Different Types of ADUs

Every ADU falls into one of six types. Knowing them matters because the type you choose changes your cost, your timeline, and even which rules apply to you.

1. Detached ADU

A separate small house, usually in the backyard. This is what most people picture. It offers the most privacy and design freedom, and it's usually the most valuable — but also typically the most expensive, because you're building a home from scratch: foundation, walls, roof, and utilities.

2. Attached ADU

A new unit built onto your existing house — sharing at least one wall — with its own entrance. Attached ADUs can save money on one wall and shorter utility runs, but sound privacy between the units takes planning.

3. Garage conversion

Your existing garage becomes living space. Because the walls, roof, and slab already exist, this is usually the least expensive path to an ADU. The trade-offs: you lose storage and parking, the size is limited to the garage footprint, and older garages often need structural and foundation work.

4. Interior conversion

Part of the existing house — a basement, an attic, an oversized bonus room — becomes a separate unit. Cost varies a lot depending on ceiling heights, egress (emergency exit) windows, and plumbing access.

5. Above-garage ADU

A new unit built on top of a garage (existing or new). Great when yard space is tight, but the garage below usually needs significant structural work to carry a second story, which raises cost.

6. JADU (Junior ADU)

A small unit — up to 500 square feet — carved out of the existing house or attached garage. A JADU needs its own entrance and a kitchen, but it can share a bathroom with the main house, and the owner generally has to live on the property. It's the cheapest option, and here's the interesting part: California lets many properties have both an ADU and a JADU.

Which one is right for you?

That depends on your lot, your garage, your budget, and your goals — which is exactly what the next few modules cover. For now, just know the menu.