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ADU Construction Cost: $218.54 per Square Foot
A 1,196 sq ft accessory dwelling unit in San Diego came to $261,375 to build, or $218.54 per square foot. That is one real estimate, not an industry average, and it is hard construction cost only — permits, design, utility connections and impact fees sit outside it. What it excludes is listed in full below. Every division is priced from the drawings, and the source PDF is linked at the bottom of the page.
Estimate dated 15 January 2025 · San Diego, CA
BUILDING AREA
1,196 SF
COST TO BUILD
$261,375
COST PER SF
$218.54
CSI DIVISIONS
16
WHERE THE MONEY GOES
All 16 divisions, priced
Finishes are the single largest line at $68,585, which is 26% of the build. Most ADU cost pages stop at a range. This is the actual estimate.
9000FINISHES
$68,585100.0%
6000WOOD, PLASTIC & COMPONENTS
$34,50150.3%
8000OPENINGS
$26,40338.5%
23000HVAC
$21,44031.3%
7000THERMAL AND MOISTURE PROTECTION
$20,20529.5%
26000ELECTRICAL
$19,51628.5%
1000GENERAL REQUIREMENTS
$18,25026.6%
22000PLUMBING
$12,02817.5%
3000CONCRETE
$10,97516.0%
12000FURNISHING
$7,37210.7%
32000EXTERIOR IMPROVEMENTS
$5,7588.4%
11000EQUIPMENTS
$5,2157.6%
10000SPECIALITIES
$5,0717.4%
2000EXISTING CONDITION
$2,6063.8%
5000METALS
$1,9992.9%
28000ELECTRONIC SAFETY AND SECURITY
$1,4512.1%
| CSI | Division | Cost | $/SF | % of build |
|---|---|---|---|---|
| 9000 | FINISHES | $68,585 | $57.35 | 26.2% |
| 6000 | WOOD, PLASTIC & COMPONENTS | $34,501 | $28.85 | 13.2% |
| 8000 | OPENINGS | $26,403 | $22.08 | 10.1% |
| 23000 | HVAC | $21,440 | $17.93 | 8.2% |
| 7000 | THERMAL AND MOISTURE PROTECTION | $20,205 | $16.89 | 7.7% |
| 26000 | ELECTRICAL | $19,516 | $16.32 | 7.5% |
| 1000 | GENERAL REQUIREMENTS | $18,250 | $15.26 | 7.0% |
| 22000 | PLUMBING | $12,028 | $10.06 | 4.6% |
| 3000 | CONCRETE | $10,975 | $9.18 | 4.2% |
| 12000 | FURNISHING | $7,372 | $6.16 | 2.8% |
| 32000 | EXTERIOR IMPROVEMENTS | $5,758 | $4.81 | 2.2% |
| 11000 | EQUIPMENTS | $5,215 | $4.36 | 2.0% |
| 10000 | SPECIALITIES | $5,071 | $4.24 | 1.9% |
| 2000 | EXISTING CONDITION | $2,606 | $2.18 | 1.0% |
| 5000 | METALS | $1,999 | $1.67 | 0.8% |
| 28000 | ELECTRONIC SAFETY AND SECURITY | $1,451 | $1.21 | 0.6% |
| Total | $261,375 | $218.54 | 100% |
SCOPE
What this number does not include
$261,375 is hard construction cost — the 16 divisions above and nothing else. Each item below was checked against the full text of the source estimate and is genuinely absent from it. Budget for these separately.
- Permit and impact fees
- The estimate carries $1,000 for permits, priced as 20 hours of labour at $50/hr. That is the cost of pulling them, not what the city charges.
- Design and engineering
- No architectural, structural or MEP design fees appear anywhere in the estimate. It prices construction from drawings that already exist.
- Utility connections
- No sewer, water, gas or electrical service connection or upgrade. One line relocates the existing electric meter; that is all.
- School and development impact fees
- Not present. These are jurisdiction-specific and can be substantial for a new dwelling unit.
- Survey and soils reports
- No boundary survey, geotechnical work or testing.
- Solar
- No photovoltaic system is priced. Check what your jurisdiction requires for a new dwelling.
None of these are small. If you are working out whether an ADU pencils, treat $261,375 as the construction line in a larger budget, not the budget.
COST BY SIZE
What other ADU sizes work out to
Scaled from the measured $218.54 per square foot. Only the 1,196 sq ft row is a real estimate — the rest are that rate applied to other sizes. All of them are hard construction cost, excluding everything listed above.
| Size | Cost to build | Typical bid | Basis |
|---|---|---|---|
| 500 sq ft | $109,270 | $137,680 | Scaled |
| 800 sq ft | $174,832 | $220,288 | Scaled |
| 1,000 sq ft | $218,540 | $275,360 | Scaled |
| 1,196 sq ft | $261,375 | $329,332 | Real estimate |
| 1,200 sq ft | $262,248 | $330,432 | Scaled |
| 1,500 sq ft | $327,810 | $413,040 | Scaled |
Treat the small end as optimistic. A kitchen, a bathroom, a service panel and a meter cost roughly the same whether the unit is 500 sq ft or 1,200, so those fixed items are spread over fewer feet and the real rate per square foot climbs as the unit shrinks. A 500 sq ft ADU will usually come in above $218.54 per foot, not at it.
COST VERSUS BID
Why the quote is higher than the cost
$261,375 is what the work costs. The suggested bid was $329,332 — $275.36 per square foot. The estimate itemises the difference rather than rolling it into a single margin:
| Total trade cost | $261,375 | $218.54/SF |
| Overhead and profit | $52,275 | $43.71/SF |
| Insurance | $3,921 | $3.28/SF |
| Contingency | — | — |
| Tax | $11,762 | $9.83/SF |
| Suggested bid | $329,332 | $275.36/SF |
Overhead and profit is the large one, at 20% of trade cost. Contingency was carried at zero here, which is a choice rather than a default — on a job with more unknowns you would expect it funded. If you are a homeowner collecting quotes, the number a builder hands you is the bid. If you are the contractor, the cost is the figure you have to defend.
HOW FAR THE NUMBER MOVES
Same square footage, 2.7× the cost
We estimated another 1,196 sq ft residential project, the Heinzsman Residence, at $703,668 — $588.35 per square foot against this ADU’s $218.54. Identical area, 2.7× apart. Square footage alone does not set the number; specification does.
| Project | Area | Cost | $/SF |
|---|---|---|---|
| College Ave ADU (this page) | 1,196 SF | $261,375 | $218.54 |
| Heinzsman Residence | 1,196 SF | $703,668 | $588.35 |
Both estimates cross-check: the per-square-foot figure printed on each PDF matches the figure derived from that PDF’s own total and area. The matching square footage is coincidence — the two projects share no scope, rates or divisions.
ESTIMATE YOUR OWN
ADU cost calculator
Applies this project’s measured rates to a size of your choosing. It is arithmetic on one data point, not a quote.
Construction cost
$174,832
Typical bid
$220,288
Hard construction cost only, at this project’s measured $218.54/SF. It excludes everything in the list above, and small units run higher per foot than this rate suggests. Treat it as a starting bracket, not a budget.
OTHER MARKETS
How far this travels
This is a San Diego estimate, and San Diego is an expensive labour market with California’s energy and seismic requirements priced in. The following is general guidance rather than anything measured here: labour rates, code requirements, site access and finish level are what move the rate most between markets. A tight infill lot a pump truck cannot reach costs more than the drawings suggest, anywhere in the country.
SOURCE
Check the estimate yourself
Every figure on this page comes from one estimate for a 1,196 sq ft ADU at 4768 College Ave, San Diego, prepared by The ACE Services and dated 15 January 2025. Quantities were taken off the permit drawings and unit-priced by CSI division. The divisions add to $261,375, which is the stated total exactly, and the printed bid per square foot matches the figure derived from the total and the area.
Construction pricing moves. This estimate is a January 2025 basis for one San Diego project — check it against current local pricing before you rely on it.