ADU Cost Breakdown in California: Where Every Dollar Goes (2026)

The six categories of every ADU budget

Every ADU budget — prefab or site-built, Sacramento or Sunnyvale — divides into six buckets: (1) design, engineering, and Title 24; (2) permits and government fees; (3) site work, foundation, and utility connections; (4) the shell: framing, roofing, windows, exterior; (5) MEP systems: electrical, plumbing, heating/cooling; (6) interior finishes including kitchen and bath. Owners obsess over bucket six; budgets die in bucket three.

Where the dollars go: 600 sq ft detached, Bay Area

• Design, engineering, Title 24: $15K–$30K (~7%)
• Permits & city fees: $8K–$20K (~5%) — impact-fee-exempt under 750 sq ft
• Site work, foundation, utilities: $45K–$85K (~22%)
• Shell — framing, roof, windows, siding: $60K–$90K (~26%)
• MEP — electrical, plumbing, HVAC: $40K–$60K (~17%)
• Interior finishes, kitchen, bath: $55K–$85K (~23%)
• Total: $223K–$370K → typical contract range $250K–$330K

The hidden costs that ambush first-timers

• Electrical panel upgrade ($4K–$12K): a second dwelling on a 100A service rarely pencils without one.
• Sewer distance ($8K–$30K): every foot of trench is money; deep laterals sometimes need ejector pumps.
• Slope ($20K–$80K): stepped foundations and retaining walls reprice the whole project.
• Tree ordinances ($2K–$15K): protected trees constrain siting or require arborist-supervised work.
• Solar ($8K–$15K): required by Title 24 on new detached units — plan it, don't discover it.

Why conversions cost half

Garage conversions ($125K–$220K) skip buckets three and four almost entirely — foundation, framing, and roof exist. That's the whole secret: you're buying buckets five and six plus sealing and insulation. When a structurally sound garage sits on your lot, it's the highest-yield square footage in California construction. The trade-off is size (typically 350–480 sq ft) and the occasional slab surprise, which proper feasibility catches before contracts are signed.

Keeping the budget honest

Three rules. First, demand all-in quotes: design, permits, utilities, finishes, site restoration — a quote excluding 'permits and utilities' is hiding $30K–$80K. Second, fix the price: cost-plus ADU contracts in a hot trades market are open checkbooks. Third, verify the site before believing any number: a feasibility check that walks your utilities, panel, and setbacks converts internet averages into your actual project cost — the only honest way to start.

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