Building an accessory dwelling unit in San Diego typically costs $375 to $600 per square foot for construction, plus design fees of $8,000–$25,000 and permit costs of $6,000–$20,000 on top, depending on jurisdiction and project complexity.
Where your ADU price per square foot in San Diego lands within that range depends mostly on size, site conditions, and finish level — here’s how each of those actually plays out.
Where you land in the $375–$600/sf range
| Size | Typical position in the range | Why |
|---|---|---|
| 400–500 sq ft | Toward the higher end ($500–$600/sf) | Design and permit costs are spread over less floor space |
| ~750 sq ft | Middle of the range ($425–$500/sf) | A common balance point for size vs. fixed costs |
| ~1,000–1,200 sq ft | Toward the lower end ($375–$450/sf) | Fixed costs are spread over more floor space |
Design and permit costs don’t scale directly with square footage the way construction does — that’s a fixed-ish range whether the finished unit is 450 or 1,150 square feet, which is the main reason smaller units cost more per square foot to build. More on that below.
Why a smaller unit often costs more per square foot
A compact 450 sq. ft. unit and a larger 1,150 sq. ft. unit both need a foundation, a kitchen, a bathroom, a permit set, and a design package — none of that shrinks proportionally with the floor plan. Spread a $15,000 design-and-permit cost over 450 square feet and it adds about $33/sf. Spread that same cost over 1,150 square feet and it adds about $13/sf. That gap alone explains a meaningful chunk of why the per-square-foot rate drops as the floor plan grows, even before construction efficiencies come into play.
What’s included in the design fee
Eco’s design fees run $8,000–$25,000 total. That figure normally includes the permit submission and any city corrections, plus interior design, 3D rendering, and material selection — all bundled into one package alongside the architectural drawings and engineering. If you’d rather see those pieces broken out separately, that’s available too: permit management alone typically runs $1,500–$3,500, and interior design/3D rendering/material selection runs $2,500–$6,000 depending on project size.
What permits cost, and why the range is wide
Average ADU permit cost in San Diego runs $6,000–$20,000, and the exact number depends heavily on which jurisdiction the property sits in — the City of San Diego, Carlsbad, Encinitas, Chula Vista, Oceanside, and the unincorporated county each set their own fee schedule.
For the City of San Diego specifically, the official fee schedule (Information Bulletin 501, effective May 2026) sets:
- ✓Detached unit up to 500 sq ft: plan check base $3,512.92 + $6.98/sq ft, plus inspection base $2,228.29 + $4.46/sq ft
- ✓Attached unit or JADU over 1,100 sq ft: plan check base $8,085.26 + $4.10/sq ft, plus inspection base $8,401.90 + $4.21/sq ft
- ✓Additional standard fees apply: a $737 General Plan Maintenance Fee, a $58 Lead Hazard Prevention fee on pre-1978 structures, plus state seismic/building standards fees and water/sewer plan-check fees where applicable
Structural, energy-compliance, or other required consultant reports on top of these base fees are part of how the range extends toward the higher end on more complex projects.
Where the money actually goes
Across a typical project at Eco’s rates, construction makes up roughly 90–93% of the total cost, with design — architecture, engineering, interior design, and rendering — at roughly 4–5%, and permitting at roughly 3–4%. That ratio holds fairly steady whether the unit is 450 or 1,150 square feet; the number that actually shifts most between projects is the construction share itself, driven by size, site conditions, and finish level.
The reason this is worth knowing: a design fee that looks large in isolation ($8,000–$25,000) is a small fraction of the total project — it’s the piece that determines whether the other 90%+ gets spent correctly the first time.
One team, not six separate hires
A typical ADU project means coordinating an architect, a structural engineer, an interior designer, a permit expediter, a general contractor, and a project manager — six different relationships, six different timelines, six different people to keep in sync (and six different places for something to fall through the cracks). Eco’s design fee bundles architectural design, engineering coordination, permit management, interior design, and construction into one team and one point of contact, so that coordination isn’t something you’re doing yourself.
What’s included and what’s not
Included in the ranges above: design and engineering, permit management and city fees, standard site work and utility connections, and construction through standard finishes.
Typically not included: furniture and appliances beyond what’s specified in the contract, landscaping, and any utility-company-required upgrades outside the property line. Confirm the exact scope against your specific agreement — every project’s inclusions are spelled out in the proposal.
Hidden costs that can move the number
These don’t apply to every project, but they’re worth checking for on yours before finalizing a budget:
- ✓Grading and drainage on sloped lots
- ✓Sewer or water line distance — a longer run from the street costs more than a short one
- ✓Electrical panel upgrades, if the existing panel can’t support the new unit
- ✓Retaining walls, where grade changes require them
- ✓Fire access requirements, which vary by site and jurisdiction
- ✓Coastal zone review, for properties in or near the Coastal Zone
- ✓Garage structural corrections, for conversions where the existing slab, framing, or ceiling height needs work before it can become livable space
The feasibility phase of a project exists specifically to catch these before they show up as surprises mid-construction.
How to keep costs down without cutting quality
- ✓Lock in finish selections during design, not room by room during construction — one of the more effective ways to avoid budget creep
- ✓Get an honest read on your garage’s condition early if a conversion is on the table — it’s only the cheaper option if the existing structure actually supports it
- ✓Ask about your specific jurisdiction’s fee schedule rather than budgeting off a countywide average
- ✓Build in a contingency for site conditions that only show up once work starts, especially on sloped or older lots
Frequently asked questions
Getting a real number for your property
Every ADU cost figure in San Diego still has to meet your actual lot, your actual garage (if a conversion is on the table), and your actual finish goals. That’s what the feasibility phase of our process is for.
Schedule a consultation and we’ll walk your property with you to turn these ranges into a number specific to your project.
Related reading
- ✓ADU Builders in San Diego (service page)
- ✓San Diego ADU Permit Process: What Homeowners Should Expect
- ✓Garage Conversion vs. Detached ADU in San Diego
- ✓How Long Does It Take to Design, Permit and Build an ADU?
- ✓ADU Financing in San Diego
- ✓Planning an ADU for Rental Income in San Diego
- ✓ADU vs. Room Addition: Which Is Right for Your Property?
