San Jose ADU cost ranges from about $100,000 for a compact junior ADU to more than $600,000 for a large detached unit with upgraded finishes. The main price drivers are unit size, conversion versus new construction, site access, utility work, and labor.
| Item | Low | Average | High | Notes |
|---|---|---|---|---|
| Garage conversion | $140,000 | $220,000 | $320,000 | Existing garage, about 400-600 sq ft |
| Detached ADU | $280,000 | $420,000 | $650,000 | New foundation and utility connections |
| Attached ADU | $220,000 | $340,000 | $500,000 | About 500-800 sq ft |
| Junior ADU | $80,000 | $130,000 | $190,000 | Created within the primary home |
Assumptions: San Jose-area construction, standard finishes, permitted work, and typical site access; land purchase is excluded.
What a San Jose ADU typically costs to build
For a new detached ADU, a practical planning range is $350-$650 per finished square foot, with many standard projects landing near $450-$550 per square foot. A 600-square-foot unit may therefore cost about $270,000-$390,000 before unusual grading, retaining walls, or premium finishes. Per-square-foot quotes can exclude design, permits, utility fees, and site work, so the total contract matters more than the headline rate.
Conversions often cost less because the shell already exists, but a garage may need foundation upgrades, insulation, plumbing, and a new electrical service. A basic garage conversion commonly totals $140,000-$320,000. A JADU generally costs less because it uses space inside the home and is limited to 500 square feet under California rules.
For budgeting, a standard 600-square-foot detached unit in San Jose often falls around $350,000-$500,000 including design and ordinary site work.
Where design, construction, and utility charges go
Construction is only one part of the estimate. Confirm whether the proposal includes architectural and engineering plans, permit processing, utility connections, appliances, landscaping, and final inspections. The ranges below are broad allowances, not guaranteed city charges.
| Quote component | Low | Typical | High | Basis |
|---|---|---|---|---|
| Design and engineering | $12,000 | $25,000 | $45,000 | Plans, structural and energy documents |
| Construction labor | $90,000 | $170,000 | $280,000 | Roughly 700-1,200 hours |
| Materials and fixtures | $70,000 | $125,000 | $210,000 | Standard to upgraded finish package |
| Permits and utility work | $8,000 | $25,000 | $60,000 | Fees, trenching, meters, service changes |
| Site work and contingency | $10,000 | $30,000 | $75,000 | Access, drainage, demolition, unknowns |
Labor rates vary by trade and contractor; a planning allowance of $75-$150 per hour is common for skilled Bay Area work, while a full crew cost is usually embedded in the project bid. Ask for an itemized quote that separates construction, design, fees, and utility work.
How square footage and site conditions change the quote
Size affects the total, but kitchens, bathrooms, and utility runs make small units expensive per square foot. Increasing a detached unit from 500 to 800 square feet can add roughly $100,000-$180,000 when it requires more foundation, framing, finishes, and mechanical capacity.
Site conditions can add another $15,000-$60,000 or more. A narrow side yard may require hand excavation or smaller equipment; a long sewer or water run can add thousands. Slopes, poor soil, mature trees, drainage changes, and electrical-panel upgrades also affect bids. A run longer than 50 feet, or a panel that cannot support the added load, deserves an early utility assessment.
Before comparing prices, make sure each builder has priced the same floor area, foundation, utility routes, and finish level.
How San Jose permit rules affect ADU expenses
San Jose projects need plan review and permits, and costs depend on the design, site, and required inspections. California generally prohibits impact fees for ADUs under 750 square feet; fees for larger units may be proportional to the primary dwelling’s size. This does not mean every permit, plan-review, school, or utility charge disappears, so buyers should request a written fee estimate from the city and utility providers.
Utility work is especially variable. A unit may share existing service where allowed, or need new electrical capacity, sewer connections, or separate metering. These choices can change the estimate by $5,000-$30,000 or more. Verify fees and connection requirements for the specific parcel before signing a fixed-price construction contract.
Three San Jose ADU quotes with different scopes
A 450-square-foot garage conversion with one bathroom, a compact kitchen, and standard finishes might price at $170,000-$250,000. The lower end assumes a sound slab, short utility runs, and no major electrical upgrade; the upper end allows for structural corrections and more extensive plumbing.
A 600-square-foot detached ADU with a new foundation and one bathroom may total $350,000-$500,000. A 750-square-foot two-bedroom unit with upgraded cabinets, difficult access, and longer trenching may reach $475,000-$650,000. These examples assume ordinary permitting and do not include land costs or financing.
Compare scope line by line: a lower quote may omit appliances, landscaping, permit coordination, or required utility upgrades.
How long ADU construction labor usually takes
Design and city review can take several months, and construction often takes about 6-10 months after permits are issued. A garage conversion may take 4-7 months of construction, while a detached unit commonly needs 7-10 months because it requires foundation, framing, and exterior work. Complex plan revisions or inspections can extend the schedule.
Crews may include a general contractor, framers, electricians, plumbers, roofers, and finish trades. Delays between trades or material orders can increase overhead or temporary housing costs. Ask bidders to state the assumed start date, construction duration, payment schedule, and how delays are handled.
Which scope choices can lower a San Jose ADU bill
Keeping the design compact and rectangular can reduce foundation, roof, and framing costs. Stacking the bathroom and kitchen near existing plumbing may limit trenching, while standard-size windows, doors, and cabinets usually cost less than custom selections. A conversion can be economical when the existing structure is sound, but replacing a deteriorated garage shell may make new construction more practical.
Obtain at least three comparable bids using the same plans and finish schedule. Complete a boundary or utility survey early, and avoid changing layouts after permit review begins. Keep a 10%-15% contingency for hidden conditions, especially for older garages or difficult yards. Preserve required safety and code work, and trim upgrades that do not improve the unit’s intended use.