Detached, attached, garage conversion, junior ADU, and multifamily options: what each one is, how big it can be, and who it fits.
The short answer
California law recognizes four main ADU types: a detached backyard unit, an attached addition, a conversion of existing space like a garage, and a junior ADU (JADU) of up to 500 sq ft inside your house. Apartment and duplex owners get two more: units in unused space and detached units on the lot.
detached ADU your city must approve with 4-foot side and rear setbacks (Gov. Code § 66323)
maximum size of a junior ADU, built inside a single-family home (Gov. Code § 66313)
a single-family lot can combine: a conversion ADU, a JADU, and a new detached ADU (HCD, 2026)
detached ADUs next to an existing apartment building, capped at its unit count (§ 66323)
Most people pick an ADU type by looking at their backyard. That's backwards. The type decides how big you can build, whether you need parking or solar panels, and even whether you have to live on the property.
Several sections of California's ADU law were amended again on January 1, 2026. Here's each type under the current rules, what it's good for, and where the costs hide.
What does ADU mean?
ADU stands for accessory dwelling unit: a second, smaller home on the same lot as a house or apartment building. You may know it as a granny flat, in-law unit, or backyard cottage.
Under Government Code § 66313, an ADU provides "complete independent living facilities for one or more persons," including a place to sleep, cook, and bathe. You'll sometimes see "alternative dwelling unit" online, but the legal term in California is accessory dwelling unit.
What are the types of ADUs in California?
Detached, attached, converted, and junior ADUs on single-family lots, plus converted and detached ADUs on multifamily lots. Here's how they compare under state law as of October 2026.
| Type | What it is | Size | Height | Parking |
|---|---|---|---|---|
| Detached | New freestanding home | 800 sq ft always allowed; local caps no lower than 850 (1,000 for 2+ bedrooms) | 16 ft; 18 ft near transit, +2 ft for roof pitch | Max 1 space, often none |
| Attached | New addition to the house | Up to 50% of the house, same 850/1,000 floor | 25 ft or the house limit, if lower | Max 1 space, often none |
| Conversion | Garage, shed, basement, or other existing space | No limit | Existing structure | None |
| Junior ADU | Unit inside a single-family home | Up to 500 sq ft | Inside the house | None |
| Multifamily conversion | Unused space in an apartment building | At least 1, up to 25% of units | Inside the building | None |
| Multifamily detached | New units beside an apartment building | Up to 2 (proposed building) or 8 (existing) | 16 or 18 ft, +2 near transit | None |
Gov. Code §§ 66314, 66321–66323, 66333–66334, checked October 8, 2026; HCD ADU Handbook, March 2026. Cities can be more generous, never stricter.
What is a detached ADU?
A detached ADU is a newly built home that stands on its own on the same lot as the main house. It's the classic backyard cottage.
State law protects it well. Your city must approve a detached ADU of up to 800 square feet with 4-foot side and rear setbacks, and lot coverage or front setback rules can't block it (Gov. Code §§ 66321, 66323). Cities without their own limits fall back on a 1,200 sq ft cap in state law.
- Pros: the most privacy for tenants or family, and your house stays untouched during the build.
- Cons: it needs its own foundation, utility lines across the yard, and backyard space.
- Cost drivers: distance to water, sewer, and power, the foundation, delivery access, and solar. New detached ADUs need solar under the 2025 Energy Code, but the panels can go on your main house.
Factory-built (modular) is a way to build, not a legal type. In the California Energy Commission's words, "Factory-built housing is a factory-constructed version of a site-built residential building that is manufactured and then transported to its permanent installation location." Bay Modular's ADU models are detached units built in our Lathrop factory, from the 256 sq ft Small Studio to the 640 sq ft 2 Bedroom ADU, all under the 800 sq ft the state protects. Each is delivered in a day and installed in about a week. See the models.
What is an attached ADU?
An attached ADU is a new addition connected to your house, on the side, on top, or below, that works as a separate home.
HCD's examples include an ADU over an attached garage or a new second story. It can't exceed 50% of your home's floor area (§ 66314), but a city can't use that rule to cap you below 850 sq ft. Height goes up to 25 feet or your zone's limit, whichever is lower.
- Pros: shorter utility runs, less yard used, and no solar requirement, since the Energy Commission treats attached ADUs as additions.
- Cons: construction happens on your home, and new work has to tie into old.
- Cost drivers: the condition of your existing walls, roof, and foundation.
What is a garage conversion ADU?
A conversion ADU turns existing space, such as a garage, shed, basement, or part of your house, into a separate home.
Conversions have no size limit and need no setbacks, because they sit inside an existing structure (HCD, March 2026). A converted detached structure can grow by up to 150 sq ft, but only for an entry, such as a staircase.
Watch out
Many people assume they'll have to replace the garage parking they lose. They won't. A city can't require replacement spaces when a garage is converted to an ADU (Gov. Code § 66314(d)(11)).
- Pros: no new footprint and usually the simplest permit.
- Cons: you lose the garage, and you keep the old structure's shape and ceiling height.
- Cost drivers: bringing the space up to code for a home: insulation, windows, plumbing, sometimes the foundation.
What is a JADU (junior ADU)?
A junior accessory dwelling unit (JADU) is up to 500 sq ft and built entirely within the walls of a single-family home, including an attached garage.
It needs its own entrance and an efficiency kitchen but can share a bathroom with the house. One per lot, and never in a detached garage or shed (Gov. Code § 66333).
- Pros: the lightest construction, no parking (§ 66334), and no impact fees (§ 66311.5). HCD notes a 220-volt circuit isn't required for its kitchen.
- Cons: it's small. If it shares a bathroom, the owner must live on the property. Rentals must be longer than 30 days.
Can you build an ADU on a multifamily property?
Yes. Duplex and apartment owners can convert non-livable space and build detached units, and combine both.
Storage rooms, attics, basements, or garages become homes: at least 1, up to 25% of existing units.
Gov. Code § 66323(a)(3)Up to 2 with a proposed building; up to 8 with an existing one, never more than its unit count.
Gov. Code § 66323(a)(4)A duplex counts as multifamily under state ADU law, so it can't add a JADU, which is only for single-family homes.
Which ADU type fits you?
Match the type to your goal. And you may not have to choose: HCD says a single-family lot can combine a conversion ADU, a JADU, and a new detached ADU.
- Rental income or privacy for family: a detached ADU.
- An unused garage: a conversion.
- A small yard but room to build up: an attached ADU.
- A low-cost space for a relative or caregiver: a JADU.
- A duplex or apartment building: multifamily conversions, detached units, or both.
Pro tip
Size is a money decision. An ADU of 750 sq ft or less pays no local impact fees (Gov. Code § 66311.5). For a full budget, see what an ADU costs in California; for recent changes, our guide to California ADU laws.
Not sure what fits your lot?
We review satellite imagery, mark the buildable area and delivery access, and check your city's ADU rules. Free, with no obligation.
Frequently asked questions
What does ADU stand for?
ADU stands for accessory dwelling unit, a second, smaller home on the same lot as a house or apartment building, with its own kitchen, bathroom, and sleeping area.
What's the difference between an ADU and a JADU?
A JADU is up to 500 sq ft, sits inside a single-family home, and can share its bathroom. An ADU can be detached, attached, or converted and can be much larger.
How big can an ADU be in California?
Your city must allow at least 850 sq ft, or 1,000 sq ft with more than one bedroom. State law caps a new detached ADU at 1,200 sq ft where a city has no ordinance, and cities may allow more. Conversions have no size limit.
Do I need a parking space for my ADU?
Often not. No parking can be required within a half mile walking distance of public transit, for conversions, or where a car share vehicle is within one block. Otherwise, the maximum is one space per unit or bedroom.
Is a modular or prefab ADU a separate type?
No. Factory-built is a construction method. A modular unit in your backyard is a detached ADU under the same rules as one built on site.
Do I have to live on the property?
No. Cities can't require owner-occupancy for an ADU (Gov. Code § 66315). The exception is a JADU that shares a bathroom with the main house.
Find out which ADU fits your lot
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About Bay Modular Team
Bay Modular is a California factory-built housing manufacturer (HCD #1585366). We design and build backyard ADUs in our factory in Lathrop, CA, and install them across California. More about us
Sources
- California Government Code § 66313, § 66314, § 66321, § 66322, § 66323, § 66333, § 66334, § 66315, § 66311.5. Accessed October 8, 2026.
- HCD, Accessory Dwelling Unit Handbook, March 2026.
- California Energy Commission, 2025 Energy Code ADU FAQs. Accessed October 8, 2026.
- Bay Modular Pricing & Financing, October 2026.
This guide is general information, not legal advice. Local rules vary; confirm details with your city's planning department.
