The first thing every San Diego homeowner asks us is also the one thing most ADU builders refuse to answer straight: what does it actually cost?
Here's the honest read for 2026.
The quick answer
For a detached ADU in San Diego County in 2026:
- 1 BR / 1 BA, 500–650 sqft — $195,000 to $400,000 all-in
- 2 BR / 1 BA, 700–900 sqft — $240,000 to $475,000 all-in
- 2 BR / 2 BA, 900–1,200 sqft — $270,000 to $540,000 all-in
- Garage conversion — $105,000 to $260,000 all-in
"All-in" means design, permits, and construction. It does not include financing costs, furniture, or landscaping beyond what's required for your certificate of occupancy.
Where the money actually goes
On a typical $400K detached ADU in San Diego, here's the breakdown by percentage:
- Design + architectural (6–9%) — $24K to $36K. Architectural drawings, interior design, any VR or 3D visualization. Higher on custom, lower on catalog plans.
- Permits + city fees (5–9%) — $20K to $36K. Plan check, building permit, school impact fees, utility impact fees, SDG&E capacity. Varies dramatically by jurisdiction.
- Sitework + foundation (12–18%) — $48K to $72K. The biggest hidden variable. Slope, soil conditions, and utility tie-ins can all push this up.
- Shell + framing (22–28%) — $88K to $112K. Lumber, trusses, roofing, windows, exterior cladding.
- Mechanical, electrical, plumbing (15–20%) — $60K to $80K. Often includes main-panel upgrades, which can add $8K–$15K on their own.
- Finishes (20–25%) — $80K to $100K. The range where your taste and your budget have their biggest argument.
- Soft costs + contingency (8–12%) — $32K to $48K. Insurance, inspections, and the 5–10% contingency every professional builder insists on.
What the "cheap ADU" pitches leave out
The $150K detached ADU you see advertised is almost always one of three things:
1. In a different market. Sacramento or Phoenix cost bases aren't San Diego cost bases. 2. Without permits. Unpermitted "ADUs" are a liability — they don't show up on title, they don't raise your appraisal, and they're a problem when you sell. 3. A floor-plan quote, not a delivered-building quote. The most common exclusion: foundation. The second: site utility work. The third: sales tax on materials.
When someone quotes you a shockingly low number, ask them for a line-item list of what's not included. That's where the truth lives.
Cost drivers specific to San Diego
A few things make San Diego pricier than the California average:
- Labor — SD construction wages run ~8–12% above the state average.
- Coastal rules — if you're west of the 5, expect higher design, engineering, and permit costs.
- Soil — much of the county has expansive clay that requires deeper foundations.
- Fire severity zones — East County and inland North County require fire-rated assemblies.
Cost drivers that save you money
- Alley access — simplifies utilities and construction logistics, often shaves 3–6% off total.
- Flat lot — straightforward foundation.
- Adequate main electrical service — no panel upgrade required.
- Existing sewer lateral capacity — avoids a street cut.
The ADU PALS Complete Financial Report
Our first deliverable is a Complete Financial Report (CFR) built around your actual lot, not an industry average. It includes your projected rent, projected ROI, ballpark build cost, and projected resale lift. Two weeks, no cost, no pressure.
Send us your address and we'll get started.
