ADU Cost 2027: Detached, Garage Conversion, Attached and Prefab Units Compared
By Virginia Viadas
Quick answer: An accessory dwelling unit (ADU) costs about $60,000 to $300,000+ in 2026–2027, depending on type and location. Garage conversions are the cheapest, at $60,000 to $150,000 nationally. Detached new-construction ADUs cost $110,000 to $285,000 nationally, and $240,000 to $400,000 in Los Angeles and San Diego. Prefab units start at $80 to $250 per square foot for the unit alone, but $150 to $450 per square foot once installed with site work.
Key numbers at a glance
- Garage conversion: $60,000–$150,000 (national)
- Detached new construction: $110,000–$285,000 (national)
- Prefab, installed: $150–$450 per sq ft
- California median (Terner Center survey): about $150,000, or roughly $250 per sq ft
- Most common total for new detached units: $150,000–$300,000
What is an ADU?
An ADU is a second, self-contained home on a single-family lot, with its own kitchen, bathroom and entrance. Common forms include a backyard cottage, a converted garage, an attached unit, or a junior ADU (JADU) carved out of the main house. Owners build them for rental income, for multigenerational living, or to house aging parents.
How much does an ADU cost by type?
| ADU type | National cost range | Cost per sq ft | Best for |
|---|---|---|---|
| Garage conversion | $60,000–$150,000 | $50–$200 | Lowest total cost; sound existing slab |
| Basement conversion | $60,000–$150,000 | Varies | Homes with dry, tall basements |
| Attached ADU | $150,000–$280,000* | Varies | Lots without backyard space |
| Prefab / modular | $100,000–$350,000+ | Unit $80–$250; installed $150–$450 | Flat lots, short utility runs |
| Detached new construction | $110,000–$285,000 | $200–$400+ | Maximum privacy and rent |
Sources: Angi (Sept. 2026); DwellingIndex (Sept. 2026); HomeGuide. *Attached range from San Diego builder data (Subwork It Contracting, 2026).
How much does an ADU cost in California?
California has the most ADU construction in the country, and also some of the highest prices:
| Market | Garage conversion | Prefab | Detached new build | Source |
|---|---|---|---|---|
| Los Angeles | $100,000–$175,000 | $120,000–$250,000 | $240,000–$330,000 typical | ADUScale (2026) |
| San Diego | $50,000–$150,000 (JADU) | $150,000–$220,000 | $260,000–$400,000 | Subwork It Contracting (2026) |
| California statewide (median) | — | — | About $150,000 (~$250/sq ft) | Terner Center survey, via Better Place Design & Build |
Within a single metro, lot conditions move the price. San Diego builder data puts a 600 sq ft detached unit at about $255,000–$280,000 on flat inland lots in Escondido or El Cajon. The same unit in coastal La Jolla or Del Mar runs $370,000–$450,000+.
Is a prefab ADU really cheaper?
Not always. The factory price is only part of the bill. DwellingIndex puts prefab unit-only prices at $80–$250 per sq ft, but installed totals at $150–$450 per sq ft once you add:
- Foundation ($5–$40 per sq ft, according to Angi)
- Crane and delivery
- Utility connections. San Diego builder data puts a new water/sewer lateral at $8,000–$18,000 and an electrical panel upgrade at $3,500–$8,000.
- Permits, plan review and impact fees
- Site grading and access work
Prefab's real advantage is predictability. Fixed-price factory contracts reduce the risk of change orders. That matters in 2026, when ABC reported that construction input prices were up 8.9% year over year in August.
Why is a garage conversion the cheapest ADU?
The foundation, roof and most walls already exist. Builder data from ADU Pilot puts a California garage conversion at about $100,000 to $175,000. A detached new build of the same size costs $250,000 to $400,000, roughly 30% to 50% more.
The trade-offs: you lose parking, layouts are limited to the garage footprint, and older garages may need structural retrofits that erase the savings.
Does an ADU pay for itself?
It depends on local rents. In Los Angeles, ADUScale reports that garage-conversion rentals typically bring in $1,800–$2,400 per month, and comparable detached ADUs $2,400–$3,400. Divide your all-in cost by expected annual net rent to estimate the payback period. In many California markets, that works out to roughly 7 to 15 years.
Financing matters too. Freddie Mac's 30-year fixed rate averaged 7.03% on September 24, 2026, so a cash-out refinance to fund an ADU costs more than it did a year ago. Our 2026 housing market outlook covers the rate picture in more detail.
What should you check before building an ADU?
- Local zoning. ADU rules on size limits, setbacks, owner-occupancy and parking vary by state and city. Confirm with your planning department first.
- Utility capacity. Undersized sewer laterals or electrical service are the most common budget-breakers.
- Rental rules. Some cities limit short-term rental use of ADUs.
- Material pricing. Lumber and cabinet tariffs changed in 2026. See our guide on lumber and cabinet tariffs and remodeling budgets.
- Contract protection. On a multi-month build, an escalation clause clarifies who absorbs price increases.
Frequently asked questions
What is the cheapest type of ADU to build?
A garage conversion or junior ADU inside the existing house is cheapest, typically $50,000 to $150,000, because the structure and foundation already exist.
How much does a 600 sq ft ADU cost?
A 600 sq ft detached ADU costs roughly $150,000 to $280,000 in average-cost markets. In coastal California it runs $255,000 to $450,000, depending on lot conditions and finishes.
How long does it take to build an ADU?
Garage conversions typically take 4 to 8 months from contract to occupancy. Detached and prefab units often take 8 to 14 months, including permitting.
Does an ADU increase property value?
Generally, yes, especially in markets with strong rental demand. The increase depends on local comparable sales and whether the ADU is legally permitted.
Are prefab ADUs worth it?
Prefab ADUs are worth considering for flat lots with easy crane access and short utility runs. On sloped or tight lots, site work can erase the savings.
Sources
- Angi, "How Much Does an ADU Cost to Build? [2026 Data]": https://www.angi.com/articles/how-much-do-adu-costs.htm
- DwellingIndex, "Prefab ADU Cost: Real All-In Prices (2026)": https://dwellingindex.com/prefab-adu-cost/
- Sweeten, "ADU Cost: A Practical Guide to Planning Your 2026 Build": https://sweeten.com/blog/home-renovation-cost-guides/adu-cost-guide/
- ADUScale, "ADU Cost in Los Angeles 2026": https://aduscale.com/los-angeles/cost/
- Subwork It Contracting, "ADU Cost in San Diego 2026": https://subworkitcontracting.com/blog/adu-cost-san-diego/
- ADU Pilot, "Garage Conversion ADU" (2026): https://www.adupilot.com/garage-conversion-adu
- Better Place Design & Build (citing Terner Center survey): https://betterplacedesignbuild.com/blog/detached-garage-adus-the-ultimate-guide/
- Freddie Mac, PMMS, Sept. 24, 2026: https://freddiemac.gcs-web.com/news-releases/news-release-details/mortgage-rates-average-703