ADUs — accessory dwelling units — have become one of the most requested projects we get at Complete Builders Inc. And with good reason: San Diego's permitting process has been streamlined, rental demand is strong, and a well-built ADU can add significant value to your property. But the cost question is where most homeowners get confused fast.
Here's the honest breakdown of what an ADU actually costs in San Diego in 2025 — by type, by size, and by what moves the number in either direction.
ADU Cost by Type
ADU cost varies dramatically depending on what type you're building. The three most common in San Diego are detached new construction, garage conversions, and attached ADUs.
| ADU Type | Typical Size | Cost Range (All-In) |
|---|---|---|
| Garage Conversion | 400–600 sq ft | $80,000 – $150,000 |
| Attached ADU | 400–800 sq ft | $130,000 – $220,000 |
| Detached ADU (new) | 400–1,200 sq ft | $180,000 – $350,000+ |
| Two-Story Detached ADU | 800–1,200 sq ft | $280,000 – $420,000+ |
All-in means all-in: These ranges include design, permits, construction, and finish work. They do not include furniture, landscaping, or appliances. Always ask your contractor to confirm what's included — vague scopes lead to budget surprises.
Garage Conversions: The Most Cost-Effective Option
A garage conversion is almost always the least expensive ADU path because the structure already exists. You're adding insulation, framing interior walls, running plumbing and electrical, adding HVAC, and finishing the interior — but you're not building a foundation or exterior framing from scratch.
The condition of the existing garage matters a lot. An attached two-car garage in good condition with easy access for plumbing is the best-case scenario. A detached single-car garage with a cracked slab and poor drainage is the worst. We inspect every garage before giving a number because there's real variance.
What drives garage conversion costs up:
- Replacing or repairing the existing slab
- Long plumbing runs to reach an existing sewer line
- Upgrading the electrical panel to accommodate the new unit
- Adding a full bathroom (vs. a ¾ bath)
- Installing a mini-split HVAC system
Detached ADUs: The Most Valuable but Most Expensive
A new detached ADU is essentially building a small house on your existing lot. You're paying for foundation work, framing, exterior finishes, roofing, all mechanicals, and a full interior build-out. In San Diego, detached ADUs run $200–$300 per square foot all-in for a standard finish level.
The biggest cost variable on detached ADUs is site conditions — specifically, how difficult it is to connect to existing utilities. A lot with easy sewer and water access can save $10,000–$25,000 compared to a sloped lot requiring significant grading and longer utility runs.
Permits and Soft Costs
San Diego has streamlined ADU permitting significantly since 2020, but you're still looking at real costs. Budget $15,000–$30,000 for design and permitting on a typical detached ADU. This includes architectural drawings, structural engineering if required, and city permit fees.
San Diego permit timelines for ADUs currently run 3–5 months for most projects. Factor this into your overall timeline — you're not breaking ground on day one.
Will an ADU Pay for Itself?
In San Diego's rental market, yes — typically. A well-located 500 sq ft ADU in neighborhoods like North Park, South Park, or Hillcrest rents for $1,800–$2,400/month. At $200,000 all-in with a $2,000/month rental income, you're looking at roughly an 8-year payback period before factoring in property value appreciation.
In higher-demand coastal areas like La Jolla or Pacific Beach, rents run $2,500–$3,500/month for a comparable unit, shortening that payback window considerably.
Important: San Diego does not currently require owner-occupancy for most ADU types, which means you can rent both the main house and the ADU simultaneously. Check with the City of San Diego for the latest rules — they've been updated several times in recent years.
How We Price ADU Projects at Complete Builders Inc
Every ADU estimate we provide is fully itemized — site work, foundation, framing, roofing, plumbing, electrical, HVAC, insulation, drywall, flooring, cabinetry, and finish work all broken out separately. We handle the majority of this work with our own licensed in-house crew, which means tighter cost control and a single point of accountability from permit to move-in.
We've built ADUs across San Diego County — detached units, garage conversions, attached additions, and two-story structures. If you're early in the planning process and want a realistic number before committing to design, call us. We can usually give you a rough range after a 20-minute site visit.
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