ADU cost guide
How Much Does an ADU Cost in San Jose?
How much an ADU costs in San Jose depends first on its type, then on its size, utilities, site work and city fees; our ADU builders page gives the range: “ADU costs typically range from $150,000 to $350,000+ depending on size, type, and finishes. Garage conversions are generally the most affordable option.” This guide explains what moves a project within that range or past it.
The type sets how much is new construction and how much reuses walls, a roof and a slab you already own. For a price on your lot, call QA Builders at (669) 213-8077 or request a site visit.
ADU cost by type: junior ADU, garage conversion, attached or detached
| Type | What you pay for | What pushes the cost up or down |
|---|---|---|
| Junior ADU (JADU), up to 500 sq ft inside the house | Interior work, an efficiency kitchen and a separate entrance; no new structure and no separate electric meter | Sharing the house’s bathroom saves building one, but then the owner has to live on the property |
| Garage conversion ADU | Insulation, floor, kitchen, bathroom and utilities inside a shell that already exists | How far the kitchen and bathroom sit from the sewer line, and the condition of the slab |
| Attached ADU | A new foundation, walls and roof joined to the house, plus a full kitchen and bathroom | The tie-in to the existing roof and walls; a larger attached unit can bring fire sprinklers for the whole house |
| Detached ADU | A complete small house: foundation, framing, roof, kitchen, bathroom, and every utility run across the yard | The distance to the street, the sewer line and the electrical panel |
| Two-story ADU | Everything a detached unit needs, plus a stair and a second floor | More structure and engineering, in exchange for more space on a small yard |
The more of the unit that already exists, the less there is to build, which is why a detached unit, where every part is new, sits at the higher end. For the conversion route in detail, see our garage conversions page.
How size changes ADU cost, and why there is no per-square-foot price here
A bigger unit costs more to build, but not in proportion. A kitchen, a bathroom, the utility connections, design and permits are needed whatever the unit’s size, so a small ADU carries them over fewer square feet and its cost per square foot swings widely with size. That is why this guide quotes only the published range above and no per-foot figure: type, size and site work move an ADU’s price far more than its floor area alone.
Size also sets the city’s maximum and a fee line. Whether the City or the State standards suit your lot decides how large the unit may be (the limits are on our ADU builders page). School and parkland impact fees apply only to larger ADUs, so keeping the design under 750 square feet keeps them off the bill; the city’s own pages word the limit differently at exactly 750 square feet, so confirm it with the City before you fix the plan.
Bedroom and bathroom counts have no maximum, but each extra bathroom adds plumbing, fixtures and tile, so a two-bedroom, two-bath unit costs more than a one-bath unit of the same size.
Lot items that change an ADU estimate
These depend on your property, and they are where ADU budgets most often move after the first estimate:
- A new electric service. A full ADU needs its own PG&E meter, which can mean a panel upgrade and a trench; a JADU stays on the house’s service.
- Gas or no gas. San Jose currently allows either for a detached ADU; an all-electric design avoids running a gas line across the yard.
- Fire access. A lot that fails the city’s distance tests from the street or the nearest hydrant may need a Fire Variance, which carries its own fee and added safety measures.
- Fire sprinklers. Required only in some cases, but when they are, they apply to the ADU or, for some attached units, to the whole house.
- Hazard zones and old pools. A geologic hazard clearance, flood design or a geotechnical report and foundation design each add a consultant and a line to the budget.
- Trees. A protected tree removed for the ADU has to be replaced, which is a planting cost rather than a permit fee.
- Parking. State law bars ADU parking requirements within half a mile walking distance of public transit and for ADUs inside the house or an existing accessory structure, and garage spaces lost to an ADU never have to be replaced, so a new parking pad is often not part of the budget.
The rules behind each item are set out under fire access and utilities on our ADU builders page.
City fees and how long approval takes
The building permit fee is set by the ADU’s size under the city’s Building Fee Schedule, and San Jose’s Building Fee Estimator gives a general idea before you apply. Plan review fees are invoiced when the application is accepted; inspection fees, any impact fees and the remaining review fees are paid before the permit is issued.
The city says the first round of building review typically takes 20 business days when plans are complete and accurate. State law adds deadlines: a permitting agency must decide within 15 business days whether an ADU application is complete, and must approve or deny a complete application within 60 days when there is already a house on the lot.
Preapproved plans can lower both cost and wait. San Jose describes its preapproved ADU plans as the fastest and lowest-cost permit path; when the site-specific documents are accurate and complete, the permit may be issued on the day of the plan review meeting. They are open only to residential lots with an existing single-family home or duplex outside geohazard, landslide, flood and wildland-urban interface zones.
Rental payback and property tax
Our ADU page states: “A well-designed ADU in San Jose can generate $2,000-$4,000+ per month in rental income, often covering your mortgage.” and “ADUs can increase your property value by 20-30%, making them one of the best ROI home improvements.”
To judge your own payback, set the rent you expect against what the ADU adds every year: higher property tax (the city notes that taxes rise based on the value of the ADU), insurance, any utilities you pay for the tenant, upkeep and the weeks between tenants. State law requires an ADU to be rented for terms longer than 30 days (Government Code 66323(e)), so plan on monthly tenants rather than nightly guests. A smaller unit that stays clear of impact fees costs less to build, which shortens payback even if the rent is lower.
Savings, a home equity line of credit, a cash-out refinance and construction or renovation loans are the usual ways to pay for the build; your lender can say which suits you. For the reasons beyond money, read the benefits of building an ADU.
How to get an ADU quote you can rely on
- Complete the city’s ADU Universal Checklist. It confirms the main home is permitted, finds open code cases and lists your lot’s flood, geohazard, historic and fire designations.
- Choose City or State standards with your designer, since the choice sets size, height and siting.
- Request the water flow letter before plans are final.
- Ask each contractor to list design and engineering, Title 24 energy calculations, utility work to the meter and sewer, fire items, permit and impact fees, and the allowances for finishes.
- Agree who applies for the permit. The city highly recommends that the contractor sign the application, because whoever pulls the permit bears the liability for the construction work.
If you want more room for the household rather than a separate unit, compare the numbers with our home addition cost guide.
Build your ADU with a San Jose general contractor
QA Builders (CSLB License #1073096) builds detached ADUs, attached ADUs, two-story ADUs and garage conversions across San Jose and the Santa Clara, San Mateo and Alameda County cities we serve, including Sunnyvale, Mountain View, Los Gatos, East Palo Alto, Newark and Redwood City. We handle everything in-house (design, permits, construction and finishing) so you have one point of contact and one team accountable for your project’s success.
Learn how we build on our ADU builders page, then contact us for a free consultation and estimate at (669) 213-8077 or online through the contact page.