How much does a detached ADU cost in San Jose in 2026?

Permitted detached ADU construction in San Jose runs roughly $350 to $500 per square foot in 2026. A standard 750 square foot detached unit typically finishes at $300,000 to $450,000 including design, permits, and construction. Excalibur Builders (CSLB 1037968) quotes full projects at $375 to $575 per square foot depending on site conditions and finish level.

What does a garage conversion ADU cost in San Jose?

A full garage conversion ADU with kitchen and bathroom in San Jose typically runs $120,000 to $180,000 in 2026. A one-car garage conversion lands around $80,000 to $120,000 and a two-car garage around $120,000 to $150,000, before utility upgrades.

Do ADUs under 750 square feet pay impact fees in San Jose?

No. Under state law in effect since January 1, 2020, San Jose ADUs smaller than 750 square feet are exempt from parkland and school impact fees. Units of 750 square feet or larger pay school district developer fees, set at $5.38 per square foot statewide for Level 1 in 2026.

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ADU and addition cost in San Jose 2026: real numbers

A permitted 750 square foot detached ADU in San Jose finishes at $300,000 to $450,000 in 2026. Not the $150,000 the internet keeps promising. This page lays out the real ranges we see, the fee lines most bids skip, and exactly what we quote as a licensed General B contractor.

The 2026 cost table for San Jose

Project	Typical size	All-in range	What drives the spread
Detached ADU, studio	400 to 499 sq ft	$160,000 to $250,000	Foundation type, utility trench length, finish level
Detached ADU, 1 bedroom	600 to 749 sq ft	$220,000 to $370,000	Staying under 750 sq ft saves the impact fees
Detached ADU, 2 bedroom	800 to 1,000 sq ft	$280,000 to $450,000+	Second bath, school fees kick in at 750 sq ft
Garage conversion ADU	250 to 600 sq ft	$80,000 to $180,000	Slab condition, kitchen scope, panel capacity
Attached addition	Per sq ft	$300 to $600 per sq ft	Tie-in complexity, kitchen or bath count, roofline
Second-story addition	800 to 1,200 sq ft	$350,000 to $500,000+	Foundation reinforcement, full reroof, living elsewhere during the build

Ranges compiled from published 2026 San Jose and Bay Area builder pricing guides, cross-checked against our own signed proposals. Construction plus design and permits. City and utility fees are itemized separately below because most published numbers quietly leave them out.

Per square foot, permitted detached ADU work in San Jose prices at $350 to $500 in 2026. Bay Area labor runs 30 to 40 percent above the California average, and that gap never shows up in national cost articles. Second stories carry two extra loads that surprise owners. Most older San Jose homes need $10,000 to $30,000 in structural engineering and foundation reinforcement before the second floor can go on, and the house is unlivable for part of the build, which means $3,000 to $6,000 per month in temporary housing.

The costs nobody itemizes

These lines exist on every real project. A bid that never mentions them is not cheaper. It is incomplete.

City permit and plan check. Published 2026 guides put San Jose ADU building permit and plan check fees at roughly $3,500 to $6,000 for units under 750 sq ft and $6,000 to $14,000 above that. The city runs a public Building Fee Estimator at permits.sanjoseca.gov, and we run it on every proposal so the number is the city's, not a guess.
Impact fees, and the 750 sq ft line. Under state law in effect since January 1, 2020, San Jose ADUs smaller than 750 sq ft pay no parkland and no school impact fees. Cross that line and the fees return. Keeping a 1 bedroom at 749 sq ft instead of 800 is often worth $5,000 to $15,000.
School district developer fees. The statewide Level 1 fee was reset to $5.38 per square foot of new residential area on January 28, 2026. It applies to ADUs of 750 sq ft or more and to additions that add more than 500 sq ft of assessable space. On a 1,000 sq ft addition that is $5,380 payable before the permit issues.
PG&E service work. An ADU can trigger a panel upgrade, new service wires, or a second meter. In our experience the typical range runs from $0 when the existing panel has capacity to $30,000 or more when PG&E requires new service or transformer work, and the application itself can add 3 to 9 months. This is the single most under-scoped line in the bids we review.
Title 24 energy compliance. The report itself is cheap, $250 to $700 from most California providers. What it mandates is not. Heat pump equipment, insulation upgrades, and in some cases solar sizing flow from this document into your hard costs.
Sewer and water connection. Many San Jose ADUs can share the primary home's lateral. When a new or upsized lateral is required, the typical range we see is $5,000 to $15,000 in trenching and connection work, plus city sewer connection charges that depend on the parcel. We confirm this with the city before we price it, not after.

Why the internet's $150,000 number is wrong here

National cost articles average projects in Texas, Ohio, and rural California together and land near $150,000 for an ADU. San Jose is not that market. Permitted detached units here routinely finish between $300,000 and $450,000, driven by labor rates 30 to 40 percent above the state average, seismic and Title 24 requirements, and dense-lot logistics. The $150,000 figure is real somewhere. It is not real within 50 miles of this city, and any local bid built on it will fail in change orders.

What we quote, and what it includes

Excalibur proposals for ADU and addition work run $375 to $575 per square foot. That number covers the full path: design coordination, structural engineering, Title 24, permit submittal and plan check response, construction, and every inspection through final. City fees, school fees, and PG&E charges pass through at cost as their own line items, with the receipts attached.

Excluded, and stated as excluded in writing: furniture, landscaping beyond code-required work, and unforeseen conditions like an undocumented septic line, which get priced as a written change order before any work proceeds. You see the same 10-section breakdown the bank would see.

3 red flags in a low bid

No line for city fees, Title 24, or utilities. Those items total $15,000 to $40,000 on a typical detached ADU. A bid that omits them is understating the project by that amount, and you will pay it anyway, mid-build, as a surprise.
Allowances that cannot survive contact with a showroom. A $3,000 total finish allowance on a unit with a kitchen and a bathroom guarantees change orders. Real kitchens in this market start around $15,000 in materials alone.
PG&E never mentioned. If the bid says nothing about your panel, your service wires, or a second meter, nobody scoped the utility. That silence has cost owners we work with 6 months and $20,000. Verify any bidder at cslb.ca.gov before you sign. Our license is 1037968, checkable in 30 seconds.

Get a real number for your lot

Send the address and what you want to build. We run the city's own fee estimator, check your panel capacity question, and reply with an honest range, usually within one business day.

Request a cost consultation

Some owners land here already mid-project, stuck at plan check or in PG&E limbo with a budget that stopped making sense. That is a different service. Start at permit rescue.

Sources: City of San José Fees for ADUs and Building Fee Estimator (impact fee exemption under 750 sq ft), State Allocation Board Level 1 fee adjustment of January 28, 2026 ($5.38 per sq ft residential), and 2026 published San Jose and Bay Area builder pricing guides cross-checked against Excalibur signed proposals. Market ranges are estimates, not quotes. General information from a licensed general contractor, not legal or tax advice. CSLB 1037968.