How Much It Costs to Build an ADU in the Bay Area
ADU cost estimates online are all over the place. So instead of guessing, here are the real numbers from an active project we’re running through our design-build partnership — construction by Konstructo, design and project management by In The Deets.
This isn’t a prefab kit. It’s a fully custom, ground-up ADU in Lafayette — roughly 600 square feet — designed to match the architecture and finishes of the main house.
Construction: ~$350,000
This is the hard cost of physically building the structure — foundation, framing, roofing, and every trade that goes into it. This is what you pay the contractor.
Grading, foundation, framing $160,000
Electrical (incl. 400-amp panel upgrade) $43,000
Exterior (roofing, siding, gutters, paint) $40,000
Plumbing (kitchen, bath, sewer tie-in, water heater) $37,000
Walls (drywall, paint) $22,000
Windows & doors (install labor only) $14,000
Mechanical (mini-split, ventilation) $13,000
Insulation $12,000
Flooring (install labor + prep) $6,000
Misc (toilet rental, waste management) $2,700
Subtotal ~$350,000
Finish materials: ~$64,000
This covers everything the construction bid doesn't — appliances, cabinets, counters, tile, fixtures, and hardware — and it's the budget people most often underestimate.
Kitchen (appliances, counters, cabinets, tile, lighting) $23,000
Bathroom (vanity, tile, fixtures, glass shower door)$15,000
Whole-house (wood flooring, interior doors, hardware) $14,000
Exterior (windows, articulating patio door, ceiling fan) $12,000
Subtotal ~$64,000
Design, architecture & engineering: ~$40,000
This is the planning work that happens before and during construction, to make sure the design is buildable, permittable, and true to the house it's matching.
Covers architectural drawings, structural engineering, Title 24 compliance, interior design, and project management. Because this ADU is custom-designed to match the existing house rather than built from a standard plan, it carries more design time than a cookie-cutter unit would.
Permit fees: ~$8,000
This is what the city or county charges to review and approve the project — separate from anything paid to the design or construction teams.
Covers building permits, plan check, and municipal fees. Fee schedules vary by jurisdiction, so confirm current numbers early — Contra Costa County and Lamorinda-area jurisdictions each have their own ADU requirements.
Total: ~$462,000
Every lot and design is different, and a fully custom, ground-up unit built to match an existing home will always cost more than a stock plan — but this gives a real-world benchmark to plan against.
Thinking about building an ADU and not sure where to start? Let's talk.
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