ADU Construction Costs in California

By ADU Type

Junior ADU (JADU): $50,000–$100,000

Interior Conversion: $80,000–$150,000

Garage Conversion: $100,000–$175,000

Attached ADU: $150,000–$350,000

Detached ADU: $200,000–$450,000

Typical Cost Breakdown

Soft Costs (Design, engineering, permitting): $14,000–$55,000

Site Prep & Foundation: $15,000–$30,000

Utility Connections: $6,000–$28,000

Materials & Framing: $60,000–$100,000

Labor & Contractor: $50,000–$150,000

Finishes & Fixtures: $39,000–$101,000

Contingency (10–20%): $20,000–$70,000

Key Factors Affecting Your Budget

Location: Urban infill vs. rural plots, proximity to streets, and local impact fees.

Unit Size & Layout: Larger footprints demand more materials and labor.

Site Conditions: Steep grades, poor soils, or tight setbacks drive up excavation and foundation costs.

Finish Level: Premium cabinets, countertops, and fixtures can double your finishing budget.

Permitting & Impact Fees: Some municipalities charge up to $30,000+ in impact fees alone.

Financing & Value

California offers programs like CalHFA’s ADU Grant, which provides up to $40,000 for pre-development and closing costs.

An ADU can increase your home’s value by 20–30%, often recouping the construction investment over time.

Leverage Prefabricated or Modular Elements

Consider using factory-built wall panels, trusses, or even entire modular units. Prefab components arrive site-ready, cutting down on framing labor, reducing material waste, and often speeding up construction by weeks. That efficiency can translate to a 10–15% savings on both labor and general contractor fees.

Bundle Utility Work and Inspections

If you’re running new water, sewer, or electrical lines for your ADU, schedule those trenching and hookup tasks alongside any yardwork or landscaping you plan later. Combining site-excavation crews and inspections for multiple jobs minimizes mobilization costs and can shave both time and money off your overall project.