In San Diego, most homeowners spend roughly $180,000 to $450,000 to build a detached ADU in 2026, at about $250 to $450 per square foot depending on type and finish. Garage conversions and junior ADUs cost less; large custom detached units cost more. Here is the full breakdown, and how to estimate your number.
ADU cost by type (San Diego, 2026)
| ADU type | Cost per sq ft | Typical total project |
|---|---|---|
| Junior ADU (JADU, under 500 sq ft, within home) | $150 to $200 | $90,000 to $200,000 |
| Garage conversion (single-car) | $200 to $400 | $80,000 to $120,000 |
| Garage conversion (two-car) | $200 to $400 | $120,000 to $160,000 |
| Detached ADU – studio / 1BR | $350 to $550 | $220,000 to $300,000 |
| Detached ADU – 2BR | $350 to $550 | $300,000 to $420,000 |
A detached garage conversion costs about 20 to 30 percent more than an attached one, because new utility lines must be run underground.
What drives the price
- Size – the single biggest factor; quoted per square foot.
- Site conditions – slope, access, distance to utilities, foundation needs.
- Finishes – builder-grade vs. high-end kitchen and bath.
- Detached vs. conversion – a new foundation and utilities cost more than reusing a garage.
- Prefab vs. site-built – prefab is often more price-predictable; site-built is more customizable.
Permit and fee costs (City of San Diego, 2026)
Budget roughly $14,000 to $22,000 in total permit and agency fees (plan check, building permit, school fees, SDG&E, water/sewer).
- Building permit fees: about $3,000 to $8,000
- School fees: about $4.79 to $5.17 per sq ft, waived for units under 500 sq ft
- Impact fees are waived entirely for ADUs of 750 sq ft or less (CA SB 13), otherwise about $5,000 to $15,000
This matters: building under 750 sq ft can save $5,000 to $15,000 in impact fees alone. Full detail in our San Diego ADU Permit Guide.
Hidden costs people forget
Architectural and design plans, soil/geotech reports (if required), utility upgrades (electrical panel, sewer lateral), landscaping after the build, and financing costs.
Is an ADU worth it in San Diego?
With San Diego’s rental demand, a well-built ADU can generate meaningful monthly income and lift your property value. The only way to know your real number is a quote based on your specific lot.
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