What Drives the Price of an ADU?
Two big buckets make up any ADU budget. Hard costs are the physical construction: foundation, framing, roofing, plumbing, electrical, and finishes. These usually account for 80 to 90 percent of the total. Soft costs cover everything else, including design, engineering, permits, and project management, which make up the remaining 10 to 20 percent.
A few factors move the needle more than any others.
Size and the 750 Square Foot Rule
Bigger units cost more, but the relationship is not linear. The first 400 square feet are the most expensive per square foot because fixed costs like utility trenching and the foundation get spread across fewer feet. There is also one threshold every California homeowner should know: under state law (SB 13), ADUs of 750 square feet or smaller are exempt from all development impact fees. Cross that line and you may add $5,000 to $25,000 or more to your project. Many homeowners deliberately design at 749 square feet to stay under the cap.
Location
Where you build matters a lot. Compared to the statewide average, expect to pay 25 to 40 percent more in the Bay Area, 15 to 25 percent more in greater Los Angeles, and 20 to 35 percent more in San Diego. Inland regions like Riverside, San Bernardino, and Sacramento tend to run at or below the statewide baseline.
Construction Type
Reusing an existing structure saves money. A garage conversion already has a foundation, walls, and roof, so it costs far less than a detached unit built from the ground up. A junior ADU built inside your existing home is cheaper still. Detached units cost the most because they need a new foundation, full utility connections, and standalone systems.
Site Conditions
A flat, clear backyard close to existing utilities keeps costs down. A hillside lot, poor soil, long trenching runs, or a garage that needs demolition can add tens of thousands of dollars. These surprises are why a contingency fund matters.
ADU Types Explained
Junior ADU (JADU)
A JADU is carved out of your existing home and capped at 500 square feet. It needs a separate entrance and can share a bathroom with the main house. Because there is no new foundation, roof, or utility trenching, it is the cheapest legal path to extra living space or rental income. JADUs are also exempt from impact fees and school fees statewide. The trade-off: state law requires the owner to live in either the main home or the JADU.
Garage Conversion
Converting a garage is the most popular budget-friendly option. A single-car garage of 250 to 400 square feet typically runs $70,000 to $120,000, while a two-car garage of 400 to 600 square feet lands closer to $100,000 to $175,000. The biggest cost drivers are adding plumbing and a bathroom, upgrading the electrical panel, installing HVAC, and meeting Title 24 energy code. Watch out for older garages with cracked slabs or substandard framing, which can add remediation costs.
Attached ADU
An attached ADU is built onto your existing home and shares at least one wall. It usually costs 15 to 25 percent less than a comparable detached unit thanks to shared walls and shorter utility runs. Size is capped at 50 percent of your main home's square footage, up to 1,200 square feet.
Prefab ADU
Prefab or modular units are built in a factory and assembled on site. They go up faster, often in three to six months, but the sticker price can be misleading. Once you add site preparation, foundation, delivery, utility hookups, and permits, the all-in cost usually lands close to a stick-built unit. Budget for the prefab unit plus 70 to 120 percent of that figure for everything else.
Detached ADU
A standalone backyard unit offers the most privacy and flexibility, and it commands the highest price. New construction means a new foundation, full utility trenching, and complete standalone systems. Most detached units run $250 to $500 per square foot statewide, climbing to $375 to $600 in pricier markets like San Diego.
Breaking Down the Soft Costs
These are the line items homeowners most often forget.
Design: $8,000 to $20,000. Covers architectural plans, structural calculations, and Title 24 energy reports. Since January 2025, every California city must offer pre-approved plan sets, which can cut this cost by 30 to 50 percent.
Engineering: $2,000 to $12,000, including structural engineering, a soils report if your site requires one, and energy calculations. Hillside and unusual lots are the most likely to need a full geotechnical report.
Permits and application fees: $2,000 to $21,000 depending on your city and unit size. A useful rule of thumb is $8 to $18 per square foot of permitted area.
Impact fees: Apply only to ADUs over 750 square feet and can add $5,000 to $25,000 or more. Units at or below 750 square feet pay nothing, thanks to SB 13.
Utility fees: Cover connecting water, sewer, gas, and electric. These range from $5,000 for a unit using nearby existing hookups to $35,000 or more for a detached unit needing long trenching or new meters.
Project management: Usually built into a general contractor's fee at 10 to 20 percent of the project. Managing it yourself saves money but carries real risk.
How Much Should You Set Aside for Surprises?
Always build in a contingency. Ten percent is the minimum for a simple project on a good lot. Fifteen percent is the standard recommendation for most builds. Twenty percent is wise for hillside sites, older homes, or custom designs where the unexpected is more likely.
A Note on Why Costs Have Climbed
If you are working from older estimates, be careful. The California Construction Cost Index rose 37 percent between January 2021 and January 2024. Quotes from even a couple of years ago likely understate today's reality, which is why current data matters when you budget.
Frequently Asked Questions
What is the cheapest type of ADU to build in California?+
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Cost figures are based on 2024–2025 California market data, including guidance from the California Department of Housing and Community Development (HCD) and verified industry sources. Estimates are for planning purposes only and do not constitute a quote or professional advice.