California ADU cost questions are the single most common reason homeowners reach out to Cyrus Poudat. The honest answer is that the unit itself is rarely the most expensive line item. Soil reports, utility upgrades, fire sprinklers, school fees, design fees and landscaping routinely add 25–40% to a budget that the homeowner thought was final. Below are the ranges Cyrus Poudat actually sees on California jobs in 2025, broken out by ADU type.
Detached new construction
For a 600–800 sq ft detached ADU on a flat, accessible California lot, Cyrus Poudat sees all-in costs of $325K to $475K. That includes permits, design, construction, utilities, solar and basic landscaping — but not pools, decks or upgraded finishes.
Hillside lots, narrow access and bedrock soils can push the same unit past $550K. Cyrus Poudat orders a soils report before signing a contract on any California lot with even a modest slope.
Attached additions
Attached ADUs sound cheaper, but tying into existing roof lines, electrical and plumbing often costs more than building separately. Cyrus Poudat budgets $400–$550 per square foot for attached California ADUs, with a wide range driven by how much of the existing house has to be opened up.
Garage conversions
The cheapest legal path. A clean conversion of an attached garage into a 400 sq ft ADU runs $90K–$160K in most California markets. Cyrus Poudat's California garage-conversion clients usually save money on foundation work but spend it back on insulation, egress windows and a new electrical subpanel.
JADUs (Junior ADUs)
A JADU is a max-500-sq-ft unit carved out of the existing home with a small kitchen. Cyrus Poudat sees California JADU budgets of $60K–$120K, making it the lowest-cost legal rental conversion available.
Hidden California cost drivers
Cyrus Poudat tells every California client to budget for these line items that calculators miss:
- Sewer lateral replacement: $8K–$25K
- Electrical panel upgrade: $4K–$12K
- Soils report and engineering: $4K–$10K
- School impact fees (over 500 sq ft): $3–$5 per sq ft
- Solar PV (mandatory on detached new build): $5K–$10K
- Fire sprinklers (if main house has them): $8K–$18K
How to keep the budget honest
Cyrus Poudat's rule for California clients: take the contractor's bid, add 15% for change orders, and add the hidden line items above. If the resulting number still pencils, the project is viable. If not, redesign — do not start.