Free, no signup. The ADU Cost Calculator estimates your total all-in California ADU cost using May 2026 labor rates, current city permit fee schedules, and field-tracked change-order categories that contractor bids typically leave out: sewer lateral upgrades ($15K–$30K), hillside soils and engineered foundation ($20K–$60K), and structural retrofit on pre-1970 garages ($5K–$25K). It is not a sales quote and it is not a contractor bid. Sometimes the number lands high enough that the right answer is “don't build” — and we say that clearly, before any money moves. ADUscale is an independent Owner's Representative, not a contractor, lender, or designer.
Not a contractor bid. This calculator is informational. It is not a binding cost estimate, contractor bid, or loan offer. Verify output against a licensed contractor's written bid and an independent Owner's Representative review before signing any construction contract.
Contractor bids in California consistently understate total project cost. The pattern is structural, not fraud: contractors quote the physical work on a clean lot with the simplest assumptions, and skip the categories that turn into change orders.
Sewer lateral, water service, electrical panel, gas line. Often $15K–$50K combined on pre-1980 LA and Bay Area parcels. Routinely omitted from first bids.
Permits, design, soils reports, engineering, insurance. Typically 12–18% of total. Itemized only at signing in most contracts.
Industry standard is 10%. Most bids omit it or carry 2–5%, which means overruns hit the owner directly.
LADBS, San Diego DSD, SF DBI, and San Jose use different fee schedules. The same 600 sqft ADU can vary $8K–$20K in permit fees across cities.
Hillside foundation, HPOZ design review, coastal overlay, prevailing-wage rules in some cities. Each can add $25K–$80K above a simple flat-lot assumption.
Every field is defaulted to include the categories above. You can see exactly what's in the band before the first contractor meeting.
We pull county parcel data to flag hillside zoning, HPOZ overlay, coastal overlay, sewer lateral risk, and your city's permit jurisdiction. Address is used for the lookup only. Not stored.
Garage conversion is the cheapest path (existing structure). Prefab is the fastest path (factory-built shell). Detached new construction is the most flexible (full layout freedom). JADU is the smallest-footprint path (interior conversion, ≤500 sqft). See ADU types for a full breakdown.
California Gov Code §65852.2 preempts local rules for ADUs at or below 800 sqft. Common sizes: 400, 600, 800, 1,000. Above 800 sqft, your city's local rules apply and permit complexity rises. See ADU sizes.
Standard means builder-grade fixtures, vinyl plank flooring, basic kitchen. Premium means mid-tier appliances, quartz counters, upgraded fixtures. Ultra means custom millwork, designer fixtures, high-end appliances. Each tier adds roughly $50/sqft above the next-lower tier.
A tight start window adds a 5–10% rush premium from contractors who can guarantee crew availability. A flexible window does not reduce cost meaningfully; it slightly broadens the contractor pool you can verify against.
A band, not a quote. Median is the typical project on a clean lot with your chosen type, size, and finish. Low is the simplest scenario. High is the realistic top: hillside foundation, premium finish, or correction loops at plan check.
Calibrated against statewide 2026 bands. Detached new construction: $300–$550/sqft. Garage conversion: $250–$450/sqft. Hillside premium: $500–$700/sqft. Prefab: $200–$400/sqft delivered, before site work.
Permits, design, engineering, soils report, utility connections, insurance. Typically 12–18% of total. Itemized on the result so you can see which categories are driving your band.
Pulled from county parcel flags: sewer lateral upgrade, hillside foundation, electrical panel upgrade, soils surprise, HPOZ design review. Surfaced specifically for your address, not generic warnings.
A flagged list of the categories typical contractor bids omit, so you can compare line-by-line when a real bid arrives.
Five things left out — because honest defaults beat hidden assumptions.
California contractors carry 12% to 25% overhead and profit. Your specific bid will reflect a specific structure that the calculator cannot predict.
Most LA ADU permits go through 2–3 rounds, adding 3–6 months. Loan-carry cost during delay is not in the construction band. A 3-month delay on a $250K construction loan at 9% adds roughly $7,000.
Lumber, copper, and electrical components have moved 10–30% in single quarters since 2020. We use May 2026 prices. If your project starts 6–12 months from now, revisit the band.
Hourly versus percentage-of-construction billing produces different design fees on the same project. The calculator uses a midpoint estimate; verify against your specific architect's fee structure.
A flat ADUscale engagement is separate from the construction budget. See pricing for a full breakdown of what's included.
The project is in the band where a written feasibility analysis pays for itself before the first construction draw. The $199 Feasibility & Risk Assessment reviews your specific lot, your specific contractor pool, and the line items the calculator flags. The report identifies which change-order risks will actually hit your project and how to neutralize them in contract drafting. About 1 in 7 of our Feasibility & Risk Assessments recommend “wait” or “don't build” — that's the honest call, and the model only works if we make it. The $199 fully credits against a project engagement if you proceed.
ADUscale is a California-based build-side ADU advisory. The Cost Calculator is calibrated against county-assessor parcel data (LA County Assessor, San Diego County Assessor, San Francisco Office of the Assessor-Recorder, and equivalent county systems statewide); current permit fee schedules at LADBS, San Diego DSD, SF DBI, and San Jose Building Division; California HCD ADU permit fee benchmarks; the InspectPilot 11M-record construction inspection dataset (California ADU projects, since 2013); and May 2026 California labor and materials data. ADUscale is not a contractor, designer, lender, or escrow provider. ADUscale is paid directly by homeowners — not through placement arrangements with contractors, lenders, or suppliers.
California-based build-side ADU advisory. Last updated: May 2026.
It does not answer the eligibility question (Reality Check) or the financing question (Lock-In Calculator). Run all three in sequence before you talk to a lender or sign a contractor agreement. Sometimes the number lands high enough that the right answer is not to build — and we say that clearly, before any money moves.
