What an ADU Actually Costs in San Diego

Every ADU budget conversation starts with someone quoting a cost per square foot, and every blown ADU budget started the same way. Per-square-foot numbers describe the building; you're paying for a project. Here's the full picture for San Diego as of 2026 — ranges, not promises, because your lot is a variable no article can price.

Realistic all-in ranges by type

• Garage conversion: roughly $80,000–$150,000 for a permitted ADU with kitchen and bath. The cheapest path per rentable square foot; the full breakdown is here.
• Attached ADU (new addition): commonly $150,000–$300,000 depending on size and how much of the existing structure works in your favor.
• Detached site-built ADU: most San Diego projects land between $200,000 and $400,000 all-in for one-bed and two-bed units. Custom finishes push higher.
• Prefab/modular: unit prices look lower, but after site work, crane, foundations, utilities, and fees, all-in budgets usually land in the same neighborhood as site-built — here's the honest comparison.

The line items the brochure leaves out

• Site work & foundations — grading, drainage, slab or footings. On sloped lots this alone can swing tens of thousands.
• Utility connections — sewer and water runs (distance matters enormously), plus a main electrical panel upgrade on many older homes.
• Design & engineering — architecture, structural calcs, Title 24. Pre-approved plans cut this line significantly.
• Permits & fees — plan check and permit fees always; development impact fees waived at 750 sq ft or less (the single most valuable design constraint in ADU planning — see the permit guide).
• The boring finishers — flatwork, landscaping repair, address signage, separate utility metering if you want it. Small individually; five figures collectively.
• Contingency — 10–15% held by you, not spent in the plan. Projects without contingency don't cost less; they just borrow at the worst moment.

Does the rental math work?

In most San Diego neighborhoods, studios and one-bedroom ADUs rent strongly — the county's housing shortage is the whole reason the ADU laws exist. A garage conversion renting at market rates frequently recovers its cost within a decade; new-construction units take longer but add durable property value. The floor plan choice moves this math more than any finish decision: rent per build dollar peaks in small efficient layouts.

How to keep your project inside these ranges

The homeowners who land on budget do three things: they get the site variables (slab, sewer, panel) priced before signing, they level multiple bids against one written scope, and they release payments against verified progress. The ones who don't, discover the variables one change order at a time. Bid leveling and payment control are the core of my ADU project management — and as my file of leveled San Diego bids grows, these ranges will get sharper. For the whole journey, start at the complete San Diego ADU guide.

Ranges reflect mid-2026 San Diego market conditions and will drift; treat them as planning brackets, not quotes. A free consultation with your actual lot details gets you real numbers.