The 6 Types of ADUs in California, Explained

There are six real types of ADUs you’ll run into in California: detached, attached, JADU (Junior ADU), garage conversion, above-garage (or two-story), and basement conversion. They’re all legal ways to add a second home to a single-family lot, but they’re built completely differently, and the type you choose changes your cost, your timeline, how much of your yard you give up, and how much your daily life gets disrupted during construction. Here’s what each one actually is, and how to tell which one fits your property.

The short version: A detached ADU is a freestanding, fully independent home in the yard. An attached ADU shares a wall with the main house. A JADU is a small unit built inside the existing house. A garage conversion turns an existing garage into living space, an above-garage ADU stacks living space over one, and a basement conversion turns existing basement space into a unit. Detached ADUs give the most privacy and flexibility; JADUs and garage conversions are typically the cheapest and fastest.

The 6 types of ADUs in California

Every one of these is legal under California’s statewide ADU law, and in many cases you can build more than one on the same lot, a detached ADU plus a JADU is a common combination. What’s different is the construction method, the size cap, and the tradeoffs that come with each. For the fuller legal picture behind all six, see what is an ADU and California ADU laws 2026.

1. Detached ADU

A detached ADU is a freestanding structure, fully separate from the main house, with its own foundation, roof, walls, entrance, kitchen, and bathroom. It’s what most people picture when they hear “ADU,” and it’s the type that gives you the most privacy, the most design flexibility, and generally the strongest resale framing, since it reads as a genuine second home rather than a converted space.

Under state law, a detached ADU can run up to 1,200 square feet, with every city required to allow at least 800 square feet at 16 feet tall regardless of local zoning.

The honest tradeoff: because it’s a complete, independent structure, it’s usually the most expensive of the six types on a total-project basis, and it needs enough usable yard space to fit the unit plus required setbacks. It’s the best fit for homeowners who want a real rental unit, a private space for aging parents or adult children, or simply the most flexibility for however their needs change later. It’s also the only type Framework First builds, our full lineup runs 405 to 1,200 square feet, factory-built and crane-delivered. For the full picture, see what is a detached ADU.

2. Attached ADU

An attached ADU shares a wall with, or connects directly to, the primary house. It’s still a legally separate dwelling unit, with its own kitchen and bathroom, it’s just physically joined to the existing structure instead of standing apart from it.

The advantage is height: state law allows attached ADUs up to 25 feet, well above the 16-to-20-foot range detached units are guaranteed, which gives more room for a taller design or a roofline that matches a two-story main house.

The tradeoff is disruption. Building attached usually means tying into the existing home’s walls, roof, and utility systems, which puts construction crews working directly against, and sometimes inside, the house you’re living in during the build. It tends to be more site-dependent and less predictable than a standalone structure. Attached makes the most sense for homeowners on a tighter lot who specifically want the unit connected to the main house, or whose design needs the extra height allowance. See what is an attached ADU for the full breakdown, including setback rules.

3. JADU (Junior ADU)

A JADU, short for Junior ADU, is a small unit, up to 500 square feet, created inside the walls of an existing single-family home, typically a bedroom or similar space converted into a self-contained unit with its own entrance and at least an efficiency kitchen. It can share a bathroom with the main house.

Because it reuses existing walls, roofing, and often plumbing, a JADU is generally the cheapest and fastest of the six types to build. The catch is size, and a requirement most other ADU types don’t have: owner-occupancy. You have to live in either the main house or the JADU itself, which rules it out for someone who wants to rent out their whole property while owning it from a distance.

A JADU fits best when budget is the top priority, you already have convertible space, and you’re comfortable living on-site. It’s also commonly built alongside a full detached ADU, since state law generally allows a lot to have both. See JADU vs. ADU for a full side-by-side, including when each one wins.

4. Garage conversion ADU

A garage conversion turns an existing garage into a code-compliant living space, generally in place, keeping the same footprint and roofline. Because the building already exists, it can be the least expensive path on paper, real numbers typically run $80,000 to $160,000 depending on the garage’s condition and your local labor costs.

The honest catch: that number assumes the garage is already in solid structural shape. A code-compliant conversion still needs a foundation upgrade in most cases (garage slabs are usually too thin for habitable space), full wall and roof insulation (garages start at zero), new plumbing and electrical, and new HVAC, on top of finish work. Add those up on a garage with any real structural issues, and the “cheap” option can end up closer to a new small ADU in cost, without the benefits of new construction.

You’ll also usually lose the parking the garage provided, unless your property qualifies for one of the state’s parking exemptions (being within half a mile of transit is the big one). It’s the right call mainly when your existing garage is structurally excellent and you don’t need it for parking. See our full garage conversion cost and permit breakdown before assuming it’s the cheap option it looks like on paper.

5. Above-garage / two-story ADU

An above-garage ADU stacks living space on top of an existing or new garage instead of spreading across the yard, a common way to add a unit on a tight lot without giving up outdoor space. It’s a specific case of a broader category: a two-story ADU.

California’s height rules leave enough room for two stories in many cases. Detached ADUs are guaranteed at least 16 feet (18 to 20 feet near major transit), and attached ADUs can reach 25 feet, both of which can fit two code-minimum floors on paper. But whether a two-story or above-garage design actually gets approved comes down to your local zoning, how your jurisdiction classifies the structure (attached versus detached affects the height math), and whether the existing garage can structurally support a second story.

This is typically a site-built, multi-story project rather than a single-volume factory build, and the engineering, foundation reinforcement, and structural evaluation of the garage below make it more jurisdiction-specific and less predictable on cost and timeline than the other five types. It fits best on lots where preserving yard space matters more than construction speed or price certainty. See can an ADU be two stories and can an ADU be built over a garage for the full rules.

6. Basement conversion ADU

A basement conversion turns existing basement space into a self-contained unit, following the same conversion logic as a garage conversion: reuse an existing structure instead of building new. Where a home already has finishable basement space, this can preserve the yard and the exterior appearance of the property completely, nothing changes from the street or in the backyard.

The work itself is different from a garage conversion, though. Basements need real attention to waterproofing and moisture control, and habitable space requires proper egress, typically a window or door large enough to exit through in an emergency, along with adequate ceiling height throughout.

The bigger practical issue in our service area specifically: basements are uncommon in most Central Coast California homes, especially newer construction, so this path applies to a much smaller share of local homeowners than the other five types. Where a usable basement does exist, waterproofing and egress work can turn into a bigger unknown cost than homeowners expect going in, worth budgeting for conservatively rather than assuming the existing space is most of the job already done.

Which type fits your lot?

There’s a rough spectrum here, cheapest and fastest on one end, most private and flexible on the other. A JADU or a solid garage conversion gets you a unit for the least money and the least construction time, because most of the structure already exists. A detached ADU costs more and takes a larger footprint, but it gives you a fully independent home with the fewest ongoing compromises, no shared walls, no owner-occupancy requirement, no shared bathroom. Attached and above-garage designs sit in between, trading some privacy and predictability for a smaller yard footprint or a taller design. Basement conversions are a narrower option that depends entirely on whether your home already has the space to work with.

There’s no universally “best” type, only the type that fits your specific lot, budget, and goal. A lot with plenty of usable yard and a goal of real rental income usually points toward detached. A tight urban lot with an owner who’s fine sharing a bathroom and living on-site points toward a JADU. A structurally sound, unneeded garage points toward a conversion.

The fastest way to find out which applies to your property is a feasibility study, which checks your specific lot, zoning, and buildable area before you commit to a design. For the fuller picture on what an ADU is and how state law treats each type, see what is an ADU. To find out what fits your lot specifically, start a feasibility study.

Frequently asked questions

What’s the most common type of ADU built in California?

Detached ADUs are the most commonly built type, largely because they offer the most privacy, the most design flexibility, and don’t require owner-occupancy or sharing systems with the main house. They’re also the only type Framework First builds.

What’s the cheapest type of ADU to build?

Generally a JADU or a garage conversion on a structurally sound existing garage, since both reuse space and systems that already exist rather than requiring an entirely new structure and foundation. The tradeoff is size, JADUs cap at 500 square feet, and for garage conversions, the risk that code-required upgrades erase much of the expected savings.

Can I build more than one type of ADU on the same lot?

In many cases, yes. California law generally allows a single-family lot to add one ADU, which can be detached, attached, or a conversion, plus one JADU. Whether your specific lot supports more than one unit is exactly what a feasibility study confirms.

Do all six types have the same size limits?

No. A detached or attached ADU can run up to 1,200 square feet under state law. A JADU is capped at 500 square feet. Garage, above-garage, and basement conversions are generally limited by the size of the existing structure they’re converting, though above-garage additions can sometimes exceed that when new construction is added on top.

Which type of ADU is easiest to get approved?

All six types get the same fast, ministerial approval process under state law, a 60-day decision window with no public hearing. What varies is how much local design review or structural documentation a specific type requires. Conversions (garage, basement) and detached ADUs tend to move through review most predictably; attached and above-garage designs more often trigger extra structural or classification questions specific to your jurisdiction.

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Curious what type actually fits your yard? Start a free feasibility study to find out, or read the fuller picture in what is an ADU.