The Short Answer
ADU Cost Ranges by Type
Garage Conversion
Existing structure, fastest timeline, lowest cost of entry
$150K – $250K+Junior ADU
Conversion of existing space within home, max 500 sq ft
$80K – $180K+Attached ADU
Shares a wall with primary structure, multigenerational
$200K – $350K+Detached ADU — Flat Lot
Standalone structure on level or gently sloped site
$275K – $400K+Detached ADU — Hillside
Retaining walls, pier foundations, grading. Common in Pacifica & San Bruno
$300K – $450K+
These are total project costs — soft costs and hard costs combined. The ranges are wide because ADU costs depend on size, site conditions, finish level, and how far utility connections travel. A 450-sq-ft studio with standard finishes on flat land is fundamentally different from a 1,100-sq-ft two-bedroom with custom finishes on a slope.
Before Construction
Soft Costs — What You Pay Before Construction Starts
Soft costs are everything between your decision to build and the day your contractor breaks ground. On the Peninsula, they typically run 10 to 15 percent of total project cost — but in dollar terms, that means $25,000 to $60,000 or more. This is the number that catches people off guard, because you’re spending real money before anyone picks up a hammer.
Architectural Design
An architect or designer creates floor plans, elevations, and construction documents your city requires for permits. The range depends on whether you use pre-approved plans (least expensive), adapt a standard plan (mid-range), or commission a custom design (most expensive).
Design-Build Advantage: ACI bundles architectural design into the project contract, eliminating coordination gaps and typically costing less than hiring separately.
Structural Engineering
Required for most new construction ADUs and many attached ADUs. The structural engineer confirms that foundation, framing, and connections meet California Building Code requirements. On hillside lots, this work is more extensive due to soil conditions and lateral loads.
Geotechnical Report
Not every project needs one, but if your lot has significant slope or uncertain soil conditions, the city may require geotechnical investigation before approval. In Pacifica’s hillside neighborhoods (Pedro Point, Vallemar, Fairmont), a geotech report is common. On the Linda Mar valley floor or Daly City’s Westlake grid, it’s rare.
Title 24 Compliance
California’s Title 24 energy code requires commissioning, documentation, and third-party verification of HVAC, insulation, and windows. This cost is separate from design and engineering.
Permits & Plan Check
City building permit fees, plan check, and inspections. Costs vary by city and project scope. Some cities charge flat fees; others charge by square footage or construction cost.
Environmental & Utility Review
Utility company reviews for electrical, gas, water, and sewer connections. On steep hillside lots or where connections need to cross neighboring property, these costs can rise significantly.
Soft Cost Reality: Most homeowners underestimate soft costs by 25 to 40 percent. If you’re told “permits cost $3,000,” add design, engineering, geotechnical work, and Title 24 compliance — and you’re easily at $25,000 to $40,000 before you break ground.
During Construction
Hard Costs — Where the Real Money Goes
Hard costs are the construction itself: labor, materials, equipment, subcontractors. These costs vary wildly based on ADU type, site conditions, and finish level. A 500-square-foot garage conversion with basic finishes is fundamentally cheaper than a 1,100-square-foot detached ADU with high-end materials.
Hard costs typically break down like this:
Crew wages, site management, equipment rental, temporary facilities
Lumber, concrete, drywall, flooring, fixtures, mechanical/electrical
Electrical, plumbing, HVAC, roofing, windows
Grading, foundations, retaining walls, drainage (higher on hillsides)
Unforeseen conditions, price changes, minor scope adjustments
Peninsula Multiplier: San Francisco Peninsula construction costs run 30 to 50 percent higher than inland East Bay or South Bay suburbs. Labor is the primary driver — skilled trades in this market command premium wages. Add terrain challenges on hillside lots, and costs escalate further.
Real-World Scenarios
$175K vs. $400K — Two Real Scenarios
Budget ADU
Type: Garage conversion in Daly City
Site: Flat lot, existing structure, standard utilities already present
Design: 450 sq ft, 1 bed / 1 bath, basic finishes (laminate counters, modest tile, painted drywall)
Breakdown:
- Soft Costs: $25,000
- Hard Costs: $145,000 ($322/sq ft)
- No contingency overages
Premium ADU
Type: Detached ADU on Pacifica hillside
Site: Sloped lot, new utility runs needed, complex foundation design
Design: 1,100 sq ft, 2 bed / 1.5 bath, high-end finishes (quartz counters, tile, wood accents)
Breakdown:
- Soft Costs: $55,000
- Hard Costs: $330,000 ($300/sq ft)
- Contingency used: $15,000
Both scenarios represent realistic Peninsula projects. The “budget” project achieves its lower cost by using existing structure (garage) and flat terrain. The “premium” project costs more in total dollars but costs less per square foot because the larger footprint and higher finish level spread overhead differently. Hillside site work adds $40,000 to $60,000 upfront.
The Financial Case
Why Homeowners Build ADUs
1-bed ADU in Peninsula markets (Pacifica, San Bruno, Daly City)
New ADU typically adds 15–25% to total property value
Typical timeline for rental income to recoup construction costs