How Much Does It Cost to Build an ADU in California?

If you’re researching ADU costs in California, you’ve probably seen numbers ranging from $100,000 to $500,000 — and that spread isn’t wrong. The honest answer is that most custom ADUs in Placer County land between $150,000 and $400,000, and where yours falls depends on five factors we walk through below.

Typical ADU cost ranges in Placer County (2026)

Garage conversions: roughly $100,000–$180,000. You’re starting with a foundation, walls, and a roof, which is why conversions are the most cost-effective path to a legal, permitted dwelling.

Attached ADUs: roughly $150,000–$250,000. Sharing a wall (and often utilities) with your main home trims sitework and utility trenching costs.

Detached ADUs: roughly $200,000–$400,000+. A standalone unit up to 1,200 sq ft is essentially a small custom home — foundation, framing, full kitchen and bath, its own utility connections, and finishes to match your property.

Junior ADUs (JADUs): roughly $80,000–$150,000. Compact units up to 500 sq ft carved out of existing space inside your home.

The five factors that move your number

1. Size and complexity. Cost per square foot drops as size grows, but total cost rises. Vaulted ceilings, specialty spaces (golf simulators, theaters, gyms), and high-end finishes add real dollars.

2. Site conditions. Slope, soil, access for equipment, and distance to utility connections are the biggest surprises in ADU budgets. This is why we start every project with a site assessment before quoting anything.

3. Utilities. Trenching for water, sewer, and electrical — and whether your panel needs an upgrade — can swing a budget by $10,000–$40,000.

4. Permits and fees. California law caps impact fees for ADUs under 750 sq ft, but plan review, school fees (over 500 sq ft), and utility connection fees still apply. In Placer County, budget several thousand dollars; we handle permitting in-house so there are no surprises.

5. Design and engineering. Plans, structural engineering, and Title 24 energy compliance typically run $10,000–$25,000. (We currently take $1,000 off your design.)

Is an ADU worth the cost?

For most Placer County homeowners, yes — for three reasons. Rental income on a detached ADU in Rocklin, Roseville, or Loomis commonly covers a meaningful share of financing costs. Appraisers increasingly credit permitted ADUs toward resale value. And for families, a guest suite or in-law unit often costs less than years of assisted living or a home addition of comparable function.

How to get a real number for your property

Online ranges only go so far — your lot decides your budget. We offer a free site assessment: tell us your address and how you’d use the space, and we’ll confirm what Placer County allows on your parcel, flag any site-cost drivers, and sketch a first concept in 3D before you commit to anything.

Get your free ADU site assessment → or call (916) 671-0802.

Golden Valley Development is a family-owned, licensed and insured design-build contractor in Loomis, CA (CSLB #1030793), serving Placer County and surrounding areas.