What Is an ADU? The Complete Guide to Accessory Dwelling Units in California

An ADU — short for accessory dwelling unit — is a second, self-contained home on a residential lot. Under California Government Code §66313, it has its own kitchen, bathroom, sleeping area, and independent entrance — making it a complete, permanent residence, not a guest room, shed, or home office.

California has the most homeowner-friendly ADU laws in the country, and they got better again on January 1, 2026 with the passage of SB 543 and AB 1154. After designing and permitting more than 126 ADU projects across Los Angeles County, we have watched these rules evolve from a niche planning curiosity into one of the most powerful tools available to California homeowners.

This guide walks through what an ADU actually is — legally, structurally, and practically — with every regulatory fact tied to the specific section of the California Government Code that controls it. If you have read other ADU explainers and come away more confused than when you started, that is because most of them quietly repeat each other’s interpretations rather than going to the statute. We go to the statute.

What an ADU Is — The Legal Definition

California Government Code §66313 defines an accessory dwelling unit as “an attached or a detached residential dwelling unit that provides complete independent living facilities for one or more persons and is located on a lot with a proposed or existing primary residence.”

That definition does a lot of work. Three things matter:

• Independent living facilities. The unit must include permanent provisions for living, sleeping, eating, cooking, and sanitation. A bedroom built over the garage with no kitchen is not an ADU. A workshop with a sink is not an ADU. The unit has to be capable of being lived in as a complete home.
• On a lot with a primary residence. An ADU is always secondary. You cannot build an ADU on a vacant lot — there has to be a primary single-family or multifamily dwelling either already there or being built at the same time.
• Attached or detached. Either physical configuration qualifies. We will get to the four specific types in the next section.

The term you may also see is JADU — junior accessory dwelling unit. A JADU is a smaller, more restricted version defined separately under §66313 and §66325. We will cover JADUs in the types section.

Some other names you may hear — granny flat, in-law unit, casita, backyard cottage, secondary suite — all refer to the same thing under California law. Once a unit meets the §66313 definition, the rules in §§66310–66342 apply regardless of what the homeowner or contractor calls it.

The Four Types of ADUs in California

The HCD ADU Handbook (the California Department of Housing and Community Development’s official interpretation of the statute) sorts ADUs into four categories. Each category has different rules around size, setbacks, and what can be combined with what on a single lot.

Why this matters: The type of ADU you build determines what is possible on your specific lot. Detached new construction gives you the most design flexibility, the highest rental income, and the largest absolute property value increase — which is why it is the focus of everything we build at CALI ADU. JADUs and conversion ADUs, by contrast, are often the highest-ROI options on a per-dollar basis: because you are converting space that already exists as livable square footage, the build cost is dramatically lower than ground-up new construction. The trade-off with a JADU is that you are giving up interior space from the main house — turning a 3-bedroom into a 2-bedroom plus a JADU, for instance — which slightly reduces the appraised value of the primary dwelling. The right choice depends on whether you want maximum equity creation (detached) or maximum return on the dollars you actually deploy (conversion or JADU).

You can also combine types on the same lot, and the allowable combinations are more generous than most homeowners realize. Per the March 2026 HCD ADU Handbook, a typical single-family lot in California must be allowed up to four ADU-type units:

• One conversion ADU under §66323(a)(1) — converted from existing or proposed space (interior, garage, or accessory structure), expandable by up to 150 sqft for ingress and egress.
• One JADU under §66323(a)(1) and §§66333–66339 — created within the walls of the existing or proposed single-family home, capped at 500 sqft of interior livable space.
• One state-mandated new-construction detached ADU under §66323(a)(2) — capped at 800 sqft of interior livable space, ministerially approved with no local development standards beyond what §66323 itself allows.
• One additional “ordinance” ADU under §66314 — the standard ADU permitted by the local jurisdiction’s adopted ordinance, which the city may apply objective development and design standards to (within the size and height floors set by §66321).

The HCD Handbook is explicit on this: “In addition to the units described in Government Code section 66323, a local agency must allow at least one unit described in Section 66314.” The §66314 ordinance ADU is on top of the three §66323 state-mandated units — not instead of them.

Combined with the primary residence, an SFR lot can legally support five independently occupied dwellings. On multifamily lots, the math is even more generous: at least one ADU (or up to 25% of existing units) constructed within non-livable space, plus up to two detached ADUs on lots with a proposed primary dwelling, or up to eight detached ADUs on lots with an existing multifamily building.

For a side-by-side cost comparison of detached versus garage conversion, see our garage conversion vs. detached ADU breakdown.

How Big Can an ADU Be?

This is the question we get asked more than any other — and the answer is more nuanced than most blogs admit. California uses two different measurements that frequently get confused: interior livable space (the floor of the home you actually live on) and total floor area (which includes things like attached garages or covered porches).

Under §66321, as amended by SB 543 effective January 1, 2026, the state floor for ADU size is:

Local jurisdictions can be more generous, but not more restrictive. The City of Los Angeles, for example, allows ADUs up to 1,200 sqft total floor area on most lots — which is why our largest two-story model, The Culver, is sized at exactly 1,200 sqft. State law sets the floor; cities can choose to go above it.

SB 543 changed the measurement basis from “living area” to “interior livable space” effective January 1, 2026. The practical effect: it clarifies that interior square footage is what counts toward the 850/1,000 cap — not exterior wall thickness, not attached patios, not unconditioned mechanical chases. If you read an older guide that quotes “living area,” that terminology is outdated.

There is also no minimum size for an ADU other than what the building code requires for any habitable dwelling. Our smallest Signature Home, The Wilshire, comes in at 400 sqft — a studio with a full bathroom, kitchen, and living/sleeping area — which is plenty for one occupant and an extremely common rental size in Los Angeles.

How Tall Can an ADU Be?

State height limits for ADUs are set in §66321(b)(4) and they create a clear hierarchy. A local jurisdiction must allow at least these heights:

State law permits two-story ADUs everywhere — but the heights it guarantees are short. §66321(b)(4) explicitly states that no ADU is required to be limited to one story, and the state-mandated minimum heights cities must allow are 16 feet for detached ADUs on single-family lots (§66321(b)(4)(A)) and 18 feet for ADUs near major transit or on multifamily lots (§66321(b)(4)(B)–(C)). Those floors are tight for a true two-story home with full ceiling heights on both levels.

Our two-story Signature Homes need cities that allow taller ADUs by right. The Fairfax, The Venice, and The Culver are all over 20 feet tall — above the 16- and 18-foot state minimums. They are designed for jurisdictions that permit taller detached ADUs by right, including the City of Los Angeles, where ADUs can generally be built up to 30 feet in most residential zones. In a city that caps detached ADU height at the 16- or 18-foot state floor, our two-story lineup will not be approved as designed. Confirming the local height limit on your specific lot is one of the first things we do in a feasibility review.

That said, no factory-built ADU is two stories at any height — nothing that tall fits on the back of a truck. In cities that allow taller ADUs, site-built construction is the only path to real vertical square footage.

The clause §66321(b)(4)(D) about 25-foot attached limits trips a lot of homeowners up. That 25-foot rule applies only to ADUs physically attached to the primary house — it does not apply to detached new construction. If you read an article saying “California ADUs are capped at 25 feet,” that article is wrong about detached units.

Real-World Example: The Fairfax in the City of Los Angeles

On a 40-foot-wide lot in Mid-City Los Angeles, lot coverage and side-yard setbacks left a buildable area of roughly 600 sqft. A single-story ADU would have maxed out around 550 sqft. Because the City of LA allows detached ADUs up to 30 feet in most residential zones, we were able to build The Fairfax (840 sqft, two stories, 14’ × 32’ footprint of 448 sqft) at full two-story height. The homeowner gained an additional 290 sqft of living space — turning a 1-bedroom into a 2-bedroom rental and adding roughly $600/month in rent. The same project would not have been buildable in a city that caps detached ADU height at the 16- or 18-foot state minimum.

Setbacks — Where You Can Place It on the Lot

Setbacks are the required distances between a building and the lot lines. For ADUs, the state has set a hard ceiling on how restrictive local setbacks can be: 4 feet on the side and rear, for new construction detached ADUs (§66321(b)(3)).

This is one of the most homeowner-friendly provisions in the entire ADU statute. Before this rule was added, many cities required 5, 10, or even 15-foot setbacks — which made detached ADUs impossible on a lot of narrower urban properties. The 4-foot floor unlocked thousands of lots that previously could not accommodate one.

The statute does not specify a state-mandated front setback — front setbacks fall to local rules, which typically require an ADU to be set back behind the front face of the primary house. Practically, in Los Angeles, this means almost all ADUs are built in the rear or side yard.

For conversions of existing structures — converting a garage to an ADU, for instance — the existing setback is honored. You do not have to move a garage that is built right on the property line to convert it.

Parking — When You Need It and When You Don’t

Under §66322, a city can require at most one parking space per ADU — and even that one space is waived in five common situations. The exemptions are broad enough that the majority of ADUs in Los Angeles are built with no required parking at all.

No parking is required if your ADU meets any one of these:

• The lot is within ½ mile walking distance of public transit — the most broadly applicable exemption in dense urban LA
• The lot is in a historic district
• The ADU is part of an existing primary dwelling or accessory structure (covers garage and basement conversions)
• The lot is in an area requiring on-street parking permits, and permits are not offered to the ADU occupant
• There is a car share vehicle located within one block of the property

And critically, §66322 prohibits any city from requiring replacement parking when you convert a garage to an ADU. If you have a two-car garage and turn it into a 1-bedroom unit, the city cannot make you rebuild those two parking spots elsewhere on the lot.

When parking is required, it can be uncovered, tandem (one car behind another on the driveway), and located in the front setback — configurations that most cities historically prohibited for primary dwellings.

Who Can Build an ADU? (Spoiler: Almost Anyone)

If you own a residential lot in California with an existing or proposed primary dwelling, you can build an ADU. The statute is deliberately permissive, and it overrides a lot of older local restrictions that used to keep ADUs out of certain neighborhoods. The key provisions:

No minimum lot size. §66314(b)(1) and §66321(b)(3) explicitly prohibit cities from requiring a minimum lot size to approve an ADU. If your property meets the 4-foot setback and lot coverage limits, lot size is not a barrier — we have permitted ADUs on lots as small as 2,500 sqft.

No discretionary review. §66317 requires cities to process ADU permits ministerially — meaning the planning department checks compliance against objective standards and issues the permit. No public hearing. No design review board. No conditional use permit. No neighbors objecting on aesthetic grounds. The 60-day clock starts ticking the moment your application is complete.

No owner-occupancy required for ADUs. Owner-occupancy requirements for ADUs were eliminated statewide. As of AB 1154 (effective January 1, 2026), even JADUs only require owner-occupancy if the JADU shares sanitation facilities with the primary home — if it has its own bathroom, no occupancy requirement applies. You can buy a house with an existing ADU and rent both units immediately.

Allowed in every residential zone. Any zone that allows a single-family or multifamily residence must allow at least one ADU. Cities cannot zone ADUs out of single-family neighborhoods.

For a deeper dive into what the 2026 law actually allows, see our complete California ADU Rules guide for 2026.

Why Californians Are Building ADUs Now

The legal framework is one piece of the answer. The financial case is the other. Three forces are converging in 2026 that make ADUs the single most powerful financial tool available to LA homeowners:

1. Rental income at LA rates. A new 1-bedroom detached ADU in most of Los Angeles rents for $2,200 to $2,800 per month. A 2-bedroom runs $2,800 to $3,800. At those numbers, a financed ADU typically cash-flows positive from month one — the rent covers the loan payment with money left over.

2. Property value uplift. A permitted, code-compliant ADU typically adds $250,000 to $400,000+ to the resale value of an LA property — often exceeding the cost to build it. A homeowner who spends $259,000 building The Westwood walks away with $40,000 to $140,000 of net new equity the day construction finishes.

3. Family housing flexibility. Parents who want their adult children to live close. Adult children who want their aging parents nearby. The cost of housing in LA has made multigenerational living a practical necessity for many families — and an ADU is the only way to do it without sacrificing privacy on either side.

For a full breakdown of what each Signature Home model costs and what you actually get for the money, see our ADU cost guide for 2026. For financing structures — HELOCs, construction loans, cash-out refis — see our ADU financing guide.

How to Start an ADU Project the Right Way

After 126 projects, the single biggest mistake we see homeowners make is starting the process backwards — getting bids from contractors before they have decided what they actually want to build, on what part of their lot, with what budget.

The right order:

1. Confirm your lot can accommodate an ADU. Lot size, setback geometry, and access for construction equipment all matter. We do this in a free 30-minute feasibility consultation.
2. Choose a model that fits. Our nine Signature Home models span 400 to 1,200 sqft, single-story and two-story, with fixed all-inclusive pricing from $219,000 to $459,000. Choosing a pre-designed model eliminates 8–12 weeks of custom architecture and dramatically reduces budget risk.
3. Confirm financing. See the financing guide linked above. Most clients use a HELOC, a construction-to-permanent loan, or cash.
4. Sign a fixed-price design-build contract. Avoid generic contractors who quote low and bill change orders. A guaranteed fixed price is the only way to know what you are actually committing to.
5. Permit, build, occupy. Total timeline from contract signing to certificate of occupancy is typically 6 to 9 months in the City of LA.

If you are weighing an ADU against simply adding to your existing house, our ADU vs. home addition comparison walks through which delivers more value in different scenarios.