Published: 3 Oct 2026 · Last verified: 3 Oct 2026 · Next review: Jan 2027 Cost figures are re-checked against permit records quarterly. Changes are logged in Corrections.
Every other post on this site is anchored to Southern California. This one is not, and the Bay Area changes which line deserves your attention.
In San Diego County the swing was the permit bill. In the Bay Area, the building permit is often the smallest fee on the invoice. The swing is the stack of school, park, and impact fees that switches on above 750 square feet — and which city you are in decides whether that stack exists at all.
San José’s building permit for an 800 sq ft ADU is $5,103 (FY 2026-27 fee schedule). Fifteen miles up the road, Palo Alto charges an 800 sq ft ADU about $18,000 in development impact fees that a 749 sq ft unit does not pay (Palo Alto FY27 impact fee sheet; our arithmetic). Oakland exempts ADUs from its impact fees entirely (Ordinance 13853 C.M.S.).
So this post does two things. First, the standard six-phase breakdown for an 800 sq ft detached ADU in San José — about $400,000, or $500 per square foot (our estimate). Then the part nobody publishes: what that same plan owes in fees in five Bay Area jurisdictions, and what happens at 750.
The short answer
| 800 sq ft detached ADU · San José · 2026 | Amount |
|---|---|
| Total build cost (our estimate) | ≈ $400,000 |
| Cost per square foot (our estimate) | ≈ $500 |
| Largest single line | Systems + interior, ≈ $184,000 |
| City building permit, plan review, inspection | $5,103 |
| Fees and taxes we could source at 800 sq ft | ≈ $17,000 – $24,500 |
| Same permit at 750 sq ft | $3,291, and no school or parkland fee per the city checklist. School-fee thresholds differ by school district — each district administers its own fees under Ed. Code §17620. |
The full line-item breakdown
Six phases. The same six used on every build in this series. All dollar rows are our estimate for San José, mid-grade finishes, a flat lot with reasonable access.
| Phase | Cost | Share |
|---|---|---|
| Soft costs (design, engineering, permits, taxes, school and park fees) | $40,000 | 10% |
| Site work + foundation | $48,000 | 12% |
| Framing + structure | $60,000 | 15% |
| Building envelope | $52,000 | 13% |
| Systems + interior | $184,000 | 46% |
| Landscaping & hardscape | $16,000 | 4% |
| Total | $400,000 | $500 / sq ft |
The shape is the same as Los Angeles and San Diego: systems and interior is 46% again. One kitchen, one bathroom, one heat pump, one electrical service. What moves in the Bay Area is the rate on every line, not the structure of the spend.
Inside that $40,000 soft-cost line, here is what we could tie to an official San José source for an 800 sq ft unit:
| San José fee or tax · 800 sq ft | Amount | Source |
|---|---|---|
| Permit issuance + plan review + inspection (751–1,200 sq ft tier) | $633 + $1,950 + $2,520 = $5,103 | FY 2026-27 fee schedule |
| Citywide Planning Fee, 13% “of application initial plan review and/or permit fees” | $336 – $663 depending on which fees it is applied to | Fee schedule |
| Building & Structure tax 1.54% + CRMP tax 2.42% of valuation, at the city’s R-3 value of $175.92/sq ft | $5,573 | Development Construction Taxes · fee schedule p. 24 |
| School fee, San José Unified, $5.38/sq ft | $4,304 | SJUSD developer fee |
| Parkland fee for ADUs of 750 sq ft or more, by MLS area | $1,700 – $8,900 | City ADU parkland fee map |
| Subtotal | ≈ $17,000 – $24,500 | our arithmetic |
The rest of the $40,000 is design, structural engineering, Title 24 energy calculations, survey and utility coordination — our estimate, not a city figure.
Three more San José line items appear in the fee schedule, but we could not confirm how they apply to an ADU. Not confirmed — check with the city: the per-unit Construction Tax ($150) and Residential Construction Tax ($180) for a one-family unit; the $780 Sewage Treatment Plant Connection Fee for a single-family unit (state law limits connection fees on ADUs, Gov. Code §66311.5(b), (e)); and whether a declared construction value higher than the city’s $175.92/sq ft table raises the tax base. The city’s ADU fee estimator (beta) or the ADU Ally (408-793-5302) can answer for your parcel.
Where the Bay Area differs from Los Angeles and San Diego
| LA County | City of San Diego | San José (our estimate) | |
|---|---|---|---|
| 800 sq ft detached total | $320,000 | $344,000 | $400,000 |
| Per sq ft | $400 | $430 | $500 |
| Systems + interior share | 46% | 46% | 46% |
| Where the fee swing lives | Region | City permit bill | Impact, school, and park fees above 750 sq ft |
Why $500 per square foot, and why it is labelled an estimate. No free official source publishes a turnkey San José ADU cost. The closest public benchmark is the San Mateo County ADU Resource Center, funded by the county’s cities, which gives “a very rough placeholder” of $450 to $600 per square foot for hard plus soft costs. San Francisco’s Department of Building Inspection says adding a unit “typically costs at least $125,000 in material and labor,” with architects and engineers at about 10% of construction costs and city fees at 6–9% (SF.gov). We put San José in the lower half of the San Mateo range — Santa Clara County trade rates and lot access run slightly below the Peninsula — and about 16% above our San Diego median. For context only, UC Berkeley’s Terner Center found a statewide median of $250 per square foot in its 2021 ADU owner survey, which predates recent cost inflation and includes conversions (Terner Center).
San José’s own $175.92 per square foot (fee schedule) is a tax-valuation table value, not a construction price. Do not budget from it.
The part nobody publishes: one plan, five jurisdictions
Here is what an 800 sq ft detached ADU owes in fees in five Bay Area jurisdictions. Where a fee is a percentage of value, we apply it to $360,000 — our hard-cost estimate (the $400,000 total minus the soft-cost line) — and label it as our arithmetic.
| Jurisdiction | Building permit & plan review | Impact / park fees at 800 sq ft | School fee at 800 sq ft | Free or pre-approved plans |
|---|---|---|---|---|
| San José | $5,103 + 13% planning fee + taxes ≈ $5,573 | Parkland $1,700 – $8,900 | SJUSD $5.38/sq ft = $4,304 | Vendor plans, same-day permit possible |
| Santa Clara County (unincorporated) | ≈ $15,076 at $360k valuation; ≈ $11,270 with a free County plan (FY 2026-27 schedule; our arithmetic) | Not confirmed — check with the County | Depends on district | Free 800 & 1,200 sq ft plans |
| Palo Alto | $2,000 – $8,000 (city guidebook range) | ≈ $18,090 with a 2,000 sq ft house (FY27 rates; our arithmetic) | PAUSD $4.79/sq ft = $3,832 | — |
| Oakland | “Usually 4–6% of project value” (city) ≈ $14,400 – $21,600 on this table’s $360,000 hard-cost convention (our arithmetic); Oakland’s standalone post applies the same rule to its $322,000 modeled total → $12,800 – $19,200 | $0 — ADUs exempt (Ord. 13853) | OUSD $5.17/sq ft = $4,136 | City studio, 1-BR, 2-BR plans |
| San Francisco | City fees “6–9% of construction costs” (SF.gov) | $0 for State-program ADUs on lots with 3 or fewer existing units (Bulletin 3) | SFUSD $3.79/sq ft = $3,032 | — |
The pattern: the cities with the cheapest permits are not the cheapest places to build, and vice versa. San José’s building permit is a flat, low $5,103. Oakland and San Francisco charge a percentage of value that can run three or four times higher — but then exempt the ADU from impact fees altogether. Palo Alto’s permit is modest and its impact fees are the largest number in the table.
All figures checked 3 Oct 2026. Fee schedules change every fiscal year; treat this as a shape, not a quote.
San José: a cheap permit, then taxes and the 751 sq ft switch
San José prices ADU permits by size tier, not valuation: $3,291 up to 750 sq ft, $5,103 from 751 to 1,200 sq ft. Modular units get 10% off plan review and 20% off inspection (fee schedule). The city’s ADU checklist, Bulletin #210 (updated 16 Sep 2026), states it plainly: “750 square feet or less: Impact fees do not apply. 751 square feet or larger: School and parkland impact fees apply,” and the permit is not issued until they are paid.
The parkland fee is a flat amount per ADU by real-estate MLS area, from $1,700 (area 7A) to $8,900 (area 7B), on a city map titled “Parkland Impact Fees for Accessory Dwelling Units 750 sf or Greater” (City of San José). The map says it is based on the city’s 2017/2018 fee adjustments. Not confirmed — check with the city whether it is still the amount charged today; it is still linked from the city’s current fee page.
Boundary note: the city’s map says “750 sf or Greater,” but state law bars impact fees on ADUs of 750 sq ft or less (Gov. Code §66311.5) — at exactly 750 sq ft the two conflict, and this page follows state law.
The construction taxes are the line most ADU cost guides skip. They are excise taxes, collected at permit issuance, at 1.54% plus 2.42% of building valuation (City of San José). The state’s 750 sq ft exemption covers “impact fees” as defined in Gov. Code §66000(b) — “a monetary exaction other than a tax” — so in our reading it does not reach these taxes, or plan-review fees, at any size.
Unincorporated Santa Clara County: free 800 sq ft plans
If your parcel is outside city limits, the County offers free pre-approved plans for an 800 sq ft 2-bed/1-bath and a 1,200 sq ft 3-bed/2-bath ADU, in traditional or contemporary style (County of Santa Clara). Use one unmodified and plan review is billed at the 2-hour minimum. Not available in Wildland-Urban Interface, flood, or geologic hazard zones, or on historic properties.
County building fees are valuation-based. Under the FY 2026-27 schedule (effective 18 Jul 2026), a $100,001–$500,000 project pays $3,698.51 plus $11.30 per additional $1,000; residential plan check is 70% of that; and there is a 3% technology fee, a 0.73% training fee, a 0.205%-of-valuation planning fee, and a $2,808 planning review for residential work of 500 sq ft or more. At a $360,000 valuation that is about $15,076. With the free plan, plan review drops to 2 hours at $420, and the total to about $11,270 (our arithmetic). Fire and school fees are paid separately.
Palo Alto: the steepest 750 cliff in this post
Palo Alto exempts ADUs under 750 sq ft from its development impact fees. At 750 sq ft and above, the fee “is proportional to the size of the primary unit” (FY27 impact fee sheet, as of 17 Aug 2026). The single-family rates are steep: parks $18.75/sq ft, community centers $1.44/sq ft, libraries $0.86/sq ft, plus public safety $1,383 and general government $1,743 per unit.
On an 800 sq ft ADU behind a 2,000 sq ft house, by our arithmetic: $21.05 × 800 = $16,840 in per-square-foot fees, plus 40% of the two per-unit fees, $1,250 — about $18,090. A 749 sq ft unit pays none of it. Confirm the city’s exact proration method before you rely on this number. ADUs are exempt from Palo Alto’s citywide traffic fee and housing in-lieu fee at any size.
The city’s 2025 ADU guidebook puts ADU building permit fees at $2,000 to $8,000 and development impact fees at $10,000 to $80,000.
Oakland: percentage-of-value permits, no impact fees
Oakland says building permit fees are “usually 4–6% of project value” (City of Oakland). On a $360,000 build that is $14,400 to $21,600 — far above San José’s flat $5,103.
But Oakland’s July 2025 impact-fee rewrite kept ADUs on the exempt list for both the Affordable Housing Impact Fee and the Transportation and Capital Improvements fees, with no size limit (Ordinance 13853 C.M.S., adopted 15 Jul 2025; ordinance text, OMC 15.72.040(C)(1) and 15.74.040(C)(3)). The OUSD school fee, $5.17/sq ft, is collected by the city on the district’s behalf.
San Francisco: no impact fees on most backyard ADUs
San Francisco’s Planning Director Bulletin No. 3 (updated June 2026) exempts State-program ADUs from impact fees if they are under 750 sq ft or on lots with 3 or fewer existing units. A detached ADU behind a single-family home therefore skips city impact fees even at 800 sq ft or more. City fees still run 6–9% of construction cost (SF.gov), and SFUSD charges $3.79/sq ft of increased habitable floor area (SF Planning impact fee register, rates as of 1 Jan 2026).
The 750 square foot cliff
Under Gov. Code §66311.5(c)(1), no local agency may impose an impact fee on an ADU with 750 square feet of interior livable space or less. Above that, impact fees must be proportional to the primary dwelling. Every jurisdiction above applies some version of that line — and in the Bay Area it is worth more than anywhere else in this series.
Here is the last 50 square feet in San José, taking the city’s checklist at its word:
| From 750 to 800 sq ft · San José | Cost |
|---|---|
| Construction, 50 sq ft at $500 (our estimate) | $25,000 |
| Permit tier jump, $3,291 → $5,103 | $1,812 |
| Parkland fee, newly triggered | $1,700 – $8,900 |
| SJUSD school fee, newly triggered per city checklist | $4,304 |
| Extra construction tax on 50 sq ft | $348 |
| Marginal cost of 50 sq ft | $33,164 – $40,364 |
| Effective rate on that space | $663 – $807 / sq ft |
Those 50 square feet cost 33% to 61% more per foot than every square foot before them (our arithmetic). In Palo Alto the same step triggers about $18,000 of impact fees on its own.
The market has noticed. Of the seven plans on San José’s preapproved ADU list on 3 Oct 2026, three are 747, 748, and 749 sq ft.
Watch the wording at exactly 750. State law exempts 750 sq ft and under. San José’s checklist agrees (“750 square feet or less”). But the city’s older parkland map says ADUs “under 750 sf” are exempt, and Palo Alto’s fee sheet says “less than 750 sq. ft.” exempt and “750 sq. ft. and larger” pay. Design at 749 or be ready to cite the statute.
That does not make 750 the right answer for every lot. It means crossing it should be a decision made with the fee schedule open, not a rounding choice on the drawings. More on how impact fees work in ADU soft costs and permit fees.
School fees: the 500 sq ft rule San José’s own pages read differently
School fees follow a separate line. Under Ed. Code §17620 and Gov. Code §66311.5(c)(3), an ADU under 500 sq ft cannot be charged; above 500 sq ft, school districts may charge it. The HCD ADU Handbook reads it the same way.
San José’s pages draw the line at 750 instead. The ADU FAQ says school impact fees “do not apply to ADUs less than 750 sq. ft.,” and Bulletin #210 says school fees apply from 751 sq ft. Palo Alto, next door, says school fees apply to new units “including J/ADU” of 500 square feet or more (City of Palo Alto).
The practical point: the school district, not the city, levies and collects this fee. In San José Unified you take the city’s School Fees Form to the district office and pay there (SJUSD). A 600 or 700 sq ft ADU may still get a district invoice, roughly $3,200 to $3,800 at SJUSD’s $5.38 rate. San José is also split across several school districts, so check which one serves your parcel. Ask the district in writing before you lock the size.
Pre-approved and free plans
San José. Vendor-designed plans reviewed by the city, with a dedicated review team and same-day permit issuance possible at the plan review meeting if the site-specific documents are complete. Not available on lots in geohazard, landslide, flood, or Wildland-Urban Interface zones (City of San José). Pricing is set by each vendor.
Unincorporated Santa Clara County. Free 800 and 1,200 sq ft plans, about four weeks for first review (County).
Oakland. City studio, one-bedroom, and two-bedroom plans with a “reduced permits fee”; once everything is complete and paid, the building permit is issued within 10 business days (City of Oakland).
In every case you still pay for a site plan, Title 24 energy calculations, and usually truss engineering. Check the plan library before you hire a designer, not after.
What moves the number most in the Bay Area
1. Which side of 750. Covered above. In San José and Palo Alto it is the largest fee variable you control.
2. Which jurisdiction — and which school district. Flat-tier permits (San José) versus percentage-of-value permits (Oakland, San Francisco) versus large impact fees (Palo Alto).
3. Fire access. San José checks whether the farthest side of the ADU is within 150 feet of the curb along a 4-foot clear path, within 600 feet of a hydrant, and whether the hydrant delivers 1,000 gpm at 20 psi. Miss one and you may need a fire variance and extra safety measures; if the main house is sprinklered, the ADU must be too (Bulletin #210). The water company flow letter is, per the city, a common cause of permit delays.
4. Hazard designations. Flood, geohazard, liquefaction, landslide, and Wildland-Urban Interface zones add design requirements or clearances, and knock you out of the preapproved fast tracks (Bulletin #210).
5. Finish grade. $15,000 to $30,000 across a full unit between builder-grade and mid-grade (our estimate).
What this number does not include
| Not included in $400,000 | Typical impact |
|---|---|
| Construction loan interest | Carried 9 to 14 months |
| Property tax reassessment | Only the new construction is added |
| Vacancy before first tenant | 1 to 3 months is common |
| Property management | 8% to 10% of rent |
| Contingency | 10% to 15% — budget it, don’t hope |
A 12% contingency on this build is $48,000. One small line San José adds: credit card payments for permit fees carry a 2.66% third-party surcharge; eCheck avoids it (City of San José).
→ Full year-one ownership cost: ADU all-in cost in California
Put your own quote in these six rows
We made a blank version of the six-phase ledger. Drop your contractor’s numbers into it and see which line doesn’t match ours.
Download the worksheet — CSV (import to Google Sheets), no email required
Compare any two builds side by side →
Want the regional benchmark ranges for each phase, updated quarterly?
Methodology and sources
The six-phase dollar figures on this page are our estimates, not quotes. The fee figures are each jurisdiction’s posted rates, checked 3 Oct 2026, with our arithmetic shown where we combined them.
| Input | Source |
|---|---|
| 750 sq ft impact-fee exemption, school-fee 500 sq ft rule, ADU connection fees | Gov. Code §66311.5 (as renumbered by SB 543, eff. 1 Jan 2026) |
| Definition of “fee” (excludes taxes) | Gov. Code §66000 |
| School fees on residential construction | Education Code §17620 · HCD ADU Handbook |
| San José permit fees, planning fee, development taxes, valuation table | FY 2026-27 Building & Structure Permits Fee Schedule (eff. 10 Aug 2026) · Development Construction Taxes |
| San José ADU impact-fee threshold, fire access, hazard checks | Bulletin #210 ADU Universal Checklist (16 Sep 2026) · ADU FAQs |
| San José ADU parkland fees | Parkland Impact Fees for ADUs map |
| San José preapproved plans | Preapproved ADUs |
| San José Unified school fee | SJUSD Developer Fee |
| Santa Clara County building fees | FY 2026-27 Building Fee Schedule (eff. 18 Jul 2026) |
| Santa Clara County free plans | Pre-Approved ADU Program |
| Palo Alto impact fees and exemptions | FY27 Development Impact Fee sheet (17 Aug 2026) |
| Palo Alto permit fee range | Palo Alto ADU Guidebook 2025 |
| Palo Alto school fees | City school fee page · PAUSD School Impact Fees |
| Oakland permit-fee range, OUSD school fee | City of Oakland Impact Fees |
| Oakland ADU impact-fee exemption | Ordinance 13853 C.M.S. (15 Jul 2025) · ordinance text |
| Oakland pre-approved plans | City of Oakland |
| San Francisco ADU impact fees, fees as % of cost, SFUSD rate | Planning Director Bulletin No. 3 · SF.gov ADU permit page · SF impact fee register (1 Jan 2026) |
| Bay Area $/sq ft placeholder | San Mateo County ADU Resource Center |
| Statewide ADU cost context (2021) | Terner Center ADU owner survey |
| Phase-level cost shares for residential construction | NAHB Cost of Construction Survey |
| Per-square-foot construction price benchmarks | NAHB Survey of Construction |
On the five-jurisdiction comparison. Flat fees are quoted as published. Percentage-based fees are applied to a $360,000 valuation (our hard-cost estimate) so the rows can be compared; your declared valuation will differ. Palo Alto’s impact fees assume a 2,000 sq ft primary house. Pull the current schedule for your own jurisdiction before you budget.
Not confirmed — check with the city
- San José: which fees the 13% Citywide Planning Fee is applied to on an ADU permit; whether the $150 Construction Tax and $180 Residential Construction Tax apply to an ADU as a one-family unit; whether the $780 sewage treatment plant connection fee is charged on a detached ADU; whether the 2017/2018-based ADU parkland map is still the amount charged; whether a declared valuation above the city’s table raises the construction-tax base.
- Santa Clara County: county impact fees and fire marshal fees on an ADU.
- Palo Alto: the exact proration of per-square-foot impact fees, and the building permit fee for your specific valuation.
Check this number against your own address
- San José: look up your parcel on SJPermits.org for MLS area, school district, and hazard zones, then run the city’s ADU fee estimator from the Building Permit Fees page.
- Note the declared valuations on recent ADU permits near you.
- Compare them to the phase totals on this page.
Declared valuations run below contract price — treat them as a floor, not a total. Full detail on Methodology.
If your local numbers diverge meaningfully from ours, we want to know. See Corrections.
Our position
We don’t build ADUs. We don’t sell plans. We don’t sell your information to contractors. No figure on this site has been paid for or reviewed by anyone whose costs we publish. More on About.
Figures are Santa Clara County and Bay Area, 2026. Build costs are our estimates; fee figures are posted rates as of 3 Oct 2026. Costs vary by jurisdiction, site access, and finish level. This is construction cost information — not a quote, and not legal, engineering, or financial advice. Confirm requirements with your local building department, school district, and a licensed California contractor.
Related in this series
| If you want | Go to |
|---|---|
| The Los Angeles County hub | LA County ADU cost |
| Same 800 sq ft build in San Diego County | San Diego ADU cost |
| How permits, design, and impact fees add up | Soft costs |
| Check a contractor quote line by line | Compare |
Frequently asked questions
How much does an ADU cost in San Jose in 2026?
Our estimate is about $400,000 for an 800 sq ft detached, single-story, new-construction unit with mid-grade finishes — roughly $500 per square foot. Across the Bay Area we estimate $450 to $650 per square foot turnkey, in line with the San Mateo County ADU Resource Center’s $450 to $600 per square foot planning placeholder for hard plus soft costs.
How much are ADU permit fees in San Jose?
Under the FY 2026-27 schedule (effective 10 Aug 2026), building permit issuance, plan review, and inspection total $3,291 for an ADU of 0 to 750 sq ft and $5,103 for 751 to 1,200 sq ft. On top of that come a 13% Citywide Planning Fee, construction taxes of 3.96% of building valuation, and — above 750 sq ft — school and parkland fees. On an 800 sq ft unit the fees and taxes we could source add up to roughly $17,000 to $24,500.
Do ADUs pay San Jose construction taxes?
Very likely, at any size. The state’s 750 sq ft impact-fee exemption uses the Mitigation Fee Act definition of a fee, which excludes taxes and plan-review fees, and San Jose’s ADU process page says the residual plan review and inspection invoices include taxes. At the city’s own valuation of $175.92 per square foot, the two percentage taxes come to about $5,573 on 800 sq ft. Confirm the exact amount with the city before you budget.
Should I keep my ADU at 750 square feet or less in the Bay Area?
Model both. In San Jose, going from 750 to 800 sq ft adds a higher permit tier, school fees, and a parkland fee of $1,700 to $8,900, so those last 50 sq ft cost roughly $660 to $810 per foot by our arithmetic. In Palo Alto, about $18,000 of development impact fees switches on at 750 sq ft. In Oakland and in San Francisco’s State ADU program on small lots, ADUs are exempt from city impact fees regardless of size.
Do I pay school fees on a 600 sq ft ADU in San Jose?
Possibly. San Jose’s ADU checklist says school and parkland fees apply only from 751 sq ft, but state law lets school districts charge ADUs of 500 sq ft or more, and the district — not the city — levies the fee. Palo Alto’s own fee page applies school fees from 500 sq ft. Ask your school district in writing before you finalize the size.
Are there free ADU plans in Santa Clara County?
Yes, for unincorporated parcels. The County offers free pre-approved 800 sq ft and 1,200 sq ft plans; used without modification, plan review is billed at the 2-hour minimum ($840 at the FY 2026-27 rate) instead of 70% of the building permit fee. Inside San Jose city limits, preapproved plans come from private vendors and can get a same-day permit if the site documents are complete.