How Much Does an ADU Cost to Build? (2026)
As of 2026, a full ADU build runs $150,000 to $400,000 or more, per Maison Remodeling's California guide, and GreatBuildz's Southern California data backs that up at roughly $300 to $450 per square foot for a detached unit. Type drives the price most. A garage conversion is the cheapest way to add a full-size, standalone ADU, since the slab, walls, and roof already exist.
Builders prices each ADU with a free written estimate. We walk your lot first. Here is what the 2026 ranges cover, and what moves your price inside them.
How much does an ADU cost in California in 2026?
A full ADU build costs $150,000 to $400,000 or more as of 2026. That is per Maison Remodeling's California ADU cost guide. A detached build alone runs $200,000 to $400,000 or more. GreatBuildz's Los Angeles and Southern California data backs this up. Per square foot, a detached unit runs roughly $300 to $450.
| ADU type | 2026 market range | Source |
|---|---|---|
| Full ADU build | $150,000 to $400,000 or more | Maison Remodeling, 2026 |
| Garage conversion | $80,000 to $180,000 | Maison Remodeling, 2026 |
| Attached ADU | $150,000 to $300,000 | Maison Remodeling, 2026 |
| Detached ADU | $200,000 to $400,000 or more | Maison Remodeling, 2026 |
| Prefab or modular ADU | $150,000 to $250,000 | Maison Remodeling, 2026 |
| Junior ADU (under 500 sq ft) | $50,000 to $150,000 | Maison Remodeling, 2026 |
| Detached, per square foot | $300 to $450 | GreatBuildz, 2026 |
| Garage conversion (SoCal) | $120,000 to $150,000 | GreatBuildz, 2026 |
These are 2026 market ranges, not a quote from Builders. Your lot, your utility hookups, and your finish choices set the real number. Get a free written estimate to start.
What is the cheapest type of ADU to build?
A garage conversion is the cheapest way to add a full-size, standalone ADU. It runs $80,000 to $180,000 per Maison Remodeling, or $120,000 to $150,000 per GreatBuildz's Southern California data. The slab, walls, and roof already exist. So the budget goes into insulation, plumbing, and finishes, not a full new structure.
A junior ADU can cost even less, $50,000 to $150,000, but it is capped at 500 square feet and lives inside your existing home instead of standing alone.
Here is the full type ladder, cheapest to most expensive:
- Junior ADU (under 500 sq ft): $50,000 to $150,000
- Garage conversion: $80,000 to $180,000
- Prefab or modular: $150,000 to $250,000
- Attached ADU: $150,000 to $300,000
- Detached ADU: $200,000 to $400,000 or more
The right type depends on your lot, your goals, and your budget. See how our ADU construction service compares the options on your property.
How much does a 600 square foot ADU cost?
Take the GreatBuildz Southern California figure of $300 to $450 per square foot. A 600 square foot detached ADU then runs roughly $180,000 to $270,000. That overlaps the detached-ADU range in the table above. It can land a bit under Maison Remodeling's $200,000 detached floor, since the two guides use different math.
This is a size-based estimate, not a quote. Your utility hookups still move the number. So do your site conditions, like slope and access. Your finish level in the kitchen and bathroom moves it too.
What changed in California's ADU rules for 2026 that affects cost?
SB 543 took effect January 1, 2026. It gives cities 15 business days to say whether your ADU application is complete. Miss that window, and the city must treat it as complete anyway. Once the application counts as complete, state law gives the city 60 days to approve it.
The same law brings a real savings. ADUs and junior ADUs under 500 square feet are now exempt from school impact fees. That is one less line item on a smaller build.
We handle the full permit process for you. We submit the application. We answer the city's correction notes. We do this as the general contractor of record, so you do not have to.
Is an ADU worth the cost?
For most homeowners in our service area, yes. Median owner-occupied home values here run from $801,800 in San Dimas to $1,241,100 in Yorba Linda, per Census data. Even the top of the detached-ADU range is a modest share of a home at those values.
An ADU is a full second home on your lot. It has its own kitchen, bathroom, and entrance. Homeowners use them for aging parents, adult kids, guests, a home office, or rental income. The right use for your property depends on your goals. We walk through that with you before you commit to plans, and before you spend money on drawings.
What drives an ADU's price up the most?
Five things set your real ADU price, in the order they usually matter:
- Type: garage conversion, attached unit, or detached new build
- Size in square feet and the number of bedrooms and bathrooms
- Utility connections: water, sewer, and electric service to the new unit
- Site conditions like slope, access, and foundation needs
- Finish level in the kitchen and bathroom
Sloped lots add cost the fastest. A retaining wall can be the difference between a flat, buildable pad and a lot that cannot fit an ADU at all. We check your slope and your utility access on the first visit. This happens before you spend money on plans.
Price the exact scope with a free written estimate before you commit to anything. We back every ADU with a 2 year workmanship guarantee. Financing is available too. Ask us for details.
Get a real number for your ADU
Market ranges tell you whether a bid is fair. They cannot price your ADU. They have never seen your lot, your utility access, or your slope. We will walk the property with you and put your real number in writing.
Builders is a family owned general contractor, founded in 2017. The family that owns and runs it has 20 years in the building trades. The company is licensed under CSLB #1078391. Builders is insured and bonded. Every job carries a 2 year guarantee on our workmanship. Based in Corona, Builders serves Corona plus Yorba Linda, Anaheim, Mission Viejo, San Dimas, and Pasadena. Call (818) 836-7624 to set a time for your free written estimate. Or see the full range of remodeling work we handle.
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Is it cheaper to build an ADU or buy a prefab unit?
It depends which type you compare it to. A prefab or modular ADU runs $150,000 to $250,000 as of 2026, per Maison Remodeling's California guide. That usually beats a custom detached build, which runs $200,000 to $400,000 or more. A garage conversion is still cheaper than either, at $80,000 to $180,000, since the slab, walls, and roof already exist. A junior ADU can cost even less, but it is capped at 500 square feet inside your existing home.
How much does an ADU cost per square foot in Southern California?
A detached ADU in Southern California runs roughly $300 to $450 per square foot as of 2026, per GreatBuildz. That is a published market range, not a price from Builders. Utility hookups and finish level decide where your job lands inside that band.
Does a junior ADU cost less than a full-size ADU?
Yes, by a wide margin. A junior ADU under 500 square feet runs $50,000 to $150,000, well under every other ADU type, per Maison Remodeling's 2026 California guide. It converts space already inside your home instead of adding new structure, so it skips most of the site work a detached unit needs.