ADU cost in San Diego County: what the permits were filed at, by size

What ADUs were filed at, by size

The County records floor area on its ADU permits, so the filed value can be read against size. Bands are the floor area on the permit; values are the middle filed value in each band (County Planning & Development Services permit file, snapshot 20 October 2026, residential permits with a named firm, computed by us).

Across the 332 permits with both figures, the middle half worked out to about $102 to $135 per square foot of recorded floor area, with a middle value near $116. Values that move this closely with area look like a valuation of the space more than a builder's price; that is our reading, not a field in the file. Either way, it is the construction value on the application. It leaves out design and engineering, utility connection charges, impact fees on larger units, and site work such as grading or a long trench, so an all-in builder quote will sit above it.

The 114 permits over 1,200 sq ft are above the County's limit for an ADU (see below). The floor field is what the permit records, which can include an attached garage or other area filed with the unit; we report it as recorded.

Fees: what changed and what the law says

• The County waiver is over. "The County of San Diego trial impact fee waiver went into effect January 9, 2019, and ended on January 9, 2024." Projects started before that date did not pay the plan check fee; permits not issued by it pay the standard fees, collected at issuance (County of San Diego ADU page, read 21 October 2026).
• 750 sq ft is the state line. A local agency, special district or water corporation "shall not impose any impact fee upon the development of an accessory dwelling unit" of "750 square feet of interior livable space or less"; on a larger ADU, impact fees "shall be charged proportionately in relation to the square footage of the primary dwelling unit". Connection fees and capacity charges are not impact fees under the section (Gov. Code 66311.5(c), renumbered from 66324 on 1 January 2026).

In the County's file, 87 of 332 ADU permits with a floor area were 750 sq ft or less, about one in four. If your design is just over 750 sq ft, ask the County for the impact fees at both sizes before you fix the plan.

Size limits and pre-approved plans

From the County's ADU page: a detached ADU "shall not exceed 1,200 square feet"; an attached ADU "can be up to 50% of the total area" of the house, capped at 1,200 sq ft; a junior ADU can be "a maximum of 500 square feet". The County also offers dwelling-unit plans for detached ADUs that it describes as about 85% complete, to which you add project-specific information. Our file has 16 junior ADU permits among the 334.

How long it takes, and who builds them

The usual ADU permit took 351 days from issue to final, and 85.1% of those a year or more old had reached final. Builders are spread thin: 213 firms for 334 permits, and 79.8% of firms filed exactly one. The busiest over the last twelve months was Moe Construction with 7. 91.5% of ADU firms matched an active CSLB license by name; check yours on Check a License and look for the B (General Building) classification.

For scale, the usual addition in the county was filed at $78,705 and the usual new house at $376,183. The ADU vs addition guide compares the two.

See every San Diego County ADU builder by filed value and finished share →

Check a quote in five minutes

1. Find your planned floor area in the table and note the usual filed value for that band.
2. Open the builder on the San Diego County ADU list and compare its usual filed value with the county's middle half, $110,493 to $160,022.
3. Ask the builder in writing which fees, utility connections and site work are outside the quote.
4. If the plan is between 750 and 800 sq ft, ask the County for the impact fees at 750.

What this does not tell you

The filed value is not what owners paid, and this page has no source for all-in ADU prices. The file does not say whether an ADU is detached, attached or a garage conversion, beyond the junior ADU type. It covers the unincorporated county only; the City of San Diego and the other cities issue their own permits and set their own fees. License status is matched by business name, and an unmatched firm is not necessarily unlicensed.