The short answer
A detached, new-construction ADU in San Diego generally runs $185,000 to $500,000 all-in. Garage conversions typically land between $110,000 and $280,000. Junior ADUs — conversions of existing space within the main house — usually run $90,000 to $220,000.
Those ranges are wide because ADU cost has surprisingly little to do with square footage and a great deal to do with your specific lot. A 700 square foot ADU on a flat lot with the sewer line nearby costs dramatically less than the same 700 square feet on a sloped lot requiring a new sewer lateral and a retaining wall.
What actually drives the cost
Square footage matters less than most people expect. These are the factors that genuinely move an ADU budget:
- › Site access — whether a concrete truck and lumber delivery can reach the build area, or everything has to be carried through a side yard by hand
- › Utility distance — how far the ADU sits from the existing sewer, water, gas, and electrical service, and whether the main panel needs upgrading
- › Grading and drainage — sloped lots need engineering, retaining, and sometimes significant dirt work before anything is built
- › Foundation type — a standard slab versus a raised or stepped foundation on uneven ground
- › Finish level — the difference between builder-grade and designer-selected finishes across a whole unit is commonly $40,000 or more
- › Kitchen and bath count — plumbing fixtures are among the most expensive square footage in any building
What most ADU estimates leave out
When a number sounds too good, it is usually because it covers construction only. A complete ADU budget also includes design and engineering fees, which typically run $12,000 to $40,000; permit and school fees, which vary by jurisdiction and unit size; utility connection or meter fees; and site work.
Ask any contractor quoting an ADU whether their number includes design, permits, and utility connections. If the answer is no, you are comparing an incomplete figure to a complete one.
Does an ADU pay for itself?
In most of San Diego, a well-executed ADU generates meaningful rental income and adds appraised value to the property — but the payback period depends heavily on what you spent and what the unit rents for in your specific neighborhood.
The honest framing is this: an ADU is usually a good long-term investment on a lot that supports it easily, and a marginal one on a lot that fights you. That is exactly what a feasibility check is for, and it is the reason we do them at no cost before anyone commits.
How long does it take?
Plan on 12 to 20 months from first conversation to keys: roughly 2 to 3 months of design, 3 to 8 months of permitting depending on jurisdiction and whether the project qualifies for streamlined review, and 6 to 9 months of construction.
Anyone promising dramatically faster than that is either skipping the permit process or has not accounted for it.
Common questions
Can I build more than one ADU on my lot? +
Often, yes. State law allows an ADU plus a junior ADU on most single-family lots, and San Diego’s bonus ADU program can permit considerably more on qualifying properties. It depends on your zoning, lot size, and transit proximity.
Do I need to live on the property? +
California removed owner-occupancy requirements for most ADUs, though rules differ for junior ADUs. This area of law changes frequently, so verify current requirements before planning around them.
Will an ADU increase my property taxes? +
Yes. The ADU is assessed as new construction and added to your assessed value. Your existing home is not reassessed — only the new improvement.