At a glance
Most ADU cost figures online are ranges with no source. This one is different. In 2021 the San Diego Housing Commission built five ADUs as a pilot program, on real lots, and published what each one cost and how long each phase took. It is the most detailed public breakdown we have found, and it is worth reading in full: the pilot program report.
The pilot is not a perfect model for a homeowner. The agency picked large, flat corner lots close to electrical transformers, and it batched trade work across sites. But the line items are real, and they show where the money actually goes.
The five units
- Manufactured one-bedroom, one bath, 499 square feet: $137,875 ($276 per square foot)
- Site-built studio, one bath, 224 square feet: $116,803 ($521 per square foot)
- Site-built one-bedroom, one bath, 499 square feet: $176,333 ($353 per square foot)
- Site-built two-bedroom, two bath, 990 square feet: $296,832 ($300 per square foot)
- Site-built three-bedroom, three bath, 1,199 square feet: $342,078 ($285 per square foot)
The site-built one-bedroom, line by line
- City permits and fees: $6,153
- Survey and topography: $3,939
- Engineering: $2,364
- Architecture (modifying permit-ready plans): $6,303
- Construction and utilities: $157,574
Construction was about 89 percent of the total. Everything else, the paperwork and design that most people forget to budget for, came to $18,759.
The manufactured one-bedroom, line by line
- City permits and fees: $3,585
- Survey and topography: $725
- Engineering: $2,570
- Architecture (site plans only): $2,500
- Construction and utilities: $29,000
- Manufacturing the unit: $49,995
- Installation and site prep: $44,500
- Delivery: $5,000
Two things stand out. First, the factory-built unit was $38,458 cheaper than the site-built unit of the same size. Second, the factory box itself was only about 36 percent of the total. Installation, site prep and on-site construction together cost more than the unit. When a prefab company advertises a base price, this is the gap to ask about.
The larger units
For the two-bedroom, permits and fees were $13,636 and construction $264,669. For the three-bedroom, permits and fees were $14,509 and construction $307,288. The report says the large jump in permit costs came partly from city fee waivers that applied to smaller units. The studio's permits were $6,173 and the manufactured one-bedroom's $3,585.
Costs the table leaves out
The report flags costs that hit the pilot but are not in the main table, and these are the ones that sink budgets:
- Relocating electrical panels to get gas service to the ADU added about $10,000 per site, tied to supply problems at the time.
- Splitting utilities so the ADU has its own meters can add $10,000 or more.
- On one lot, separating utilities would have required trenching in the street, estimated at more than $100,000 and six extra months. The team shared utilities on that site instead.
- A fence between the house and the ADU cost about $4,000 per site.
- Moving the manufactured unit by truck cost $3,500. A crane would have cost $7,000 to $9,000.
How long each phase took
The report compares what the team expected with what happened. Design and predevelopment were expected to take one to two months and took four on every unit. Permitting was expected to take six to eight months. It took six months for the manufactured unit and twelve for each site-built unit. Construction took six months for the manufactured unit and eight to ten for the others. The manufacturer built and delivered its unit within 90 days of the deposit.
How this compares to the statewide survey
The statewide owner survey found a median of $180,000 for a detached ADU, built a few years earlier. The pilot's site-built one-bedroom is very close to that. The two- and three-bedroom units are much higher, which matches the survey's finding that two-bedroom units are the most expensive type to build.
What to take from it
- Budget for soft costs. Even on permit-ready plans, they were about $10,000 to $35,000 per unit in this pilot.
- When comparing prefab and site-built, compare the full installed price, never the factory price.
- Ask early whether your utilities can be split without street work.
- Use a contingency. The report recommends 15 to 20 percent for ADUs.
Before you call a builder: answer six questions to pick a type, rough out a budget, and save builders to My Stack as you go.
