Prefab ADUs in San Diego
Updated
A prefab ADU is built in a factory and then delivered and set on your lot. For a prefab ADU in San Diego, the factory price is only part of the bill. Site work, utility hookups, permits, and installation can add tens of thousands of dollars, so compare installed, permitted totals, not sticker prices. This page uses published cost guides, labeled national where they are, and the City of San Diego's own ADU rules.
Advertised price vs installed price
Block Renovation says a 600 square foot prefab ADU advertised at $120,000 to $180,000 often totals $200,000 to $300,000 once everything is added. That works out to "$333 to $500 per square foot all-in" [1]. In its example, a $150,000 advertised unit lands at $215,000 to $310,000 fully installed and permitted [1].
Costs that sit outside the unit price, per Block Renovation [1]:
| 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 the city [1].
Installed prefab prices by size
Maxable, a California ADU marketplace citing ADU Compare, lists these installed ranges [2]:
| 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 |
For comparison, HomeGuide's national guide from 2023 lists prefab modular ADUs at $80 to $160 per square foot [3]. That figure is older and national, and newer installed totals run much higher [1][2].
Is prefab cheaper?
Sometimes. Block Renovation says prefab might save 10 to 20 percent compared to site-built, but only when site conditions are ideal [1]. A comparable site-built ADU runs about $300 to $500 or more per square foot in its example [1]. A tight backyard, a long utility run, or a slope can erase the savings.
California and San Diego rules still apply
- A building permit is required for every ADU in the City of San Diego, with no exemptions. Factory-built units are not an exception [4].
- New attached or detached ADUs in the City can be 150 to 1,200 square feet [4].
- Setbacks still apply, including 4 foot side and rear setbacks in a Very High Fire Hazard Severity Zone [4].
- Maxable notes that loads wider than 14 feet need a police escort in California, which adds a fee [2].
- Maxable also notes that California generally requires solar panels on new ADUs other than garage conversions, unless the lot is too shady [2].
Questions to ask a prefab company
- Is the price for the unit only, or installed and permitted?
- Who handles the foundation, utilities, and permit, and are they in the contract?
- Will the unit fit through access to my backyard, or will it need a crane?
- What is the delivery date, and what happens if it slips?
- Who is the local licensed contractor for the site work?
FAQ
How much does a prefab ADU cost in San Diego?
We did not find a San Diego survey of installed prefab prices. Published California figures put many installed prefab ADUs at about $190,000 to $437,000 depending on size [2]. Block Renovation's 600 square foot example lands at $200,000 to $300,000 all in [1].
Do I still need a permit for a prefab ADU?
Yes. The City of San Diego says a building permit is required to create any ADU or JADU, with no exemptions [4]. Your contractor or prefab company should tell you who submits it and who pays the fees.
Compare more numbers in our ADU cost guide, or request quotes from an independent local contractor.
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
- 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
- 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/
- HomeGuide, "How Much Does It Cost to Build an ADU?," November 13, 2023. Fetched 2026-11-11. https://homeguide.com/costs/adu-cost
- 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