ADU Cost in California (2026)

California ADU cost by size

These detached-unit estimates use a cedar exterior, shed roof, and the model's soft-cost allowance. They are computed from the same engine as the calculator below.

What is included in the estimate

• A hard-cost base of $315-$355 per sq ft, depending on exterior finish, plus the modeled roof and openings.
• A 18% allowance for design, engineering, and permits, with an uncertainty band around the resulting midpoint.
• Ground-up detached construction. Conversion and prefab scopes need their own estimates.

Not included: land, financing, unusually difficult utility runs, major grading or retaining walls, access constraints, remediation, and local charges beyond the model's general soft-cost allowance.

Why ADU cost per square foot changes with size

The modeled all-in range runs from about $378 to $493per sq ft across the sizes above. A smaller unit still needs a kitchen, bathroom, service connections, plans, and inspections, so those fixed costs are divided by fewer square feet. A single per-square-foot number is most useful only when the unit type, finish, and site scope match.

Regional differences: what the model knows

The current construction model does not apply an unverified regional price multiplier. Its build range is therefore the same across Los Angeles, San Diego, and Santa Clara counties. Rent and payback do vary by county because the calculator uses county-level HUD Fair Market Rent anchors. That makes the build range a starting point, not evidence that contractors charge the same amount in every California market.

Permit fees and other soft costs

California does not have one statewide ADU permit price. HCD's March 2026 handbook says an ADU with 750 sq ft or less is exempt from impact fees charged by local agencies, special districts, and water corporations; larger ADU impact fees must be proportional. The handbook separately discusses school fees and utility connection or capacity charges, so an impact-fee exemption does not mean every fee is zero.

Confirm the fee schedule with the agency that will review your lot. Start with the California ADU rules hub and the available city guide before treating an allowance as a budget.

Prefab ADUs and garage conversions

A prefab module price is not necessarily the installed project price. Ask whether the quote includes design, foundation, transport, crane, utility trenching, finish work, permits, and site restoration. Garage conversions reuse a structure, so this ground-up detached-unit model is not the right estimator; structural, moisture, fire, and utility upgrades can dominate that scope.

What to ask for in an ADU quote

• Use the same plans, specifications, and scope for every bidder.
• Separate design, permits, utility work, site work, and construction.
• List allowances, exclusions, owner-supplied items, and change-order rules.
• Confirm the schedule, payment milestones, insurance, and warranty.
• Check the contractor's license and compare at least three written bids.

The final item follows the California Contractors State License Board's consumer guidance. A low headline price is not comparable if it omits work another bid includes.

California ADU cost questions, answered

Sources and estimate basis

• California HCD Accessory Dwelling Unit Handbook, March 2026 - state-law and fee rules
• HUD FY2026 Fair Market Rent schedule - county rent anchors used by the calculator
• California CSLB contractor-selection guidance - bid comparison and license checks

Cost figures are LotFoundry planning estimates generated from the product's finance assumptions; they are not figures published by HCD, HUD, or CSLB. Last reviewed 2026-10-07.