As of October 2026, ranges published by San Diego builders put most garage conversions at about $100,000 to $225,000, new detached ADUs at about $250,000 to $460,000, and junior ADUs at roughly $90,000 to $200,000. Design, permit fees and utility work can sit on top. Your lot and finish level decide the rest.
What does each ADU type cost in San Diego?
The table compares two local contractors' published planning ranges, as of October 2026. Neither is our pricing, and both sources say their figures are not quotes.
| Type | Realm (March 2026) | Lars Remodeling (October 2026) | What usually drives it |
|---|---|---|---|
| Garage conversion | $100,000 to $210,000 | $150,000 to $225,000 or more | Slab, framing and roof condition of the existing garage |
| Detached ADU | $262,000 to $460,000 | $250,000 to $450,000 or more | New foundation, structure, roof, solar and long utility runs |
| Attached ADU | $160,000 to $280,000 | $200,000 to $350,000 or more | Tying new walls and roof into the existing house |
| Junior ADU | $90,000 to $200,000 | $120,000 to $200,000 or more | How much plumbing has to move to add a kitchenette |
Treat the upper figures as soft ceilings, not caps. A canyon-edge lot, a long trench or a high-end kitchen can push a San Diego project past them.
What gets added on top of construction?
- Design and engineering. Floor plans, structural calculations and Title 24 energy documents. A soils report is added when the site calls for one.
- City fees. As of October 2026, the City of San Diego charges a General Plan Maintenance Fee on ADU and junior ADU permits, with no exemption for small units, according to a city notice from 2022; the amount is adjusted each July. Plan-check and inspection fees also apply, and school fees can apply.
- Utilities. Realm notes that trenching new water and electrical lines to a backyard unit can add $14,000 or more. A panel upgrade or relocation can add more.
- Solar. As of October 2026, newly built detached ADUs generally must include solar under the California Energy Code. Conversions and additions are not subject to that requirement.
- Contingency. Realm recommends holding 10% to 20% in reserve for surprises found once walls or soil are opened.
Does staying under 750 sq ft save money in San Diego?
Usually, yes. As of October 2026, state law bars impact fees on ADUs of 750 sq ft or less of interior livable space and on junior ADUs of 500 sq ft or less. Above 750 sq ft, fees can be charged in proportion to the size of the main home. The city's 2022 fee notice confirms the development impact fee exemption for units under 750 sq ft.
One San Diego exception: under the 2025 Bonus ADU reforms, the city adopted a Community Enhancement Fee that applies to bonus and affordable ADUs under 750 sq ft. It funds infrastructure in communities of concern. If you are not using the Bonus ADU program, it does not apply.
Why can two San Diego quotes for the same ADU be far apart?
It depends on scope. One quote may cover construction only, while another includes design, permits, utilities and solar. San Diego lots add their own variables:
- slopes and canyon edges that need deeper foundations or retaining walls
- coastal zone review, which can add steps near the shore
- mapped fire hazard zones, which can change materials and access requirements
- the distance from the backyard to the sewer lateral and electrical service
- whether an existing garage slab can be reused or must be replaced
Ask each builder for a written, itemized quote so you can compare line by line: design, fees, utility work to a stated point, finish allowances, and who pays for plan-check corrections.
Are there ways to lower the cost?
As of October 2026, the City of San Diego's Planning Department lists two sets of ready-made plans it accepts: San Diego County's Standard ADU Building Plans and the City of Encinitas Permit Ready ADU plans. Starting from one can cut design time, though site work and a permit are still required. The San Diego Housing Commission also runs an ADU Finance Program with construction loans and technical help for income-qualified homeowners; check current eligibility with the Commission.
How do I get a real number for my lot?
Send us the address through the lot check form. We confirm what your lot allows, walk the site, and then prepare a written, itemized quote. Related reading: garage conversion or new build and what an ADU can rent for in San Diego.
Sources, as of October 2026:
- Lars Remodeling & Design, "What Does It Cost to Build an ADU in San Diego in 2026?" (published August 13, 2026; updated October 30, 2026).
- Realm, "The Real Cost to Build an ADU in San Diego" (March 11, 2026).
- City of San Diego Development Services, notice on reinstating the General Plan Maintenance Fee for ADUs (August 11, 2022), sandiego.gov.
- Inside San Diego (City of San Diego), "City Council Adopts Reforms to Accessory Dwelling Unit Program" (June 18, 2025).
- City of San Diego Planning Department, Housing Toolkit: Accessory Dwelling Units page, sandiego.gov.
- California Department of Housing and Community Development (HCD), ADU Handbook, updated March 2026, with its addendum on law in effect January 1, 2026, hcd.ca.gov.