ADU Types Explained: Detached, Attached, JADU, and Garage Conversions

When people say "ADU," they usually picture a little cottage in the backyard. That's one type. But California law recognizes four distinct ADU types, and each one has different size limits, cost profiles, and ideal use cases.

Picking the right type before you start planning saves a lot of wasted time with architects and contractors.

Detached ADU

A detached ADU is a freestanding structure completely separate from the main house. It can be a brand-new build or a converted existing structure (like a detached garage or workshop).

Size: Up to 1,200 sq ft for most California cities, though some allow up to 1,500 sq ft. State law requires cities to allow at least 850 sq ft for a studio.

Setbacks: Under state law, only 4 feet from rear and side property lines is required for new detached ADUs. Front setback rules still apply.

Cost range: $150,000 to $350,000+ for a new build. Converted structures (pool houses, garages, barns) can be significantly cheaper.

Best for: Homeowners who want maximum privacy and rental income potential. A fully detached unit with its own entrance and yard access commands the highest rents.

The main downside is cost. Building from scratch on your property means a full foundation, framing, roofing, and all utilities. It's the most expensive option by a wide margin compared to conversions.

Attached ADU

An attached ADU shares at least one wall with the main home. It might be an addition off the back of the house, or a conversion of an existing attached portion like an attached garage, a bonus room, or a ground-floor space.

Size: Typically limited to 50% of the main home's existing square footage, or 1,200 sq ft, whichever is less. State minimums still apply.

Cost range: $100,000 to $250,000+. Because it ties into the existing structure, foundation and framing costs can be lower than a fully detached build.

Best for: Lots that don't have enough rear yard space for a detached structure, or homes where the existing layout lends itself to carving out a separate unit.

The tradeoff is shared walls. Noise travels, and the unit is harder to make feel fully separate from the main house.

Garage Conversion

Converting an existing attached or detached garage into living space is often the most affordable path to an ADU. The bones are already there, including the slab, walls, and roof.

Size: Limited to the footprint of the existing garage. Most two-car garages run 400 to 500 sq ft, which produces a studio or small one-bedroom.

Cost range: $60,000 to $130,000 in most markets. The big variables are whether you need to upgrade the electrical panel, add insulation, and how much finish work is involved.

Best for: Homeowners on a tighter budget or those who want to test the waters with a smaller rental unit before committing to a larger project.

One frequently overlooked issue: losing garage space. If you have one car and one garage, the conversion means street parking. Run that decision through your household before you commit.

Also worth knowing: California law prohibits cities from requiring you to replace any parking spaces lost to a garage conversion within half a mile of public transit. If you're further than that, your city may require you to add a parking space somewhere on the lot.

JADU (Junior ADU)

A Junior ADU is a small unit created entirely within the walls of the existing single-family home. It's capped at 500 sq ft and must include an efficiency kitchen (full kitchen not required). A bathroom can be shared with the main house.

Size: 500 sq ft maximum.

Cost range: $30,000 to $80,000, depending heavily on the existing layout and how much reconfiguration is needed.

Best for: Homes that have an underused room, basement, or suite that can be separated with its own entrance. A good fit for parents who want to house an adult child or aging parent nearby but semi-independently.

Unlike other ADU types, a JADU requires owner-occupancy in either the main house or the JADU. This restriction doesn't apply to standard ADUs under current California law.

The big advantage is cost and simplicity. No new foundation, no roofing, minimal structural work. The big disadvantage is size. At 500 sq ft, you're looking at a tight studio at best.

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Comparing All Four Types

How to Choose

Start with your lot and your goals.

If rental income is the priority, a detached ADU almost always wins on rent and resale value, even though it costs more to build.

If budget is the primary constraint, a garage conversion or JADU can get you to a functional unit for significantly less.

If you're trying to house family, a JADU or attached ADU often works better logistically since the connection to the main house can be an advantage rather than a drawback.

And if your lot is small or your backyard limited, a garage conversion or attached addition may simply be the only viable option regardless of preference.

The best move is to look at your city's specific rules, walk your property with a contractor or ADU designer, and model out the numbers for each type before committing to one path.