ADU Cost Estimator
Pick your ADU type and size to see construction cost, soft costs, and the all-in budget range.
Runs entirely in your browser — your numbers never leave this page.
How it works
An ADU’s price is driven by three choices — type, size, and where you build:
- Conversions ($60k–$150k typical): cheapest because the shell exists — but surprise structural, plumbing and electrical work still bites.
- Attached ($100k–$216k): shares a wall and often utilities with the main house, trimming site costs.
- Detached ($110k–$285k): a standalone small home — full foundation, roof, and utility runs, so the top of the range.
- Soft costs are real: design and plans $8,000–$20,000, permits $1,350–$9,000+ (California cities higher).
- The 750 sq ft trick: ADUs at or under 750 sq ft skip city impact fees in California and several other states — design just under the line.
- Utilities before framing: trenching, panels, water/sewer taps routinely add $18k–$55k+ and never appear in per-sq-ft quotes — price them first.
Frequently asked questions
How much does it cost to build an ADU?
National average is about $180,000, with most projects landing between $60,000 and $285,000 depending on type, size and region.
What does an ADU cost per square foot?
Roughly $150–$300/sq ft nationally; high-cost metros like coastal California run $300–$450/sq ft all-in.
Is a garage conversion cheaper than new construction?
Usually yes — $60,000–$150,000 versus $110,000–$285,000 for detached — but hidden structural and utility work can narrow the gap.
How much are ADU permits?
Building permits run $1,350–$9,000+ depending on location, plus $8,000–$20,000 for design and architectural plans.
What’s the 750 sq ft ADU rule?
In California (and some other jurisdictions), ADUs of 750 sq ft or less are exempt from city impact fees — often a five-figure saving.
How much do utility hookups add?
Water, sewer and electrical connections for a detached ADU commonly add $18,000–$55,000+ before any framing begins.
Does an ADU raise property value?
Generally yes — appraisers credit permitted, well-built ADUs, and rental income potential lifts value further in tight housing markets.
Can I rent out my ADU?
Usually yes, subject to local rules — some cities restrict short-term rentals or require owner-occupancy of one unit. Check before you build.
How long does an ADU take to build?
Plan on 6–12 months including design, permitting and construction; permitting alone often takes 2–4 months.
Prefab vs stick-built ADU?
Prefab can be faster with more predictable pricing, but transport, crane and foundation costs still apply — compare true all-in quotes, not brochure prices.