ADU Cost in San Diego (2026)
Updated
An ADU in San Diego can cost anywhere from tens of thousands of dollars for a simple conversion to several hundred thousand for a new detached unit. We looked for a published survey of installed ADU prices in San Diego and did not find one. So this page uses national cost guides, labeled as national, plus permit fees from the City of San Diego's own fee schedule. Use these numbers to plan, then get real quotes for your lot.
National cost per square foot by ADU type
HomeGuide's national guide, published in November 2023, gives these average ranges [1]:
| ADU type | National cost per sq ft |
|---|---|
| Prefab modular | $80 to $160 |
| Attached | $125 to $225 |
| Detached | $150 to $250 |
| Above garage | $200 to $500+ |
| Basement conversion | $50 to $100+ |
| Garage conversion | $50 to $200 |
Newer sources report higher numbers. Block Renovation, writing in 2026, says a site-built ADU comparable to a 600 square foot prefab runs "$300 to $500+ per square foot depending on finishes and complexity" [2]. That gap is one reason to treat older national averages as a floor, not a budget.
National cost by size
HomeGuide's national ranges by size (2023) [1]:
| Size | National average cost |
|---|---|
| 400 sq ft | $40,000 to $120,000 |
| 500 sq ft | $50,000 to $150,000 |
| 600 sq ft | $60,000 to $180,000 |
| 750 sq ft | $75,000 to $225,000 |
| 1,000 sq ft | $100,000 to $300,000 |
| 1,200 sq ft | $120,000 to $360,000 |
In the City of San Diego, 1,200 square feet is the largest size allowed for a new attached or detached ADU [3].
Prefab ADUs: advertised price vs installed price
A prefab unit's sticker price is rarely the full cost. Block Renovation says a 600 square foot prefab ADU advertised at $120,000 to $180,000 often reaches $200,000 to $300,000 once site work, utilities, permits, and installation are added [2]. Here is its breakdown of costs that sit outside the unit price [2]:
| Item | Range |
|---|---|
| Site prep and foundation | $15,000 to $50,000+ |
| Utility connections | $10,000 to $30,000 |
| Crane and installation | $5,000 to $15,000 |
| Final connections and finishing | $10,000 to $25,000 |
| Landscaping and site restoration | $5,000 to $20,000 |
| Total extras | $65,000 to $160,000 |
Block adds that permits and fees in California "can exceed $20,000 depending on jurisdiction" [2]. More detail is in our prefab ADU guide.
Maxable, a California ADU marketplace, lists these installed prefab ranges, citing ADU Compare [4]:
| Prefab size | Installed range |
|---|---|
| Studio, about 350 sq ft | $190,000 to $304,000 |
| 1 bedroom, 320 to 600 sq ft | $219,000 to $320,000 |
| 2 bedroom, 567 to 800 sq ft | $290,000 to $437,000 |
| 2 bedroom, 864 to 1,000 sq ft | $343,000 to $410,000 |
Planning and site costs
HomeGuide lists these national averages for early project costs [1]:
| Item | National average |
|---|---|
| House plans | $1,200 to $5,000 |
| Land survey | $400 to $1,800 |
| Excavation | $1,500 to $5,000 |
| Grading and leveling | $1,000 to $5,000 |
| Concrete slab foundation | $2,400 to $10,500 |
City of San Diego permit fees
The City's fee schedule, Information Bulletin 501 (August 2026), sets plan check and inspection fees by project size [5]:
| Project | Plan check (base) | Inspection (base) |
|---|---|---|
| Attached ADU or JADU up to 1,100 sq ft, or detached ADU up to 500 sq ft | $3,512.92 | $2,228.29 |
| Detached ADU over 500 sq ft, or attached ADU or JADU over 1,100 sq ft | $8,085.26 | $8,401.90 |
Each row also has an added rate above its base size [5]. Other City charges can apply:
- Development impact fees apply in the City's urbanized areas, and the City offers a calculator to estimate them [5].
- State law now exempts JADUs of 500 square feet or less from impact fees [6].
- School fees apply, and the General Plan Maintenance Fee has applied to all ADUs and JADUs since August 9, 2022 [3].
- Projects with more than one ADU also pay regional transportation fees [3].
For other cities, see our pages on Chula Vista and Escondido.
How to compare ADU quotes
- Ask what the price includes: design, permits, City fees, site work, utilities, and final hookups.
- Ask for the price per square foot and the total, so you can compare builders of different sizes.
- Ask who pulls the permit and who pays impact and school fees.
- Ask if a preapproved plan could shorten review. The City reviews preapproved plans in 30 days [3].
- Ask how the builder handles changes. Get any price change in writing before the work starts.
- Check each contractor's license yourself before you sign.
Quotes that look far apart often cover different work. Line them up item by item before you decide.
FAQ
Is a prefab ADU cheaper than a site-built one?
Sometimes, but not always. Block Renovation says prefab might save 10 to 20 percent compared to site-built only when site conditions are ideal [2]. Once site prep, utilities, crane work, and permits are added, the gap often shrinks. Ask each builder for an installed, permitted total.
Why do ADU cost estimates vary so much?
Size, type, finishes, and the site all change the price. A garage conversion reuses an existing structure, while a new detached unit needs a foundation and new utility lines. National guides also lag behind current local prices, so a 2023 average may read low in 2026 [1][2].
Does a JADU cost less in fees?
Often, yes. State law now exempts JADUs of 500 square feet or less from impact fees [6]. Plan check, inspection, and other fees can still apply, so ask the City or your contractor for a full fee estimate.
Use the ADU cost calculator or request builder quotes.
A referral is not an endorsement, so check any contractor yourself before you sign.
We're a referral service, not a contractor, and independent contractors may pay us for referrals.
Sources
- HomeGuide, "How Much Does It Cost to Build an ADU?," November 13, 2023. Fetched 2026-11-11. https://homeguide.com/costs/adu-cost
- Block Renovation, "Prefab ADUs: What is the True Cost Savings," February 4, 2026 (updated June 17, 2026). Fetched 2026-11-11. https://www.blockrenovation.com/guides/prefab-adus-what-is-the-true-cost-savings
- City of San Diego Development Services, "Information Bulletin 400: Accessory Dwelling Unit/Junior Accessory Dwelling Unit," January 2026. Fetched 2026-11-11. https://www.sandiego.gov/development-services/forms-publications/information-bulletins/400
- Maxable, "How Much Does a Prefab ADU Cost?," no date shown. Fetched 2026-11-11. https://maxablespace.com/how-much-does-a-prefab-adu-cost/
- City of San Diego Development Services, "Information Bulletin 501: Fee Schedule for Construction Permits," August 2026. Fetched 2026-11-11. https://www.sandiego.gov/development-services/forms-publications/information-bulletins/501
- City of San Diego Development Services, "Regulatory Updates," no date shown. Fetched 2026-11-11. https://www.sandiego.gov/development-services/regulatory-updates