San Mateo has consistently been one of the most ADU-friendly cities in San Mateo County, with streamlined permitting and a Building Division that processes ADUs efficiently. If you've been thinking about adding an ADU to your San Mateo property, here's what to expect cost-wise in 2026.
Detached new-construction ADU
Range: $350,000 – $960,000 for 500–1,200 sq ft
Per-square-foot, San Mateo detached ADUs are running $700–$800 per sq ft in 2026 for a fully-finished build, depending on finish level and site conditions. That includes:
- Engineered foundation
- Full framing, sheathing, exterior cladding
- Insulation and Title 24 compliance
- Roof, windows, doors
- Full electrical, plumbing, and HVAC (typically mini-split heat pumps)
- Kitchen and bathroom build-out
- Flooring and interior finishes
- Permit handling and inspections
Attached ADU (addition that operates as separate unit)
Range: $280,000 – $500,000
Attached ADUs share a wall with the main house and often share a utility connection point, which can save $15,000–$40,000 over a detached unit. Trade-off: more complex structural tie-ins and sometimes more permit scrutiny.
Garage conversion ADU
Range: $140,000 – $280,000
Converting an existing attached or detached garage is the lowest-cost path to an ADU in San Mateo. You're reusing the foundation, framing, and roof. Costs go to:
- Slab evaluation and insulation
- Wall insulation and drywall
- Window installation (egress required)
- Electrical sub-feed and plumbing rough-in
- Kitchen and bathroom build-out
- Mini-split HVAC
- Permit handling
Caveat: older garages with shallow foundations sometimes require foundation work, which can add $20,000–$40,000.
Junior ADU (JADU)
Range: $80,000 – $160,000
A JADU is a smaller unit (max 500 sq ft) carved out of the existing home — typically a bedroom suite with an efficiency kitchen and its own entrance. Cheapest path to a rentable unit but requires owner-occupancy of either the main house or the JADU.
San Mateo permit and impact fees
For a typical detached ADU in 2026, expect:
- Building permit: $4,000–$7,000
- Plan check: $2,000–$3,500
- School impact fee (for ADUs over 750 sq ft): ~$5,000
- SMAWPP utility connection fees (varies)
San Mateo waives many impact fees for ADUs under 750 sq ft, which is one reason 750 sq ft is a common target.
Timeline expectations
- Design + permits: 3–5 months
- Construction (detached): 5–7 months
- Construction (garage conversion): 3–4 months
See a recent detached ADU build we completed in San Mateo for what the finished product looks like, or learn more about our San Mateo ADU construction.
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