Build cost lab · SF Bay Area
Bay Area prefab homes, by the numbers
Per-city prefab and modular build costs, permit and development impact fees, SB 9 lot-split rules, and utility connection charges — researched city by city for homeowners and small investors building on Bay Area lots.
Feasibility Calculator
Pick your city, ADU type, and size — see what California law guarantees on your lot and the researched local cost range.
ADU Cost Index
Installed costs for 8 cities, permit timelines for 12 (42–180 days), and city-by-city rules in one table.
City Guides
Prefab and ADU build cost, permit-fee, and lot-rule research for 18 Bay Area cities.
City guides
Alameda County
Marin County
What you'll find in every guide
Each city page covers what a prefab or modular home actually costs to put on a lot there — factory price plus site work — alongside the city's building permit and plan-check fees, development impact fees, SB 9 urban-lot-split status, utility connection charges, and how prefab compares with stick-built locally. Every figure traces to city fee schedules and public records — see our editorial policy.