Planning tool
ADU cost calculator
Build your ADU on screen and watch the estimate update. Pick a size, finishes, and site conditions to see a realistic all-in range for your San Diego County project.
Why is all of this a range?
Because a quote comes from a site visit and a set of drawings, and this page has neither. What it does have is your answers, and each line is priced from the ones that actually drive it. A kitchen follows your finish level and the size of the unit. A septic tank follows bedroom and bathroom count, which is how code sizes it. Trenching follows slope and how close the crew can park. On top of that, each line gets a low and a high for what a real bid can swing, wider on the lines nobody can know up front and tighter on the ones that come off a published fee schedule. Tell us the lot is steep and hard to reach and the whole range widens, because that job is genuinely less predictable.
Your estimate
$50,804 to $71,471
San Diego County
How much does an ADU cost in San Diego County?
Here is what the four common builds run in San Diego, all in, from first drawing to final inspection. Every band comes out of the calculator above at a plain spec: standard finishes, a flat lot, easy access for the crew, and power run to the unit. Change any of that and the number moves.
At a plain spec, a detached adu of about 800 square feet is $320,000 to $430,000 all in, from the first drawing to the final inspection.
| ADU type | Typical all-in range | Priced at |
|---|---|---|
| Junior ADUA bedroom inside the house turned into its own unit, up to 500 sq ft, usually sharing a wall and a bathroom plan with the main home. | $85,000 to $115,000400 sq ft, conversion | 400 sq ft, conversion |
| Garage conversionThe slab, walls, and roof already exist, so most of the budget goes to insulation, plumbing, electrical, and finishes. | $95,000 to $130,000450 sq ft, conversion | 450 sq ft, conversion |
| Attached ADUBuilt onto the side or back of the house. It shares a wall and often a utility run, which trims the total against a detached build. | $245,000 to $330,000600 sq ft, site-built | 600 sq ft, site-built |
| Detached ADUA standalone home in the yard. Its own foundation and utility runs cost more, and it rents for more. | $320,000 to $430,000800 sq ft, site-built | 800 sq ft, site-built |
These are planning ranges, not quotes. Lot slope, how far the crew has to carry material, the state of your panel and sewer line, and how far you take the finishes all move the total. Put your own numbers into the calculator above and you will land closer.
BHI's range model, October 2026. Planning ranges, not a quote for your lot.
Where your ADU budget actually goes
The bands above tell you the total. This is what sits inside it. Adding an ADU is one of the smarter ways to get more from a lot you already own in San Diego, and it helps to know which parts of the budget you control before you fall for a floor plan.
Prefab ADUs: less choice, lower price
A prefab unit is built in a factory and delivered to your yard in sections or as one piece. You give up some design freedom, but you also skip much of the mess and delay of a ground-up build. Most prefab projects land between $100,000 and $350,000 once site work and hookups are included.
Site-built ADUs: more freedom, bigger bill
If you want the unit to match your main house or fit an awkward corner of the yard, site-built is the way to go. Expect $150,000 to $400,000. You pay for the flexibility, and you get exactly what you drew.
Detached or attached
A detached unit stands on its own. That privacy usually costs more because it needs its own foundation and utility runs. An attached unit shares a wall with your house, and often a water or power line too, which can trim the total.
What size does to the price
- 500 sq ft: a compact studio or one bedroom, typically $50,000 to $150,000.
- 1,200 sq ft: a real two or three bedroom home, typically $120,000 to $300,000.
Where the money goes
- Construction and materials: 50 to 60 percent. Framing, roofing, windows, cabinets, fixtures.
- Labor: 30 to 40 percent. Good trades cost money and earn it.
- Design and engineering: 10 to 15 percent. Plans, structural work, and revisions.
- Permits and fees: 5 to 10 percent. This slice depends heavily on your city.
A useful benchmark
The Terner Center at UC Berkeley studied ADU construction across California and found costs that move widely with the unit's size, the site and the finishes, which is why this calculator gives a range for your own choices rather than one number. Read the Terner Center study
What to do with this range
Treat the estimate above as your starting point, not your final price. Once we see your lot and talk through what you want, we can tighten it up.
Questions
ADU questions we get from San Diego homeowners
Short answers to what people ask before the first site visit.
How much does it cost to build an ADU in San Diego County?
It depends on the type more than anything else. A junior ADU or a garage conversion reuses walls that are already standing, so it lands well under a hundred thousand in many cases. An attached or detached build starts from bare dirt and runs into the two and three hundreds. The table above has the bands, and the calculator will price your actual plan.
What moves the price the most?
Square footage and build type set the baseline. After that it is the site: how much the lot slopes, whether a truck can reach the back yard, how far the new unit sits from the panel and the sewer, and how far you take the kitchen and bath finishes. Two identical floor plans on two different lots can differ by tens of thousands.
Is this estimate a quote?
No. It is a planning range built from real project costs. A quote comes after we walk the lot, look at your panel and sewer, and draw something specific to your property. It starts with a call you can book from this page.
Does the estimate include design, engineering, and permits?
Yes. The breakdown on the right carries design and architecture, the boundary survey, and permits and inspections as their own lines. Prefab units show zero for design because the plans come with the unit.
How long does an ADU take to build in San Diego?
Plan on about two years from submitting your permit to the final inspection in the City of San Diego, and design comes before that. A local builder went through the City's permit records and found ADUs took 21.8 months on average to reach final inspection, and new detached ones about 24. State law starts the clock for you: the City has 15 business days to say whether your application is complete, then 60 days to decide when a house already stands on the lot.
ADU permitting data for San Diego, from the City's permit records
What do permits and city fees run?
Reported ranges for San Diego put building permits and plan check somewhere around $6,500 to $15,000, school fees around $4,000 to $8,000 on larger units, and utility connection fees anywhere from $5,000 to $20,000 depending on what your property already has. The calculator's permits line is a model, not your city's fee schedule, so treat the real invoice as the number that counts.
Do I have to pay impact fees?
Not on a small unit. State law bars impact fees on an ADU of 750 square feet or less, which is why the Impact Fee line in the calculator sits at zero until you type 751. Above that, a city or the County can charge them in proportion to your house; from January 1, 2027, with one or two ADUs on the lot, only on the area above 750 square feet (SB 1117). The City of San Diego goes further and waives its own development impact fees on the first two ADUs on a lot, at any size. School fees from 500 square feet and water and sewer capacity charges are separate and can still apply.
How much does a garage conversion cost in San Diego?
A full garage-to-ADU conversion is the cheapest way into a real one bedroom, because the slab, walls, and roof are already there. Most of the budget goes to insulation, a bathroom, a kitchen, and getting the electrical up to code. Price one in the calculator by picking Conversion and typing your garage's square footage.
What about the ADU Bonus Program?
San Diego's bonus program trades extra density for deed-restricted affordable rent, and the city narrowed it in 2025. Whether your lot still qualifies now depends on its zoning and resource area. Worth asking about early, because it changes what you are allowed to build before it changes what it costs.
How accurate is this calculator?
It is a model, and it is honest about that. It prices every project in California and shows a range 15 percent either side of the total, because that is roughly how far real bids spread before anyone has seen the lot. We tune the numbers as our own jobs close.
Keep going
More ways to run the numbers
The cost is one half of the decision. These cover the other half.
Financing
How people pay for an ADU in San Diego
Six ways to finance a build, what each one adds to your month, and the public programs open now.
Read the financing guideNext step
Have your range? Talk it through.
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