Search results for "ADU cost" often show a single flat number, but that number rarely reflects what any specific homeowner will actually pay. ADU cost depends on a combination of factors that vary property by property, and the only reliable estimate is one built around your specific lot, design, and region. What we can do here is explain the real drivers, so you understand what a contractor's estimate is actually pricing in.
Size
Square footage is the most obvious driver, but it is not linear — smaller units often carry a higher per-square-foot cost because fixed costs like a kitchen and bathroom do not shrink proportionally with unit size.
Site work and existing conditions
Sloped lots, limited access for equipment, existing utility lines, and soil conditions can all add cost before framing even starts. This is a major reason detached ADU costs vary so much across California's different terrains — a flat Sacramento-area lot and a hillside Los Angeles lot are not comparable builds.
New construction vs. conversion
A garage conversion generally costs less than new construction because the foundation, framing, and roof already exist. See our ADU vs. garage conversion comparison for the tradeoffs beyond just cost.
Utility connections
Running new electrical, water, sewer, or gas lines to a detached structure — especially one far from existing hookups — is one of the most underestimated cost categories in ADU projects.
Finish level
Cabinetry, flooring, fixtures, and appliance quality can shift a budget significantly at the same square footage. This is usually the most controllable variable for a homeowner managing cost.
Prefab vs. site-built
Prefabricated or modular ADUs can sometimes reduce labor cost and construction time, but delivery, site prep, and foundation work still apply. See our prefab ADU cost guide.
Permitting and design fees
Architectural drawings, engineering, and permit fees are real costs that apply before construction begins, and they vary by jurisdiction — see our permit process guide.
Getting a real number
Because so many of these factors are property-specific, the most useful next step is an on-site estimate from a licensed contractor. That conversation, not a generic web number, is what determines what your project will actually cost.
Regional cost differences
All of the factors above still play out differently depending on where in California you are building. See our regional cost guides for the Bay Area, San Diego, Sacramento, and Los Angeles markets for market-specific context.
Have a question about your specific project?
Call (877) 300-2747 or request a match, and mention you have questions related to “What Drives ADU Cost in California” — we will route you to a licensed contractor who can speak to it directly.