ADU Cost Calculator

About $169,500 turnkey nationally ($135,600–$228,825) with a full kitchen, one bath, separate utility connections and custom design — roughly $285 per square foot all-in. That runs above HomeGuide's published $100–$300/sq ft bands because those bands cover the build itself, while this figure also carries the kitchen, bath, utility connections, site work and architect-led design — the line items raw per-square-foot rules of thumb leave out. HomeGuide's 2026 figures put the 600 sq ft detached build alone at $90,000–$150,000; California coastal lots routinely land far above the national math.

Yes — the structure, foundation and roof already exist. A 400 sq ft (2-car) garage conversion prices at about $78,000 turnkey versus $131,500 for the same-sized detached build, roughly $53,500 less. Angi and HomeAdvisor both publish garage conversions at $20,000–$60,000; our figure is higher because a legal ADU needs a full kitchen, bath and code upgrades. The cheaper route still is a junior ADU inside the main home — a 400 sq ft JADU with a kitchenette prices at about $54,000 — at the cost of carving space out of the house rather than the garage. Two trade-offs to weigh before converting: you lose the parking, and some markets discount homes without a garage.

A JADU is a small unit carved out of an existing bedroom or space inside the main home — capped at 500 sq ft and allowed an efficiency kitchen, in some setups even sharing sanitation with the house. A 400 sq ft JADU with a kitchenette, its own bath, and stock plans prices at about $54,000 — the cheapest legal route to a second dwelling on most lots.

They're itemized lines, not buried in the per-square-foot rate: $15,000 for a full kitchen with range, fridge and dishwasher hookups, or $7,000 for a JADU-style kitchenette; $10,500 for the first full bath and $9,000 for a second. HomeAdvisor's baseline says a kitchen or bath addition "starts at about $5,000" — our figures cover mid-grade cabinets, fixtures and rough-in plumbing.

For a detached rental, usually yes — many cities require a separate water, sewer and electrical connection before issuing a certificate of occupancy. Extending the house's own systems (subpanel, shared water) runs about $2,500; fully separate connections about $10,000; new meters with a long sewer lateral up to $18,000. HomeAdvisor's line items put an electrical panel upgrade near $1,600 and a new sewer main near $3,500.

Budget $4,000 on the stock-plan route (drafted plans, survey, standard permits) or about $14,000 with an architect. HomeGuide's planning table lists house plans at $1,200–$5,000, a survey at $400–$1,800 and building permits at $500–$2,000. In California, state law bars local impact fees on ADUs under 750 sq ft; larger units and high-fee cities can add thousands beyond the permit line — that part is excluded here.

On the shell, yes: factory construction prices out at about 62% of the site-built shell, which saves roughly $37,320 on a 600 sq ft detached unit ($132,180 vs $169,500 turnkey) and cuts months off the schedule. HomeGuide publishes prefab at $80–$160/sq ft against $150–$250/sq ft site-built. The catch: delivery and crane set run a few thousand dollars, lot access must fit a truck and crane, and you give up most design flexibility. Conversions and JADUs can't use prefab at all.

Conversions run about 2–7 months per HomeAdvisor (HomeGuide pegs garage conversions at 2–12 months); detached builds with custom design and permits commonly stretch past a year. Not included in either number: city impact fees beyond the permit line, fire sprinklers where required, solar-readiness requirements, landscaping and fencing, furnishings, financing costs, and anything found during demolition.