Field guide
Six ADU types, side by side
Every California ADU is one of six types. The choice decides your budget, your timeline, and whether you'll actually rent it out. Pick the type that fits your lot — not the type a contractor wants to build.
Detached ADU
LA $330K – $520K all-in · Bay $420K – $700K all-in
A detached ADU is a standalone second unit on the same lot as the primary home — its own foundation, walls, and roof. It earns the highest rents and the highest resale lift of any ADU type, and it's the only type that lets you fully control the floor plan instead of inheriting a garage envelope.
Attached ADU
LA $280K – $460K all-in · Bay $360K – $620K all-in
An attached ADU shares one or more walls with the existing house. You save on a second foundation and one shell wall, but you inherit the main house's structural system — which decides whether the project pencils.
Garage Conversion ADU
LA $185K – $310K all-in · Bay $240K – $410K all-in
A garage conversion turns an existing detached or attached garage into a permitted ADU. It's the fastest path to a rentable unit because the shell already exists — but the existing foundation, slab elevation, and lack of insulation decide whether it's actually cheap.
Junior ADU (JADU)
LA $65K – $140K all-in · Bay $85K – $175K all-in
A JADU is a small unit (max 500 sqft) created inside the existing footprint of your primary home — usually by walling off a bedroom and adding a kitchenette and exterior entrance. It's the cheapest legal second unit to build, and the only ADU type that requires the owner to live on the property.
Two-Story ADU
LA $420K – $640K all-in · Bay $520K – $820K all-in
A two-story ADU stacks living space on a smaller footprint — the right call when the lot is narrow, when you want to preserve yard, or when you need the full 1,200 sqft without sacrificing outdoor space. Height limits and privacy rules vary by jurisdiction, so the design starts with the zoning code, not the floor plan.
Basement ADU
LA $220K – $410K all-in · Bay $280K – $520K all-in
A basement ADU converts unfinished or under-utilized below-grade space into a permitted unit. The shell exists, but egress windows, headroom, and waterproofing decide whether the cost lands closer to a garage conversion or closer to new construction.
FAQ
ADU types FAQ
Which ADU type is the cheapest to build in California?
Junior ADU (JADU) is cheapest at $65K–$175K because it's carved out of existing house space — no new foundation, no new utilities. Garage conversion is second-cheapest at $185K–$410K because the shell already exists.
Which ADU type rents for the most?
Detached ADU. In the same neighborhood at the same square footage, a detached unit typically commands 15–25% more rent than a garage conversion and 30–45% more than a JADU. Privacy is the driver.
Can I build more than one ADU on my lot?
Yes — California law allows one ADU plus one JADU on every single-family lot. On multifamily lots, you can also convert non-livable space (storage, basements, garages) into additional ADUs under separate state allowances.
What's the difference between a JADU and an ADU?
A JADU is max 500 sqft, inside the existing house, requires owner-occupancy, and can share a bathroom with the main house. An ADU is up to 1,200 sqft, can be detached or attached, doesn't require owner-occupancy (state law overrides local owner-occupancy rules for ADUs through at least 2030), and must be a fully independent unit.
Is a two-story ADU worth the extra cost?
For rental income on narrow lots: yes. For owner use with aging-in-place concerns: usually no. A two-story detached ADU costs 25–40% more than single-story but rents as a full 2-bedroom unit, which often pays back in 4–6 years vs. 7–10 for a single-story unit.
Do basement ADUs work in California?
Common in older Bay Area neighborhoods (SF, Berkeley, Oakland flats) where below-grade space is built. Rare in LA where most homes are slab-on-grade. Egress windows, headroom (7 ft min), and moisture control are the three things that decide whether the math works.
What's the smallest legal ADU?
150 sqft for a JADU, 220 sqft for a regular ADU. Anything smaller is treated as a 'sleeping space' and can't be rented as a separate dwelling.
Can I add a JADU and a detached ADU on the same lot?
Yes — that's the maximum-density stack on a single-family lot in California: main house + JADU + detached ADU = 3 legal dwelling units. Total construction cost is typically $400K–$650K including the JADU, and total rental income often covers a meaningful portion of the mortgage.