space you have, and no owner-occupancy requirement (section 66315).
• A junior ADU (JADU), under Government Code sections 66333–66339, is capped at 500 square feet of interior livable space, needs only an efficiency kitchen, and may share a bathroom with the main home. In exchange it carries rules a standard ADU does not: a recorded deed restriction, no separate sale ever, and — if it shares sanitation with the main house — an owner-occupancy requirement (as clarified by AB 1154; the full comparison is in our FAQ and law hub).

What to verify: which of the two you are permitting, stated on the plans — the deed restriction and occupancy rules attach at permit time, not at move-in. The pitfall: designing a “small in-house unit” without choosing, then discovering the JADU’s restrictions in the title paperwork at closing or refinance.

Accessible ADUs: A Design Choice, Not a Permit Category

Here is the verified negative first: California ADU law imposes no accessibility mandate on a private home’s ADU. Nothing in Government Code sections 66310–66342 conditions an ADU or JADU permit on accessibility features, and HCD’s Handbook states no accessibility requirement for ADUs — the statute’s single use of the word “accessible” is about utility access, not disability access. The familiar accessibility codes do not fill that gap for a single-family backyard unit: the ADA and California Building Code Chapter 11B govern public and commercial space, and Chapter 11A governs covered multifamily housing — neither reaches a detached or converted ADU behind a private home. One honest wrinkle: asked whether a permitting agency may impose accessibility standards on an ADU converted from existing space, HCD’s Handbook answers “Maybe” and refers homeowners to its Division of Codes and Standards — so on a conversion project, ask the building department directly whether any accessibility standard is being applied to your existing building. (Larger multifamily projects are a different analysis — if your project is part of one, confirm its accessibility obligations with the building department.)

What that means practically is that accessibility in an ADU is a specification, not a compliance exercise — and it is one of the best-value specifications there is, because almost all of it is free at the drawing stage and expensive later:

• Single-story, step-free plan — the unit, its entry, and its parking or path on one level, with a no-step entrance.
• Room to move — doorways with 32 inches or more of clear width, lever handles, a turning circle in the bathroom and kitchen, and hallways that do not pinch.
• A bathroom built for the future — a curbless (roll-in) shower, and blocking inside the walls at the toilet and shower so grab bars can be mounted in an afternoon, whenever they are needed, without opening the wall.
• Everyday details — rocker switches and reachable outlets, good lighting on the entry path, and a laundry that does not require stairs.

What to verify: that the accessibility features you want are drawn and dimensioned in the plan set — “accessible” written in a brochure is not a specification. The pitfall: assuming the permit process will require or check any of it. It will not; if it matters to your family, it has to be in the design brief. Our floor plans and the multigenerational guidance in the Handbook are the place to start the conversation.

How It’s Built: Site-Built, Prefab, Modular, Panelized, Manufactured

Construction method is the second classification question, and ADU law is silent on it: the HCD Handbook — hundreds of pages on ADU rules — never uses the words factory, prefab, modular, or panel at all. There is no “prefab ADU” permit category, fast track, or fee rule in state law. What exists instead are three legal buckets in the Health and Safety Code, and the bucket — not the brochure — decides which building code the unit was built to and who inspected it:

So what do “prefab,” “modular,” and “panelized” actually mean?

• Prefab is an umbrella marketing term with no legal definition. The same product sold as “prefab” can be factory-built housing, a manufactured home, or a kit of parts permitted as site-built — ask which bucket, and for the paperwork that proves it.
• Modular, in California, usually means factory-built housing: volumetric sections built to the state code, HCD-insignia-bearing, assembled on a foundation. It does not mean HUD-code — and if a seller’s “modular” unit turns out to be a HUD-code manufactured home, the manufactured-home rules (including its installation process) are the ones that apply.
• Panelized sits between the two: wall or floor panels are built in a factory and assembled on site. If the panels are an HCD-approved factory-built component or system — plan-approved, inspected in the plant, insignia-bearing — that portion is treated as factory-built housing, and the local department inspects its assembly and installation. If the panels are factory-made components whose work can still be inspected when they arrive, the finished dwelling is permitted and inspected locally, as a site-built ADU, in the ordinary way. Which bucket a panel package falls into depends on the manufacturer’s HCD approvals — ask for the insignia or design approval for the exact system before relying on either treatment.
• Energy rules follow the bucket. HCD’s Handbook excludes manufactured homes from the solar requirement that applies to a newly constructed detached ADU. A factory-built ADU is built to the California Building Standards Code, which includes the Energy Code — so treat it like a site-built ADU for energy purposes, and confirm the exact solar sizing and any exception for the specific project with the Energy Commission guidance and your building department.

What to verify: for any off-site product, get the legal bucket in writing — HCD insignia number for factory-built, HUD label for manufactured — and confirm with the building department how installation will be permitted, before you sign. The pitfall: comparing a factory product’s sticker price against a site-built turnkey quote. Transport, crane or set crew, foundation, utility hookups, and the ADU permit sit outside the sticker in every bucket. And a note on a related but different idea: a city’s pre-approved plan program (like the standard-plan catalogs in our Los Angeles guide) pre-approves a design for faster local plan check — it is not a factory certification, and most pre-approved plans are built site-built.

Combinations: ADU + JADU, and ADU + Addition

Can you stack an ADU and a JADU on one single-family lot? Yes — plus a detached ADU. HCD states the required combination plainly: on single-family lots, local agencies must allow at least one ADU constructed from existing space (section 66323(a)(1)), one JADU, and one newly constructed detached ADU (section 66323(a)(2)) — three units alongside the primary home, each under its own rules. The JADU in the stack keeps its own conditions (500 square feet, efficiency kitchen, deed restriction), and the detached unit takes the detached standards (800-square-foot state-exempt size, height tiers of 16, 18, or 20 feet depending on the lot’s conditions under section 66321(b)(4)). Multifamily lots have their own stack — conversion units under (a)(3) plus detached units under (a)(4) — covered in the law hub.

Can you build an addition and an ADU in the same project? Yes, in two different shapes. An attached ADU built as an addition is itself one of the standard types — capped at 50 percent of the existing primary dwelling’s floor area under the local-ordinance pathway (section 66314(d)(4)). Or the project can do both jobs at once: expand the primary home and add a separate ADU (attached, detached, or converted). Two cautions keep the shapes from blurring: lot-coverage limits in a local ordinance attach to the section 66314 pathway rather than to a section 66323 conversion unit (which is still capped at 800 square feet), and an addition permitted as part of the house is part of the house — it does not become a rentable unit later by adding a kitchenette (see the room-addition comparison in our ADU vs. Everything Else guide).

What to verify: that each unit in a combined project is labeled with its own legal category on the plans — plan check reviews each against a different rulebook. The pitfall: one blended “project size” number. The caps apply per unit and per pathway, not to the project as a whole.

Two Design Details the Law Actually Reaches: Separate Entry and Over-Parking Units

A separate entrance is not just good design — for conversions it is the statute’s text. A section 66323(a)(1) conversion ADU “must have exterior access”: its own way in and out that does not pass through the primary home. HCD restates it in the combinations table (“Must have exterior access”). For an accessory-structure conversion, the statute also excludes the added ingress-and-egress area — up to 150 square feet under section 66323(a)(1)(A) — from the unit’s floor-area calculation. JADU law points the same direction — a JADU is created within the walls of the home but with its own direct exterior access (sections 66333–66339). For new detached and attached ADUs, a separate entrance is part of what makes the unit a complete, independent dwelling under the section 66313(a) definition. Plan the entry early: where the door lands decides the path, the lighting, the address and mailbox conversation, and how the two households share the yard.

An ADU over parking — the classic unit above a garage — takes one of two legal shapes, and the difference is whether the upper story already exists:

• The story is already there (a room or storage level over the garage): converting it is a section 66323(a)(1) conversion, and the 150-square-foot ingress/egress expansion under (a)(1)(A) is available — HCD’s own guidance confirms that expansion can serve a second-story conversion above an existing garage, for example for a compliant stair.
• You are building the story new (raising a roof or adding a level over a garage or parking structure): that is new construction, reviewed under the ordinary ADU standards — for a detached unit, the height tiers of section 66321(b)(4) (16, 18, or 20 feet by condition) rather than the conversion rule that limits a conversion to the original structure’s dimensions. Our second-story ADU page covers the design and structural side; conversions of the garage below are in the garage conversion guide.

What to verify: which shape your project is — the permit record for the existing structure is what plan check will work from. The pitfall: describing a new second story as a “conversion” in the application. The word sends the project down the wrong legal track and costs a correction cycle.

Replacement After a Disaster: What State Law Does — and Does Not — Say

This section exists because the honest answer is narrower than most homeowners expect, and getting it wrong after a fire is costly. California ADU law contains no like-for-like disaster-replacement entitlement. No provision of sections 66310–66342, and nothing in HCD’s Handbook, gives an owner a right to rebuild a destroyed ADU (or home) at the same size and location outside the normal permit process — the Handbook does not address disaster rebuilding of a unit at all. Two rules that sound like replacement rights are not: the “same location and same dimensions” language in sections 66314(d)(7) and 66323(a)(1)(A) is a setback and conversion rule that presupposes an existing structure being converted, and the protections against being forced to fix nonconforming zoning conditions (sections 66322(b) and 66323(c)) stop a city from using your ADU application to demand unrelated clean-up — they do not authorize rebuilding anything.

What state law does provide is one precise, valuable rule — early occupancy of a detached ADU, in Government Code section 66328 as amended by AB 462 (effective October 10, 2025):

• Normally, a city may not issue a certificate of occupancy for an ADU before the primary dwelling’s certificate of occupancy (section 66328(a)).
• The exception: in a county subject to a Governor’s state-of-emergency proclamation issued on or after February 1, 2025, where the primary dwelling was substantially damaged or destroyed by the proclaimed event, the city shall issue the certificate of occupancy for the ADU once it has been permitted and has passed all required inspections — even before the primary home’s certificate exists. A finished detached ADU can be lived in while the main house is rebuilt.
• The exception does not apply to an ADU attached to the primary dwelling (section 66328(c)), and it waives nothing else: the ADU still needs its normal permits and must pass every required inspection.

So a replacement ADU is applied for under the same ministerial pathways as any new ADU — the section 66323 categories or the local ADU ordinance — and built to current building standards. Where a destroyed primary home is being rebuilt at the same time, agencies may process the proposed home and the ADU concurrently (and section 66328(b) is what lets the finished ADU be occupied first in a qualifying emergency). Beyond that, what a specific city allows after a specific fire — a local disaster-rebuild ordinance, an emergency order, a like-for-like program for the primary home — is a local, event-specific question that lives outside ADU law. Confirm it with that building department, in writing, before you rely on it. Our California ADU law hub tracks AB 462 and the rest of the current statute.

What to verify: whether your county is under a qualifying proclamation, whether your ADU is detached, and your city’s own post-disaster rules. The pitfall: anyone — a marketer, a neighbor, a well-meaning article — telling you state ADU law guarantees a like-for-like rebuild. It does not, and plans drawn on that assumption fail at intake.

Sources (verified November 2026): California Department of Housing and Community Development, Accessory Dwelling Unit Handbook, March 2026 edition — 66323 Units and Allowable Combinations, Types of ADUs (attached, converted, detached), Manufactured Homes, Mobilehome Parks, Nonconforming Zoning, Certificate of Occupancy, and Solar Requirements sections; Government Code Sections 66313, 66314, 66315, 66321, 66322, 66323, 66328 (as amended by AB 462, Chapter 491, Statutes of 2025), and 66333–66339; Health and Safety Code Sections 18007, 18008, 18550–18551, and 19971–19993 (Factory-Built Housing Law), with HCD’s Factory-Built Housing Handbook (HCD FBH 314); California Residential Code Sections R305.1, R310, and R311.7.2. Design loads, moisture practice, and universal-design features are described as professional practice, as flagged in the text — your project’s engineer and building department confirm them for your parcel. This page is part of the California Accessory Dwelling Unit Handbook on this site (Types of ADUs, Questions 151–200). Related: ADU FAQs · ADU vs. Everything Else · Garage Conversion Guide · Second-Story ADUs