What ADUs Cost Per Square Foot in Northern California
In 2026, a custom-built accessory dwelling unit in Northern California typically runs $350 to $650 per square foot, turnkey. A 600 sq ft detached ADU generally lands between $250,000 and $400,000 including architectural design, permits, foundation, structural, MEP (mechanical, electrical, plumbing), finishes, appliances, and utility connections. Garage-conversion ADUs are the most economical entry point — because the shell already exists, total cost is often 30–45% less than a comparable detached build. Expect $175,000 to $275,000 for a full 400–500 sq ft garage conversion including new bathroom, kitchenette, HVAC, and finishes.
Why the Range Is So Wide
Three factors move ADU pricing more than anything else: site conditions, utility connections, and finish level. A flat, cleared lot with a short trench to an existing panel is dramatically cheaper than a hillside lot requiring a new pier foundation, a 200-foot utility trench, and a separate electrical service. Finish level matters equally — quartz-and-IKEA finishes vs. custom cabinetry, natural stone, and integrated appliances can double the finish budget on the same shell.
Permit Fees and Soft Costs
California ADU legislation has capped many jurisdictional impact fees for ADUs under 750 sq ft, but you'll still see permit fees ($3,000–$12,000 depending on jurisdiction), planning review, school fees on larger units, and utility connection fees. Budget 8–12% of construction cost for soft costs (design, engineering, permits, testing, coordination).
Timeline: 7 to 12 Months, Realistically
Design and permitting typically take 3–5 months; construction runs 4–7 months. If you're rushing, garage conversions can move fastest — sometimes 5–6 months total. Ground-up detached ADUs on hillside sites with utility complications can push 12–15 months.
Financing Your ADU
Common financing paths include HELOCs against your primary residence, cash-out refinance, construction-to-permanent loans, and specialty ADU loans from lenders like RenoFi. If you plan to rent the ADU, some lenders will underwrite future rental income into the loan. Talk to a lender before design starts — the financing structure often shapes design choices.
Is an ADU Worth It Financially?
In most Bay Area and Northern California markets, a well-built ADU adds 20–35% to the appraised value of the property, often exceeding the build cost within a few years. If rented at market rates ($2,200–$3,800/month depending on size and location), the ADU can pay for itself within 8–12 years while continuing to appreciate.
Frequently Asked Questions
Can I build an ADU on any property in California?+
Nearly all single-family and multi-family properties in California can build at least one ADU under statewide law (SB 9, AB 68, AB 881, and follow-on legislation). Local jurisdictions may add setback, height, or design requirements but cannot prohibit ADUs outright.
Do I need a separate utility meter?+
Detached ADUs frequently need a separate electrical panel; a separate meter is sometimes required, sometimes optional. Water and sewer connections vary — some jurisdictions allow shared, others require dedicated.
Can I rent my ADU on Airbnb?+
Long-term rentals (30+ days) are allowed statewide. Short-term rental rules are city-dependent — some cities allow them, some prohibit, some require permits.
What's the cheapest way to add an ADU?+
Garage conversions and JADUs (junior ADUs, up to 500 sq ft, inside the existing home) are the most cost-efficient. Prefab/modular ADUs can be cheaper on paper but often lose the savings to site prep and utility work.