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How much does it cost to build an ADU in the East Bay?
I do not build ADUs and make nothing when you do. Every figure below was checked against the billing agency's own published document, and where a...
Ask what an ADU costs and most websites hand you a construction range and stop. The real bill comes in three parts: what the city charges, what the water and sewer district charges, and what the builder charges. The first two are the ones people forget, and they swing the most. The same 500 sq ft detached ADU can owe almost nothing in government fees or more than $40,000, depending on the district and the build type. Here is each bill, itemized. Every figure was checked against the billing agency's own published document in August 2026, and the two rates about to rise carry their dates.
ADU money comes in three separate bills: the city's, the water district's, and the builder's. Most articles cover only the third, but the first two can differ by over $40,000 depending on your district and your build type.
I do not build ADUs and make nothing when you do. Every figure below was checked against the billing agency's own published document, and where a number could not be verified I say so.
Questions this guide helps you answer
- How much does it cost to build an ADU in the East Bay?
- What government fees apply to an ADU?
- Is a garage conversion cheaper than a detached ADU?
- What does ADU construction cost per square foot?
- How much will an ADU raise my property taxes?
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How many separate bills does an ADU involve?
Three, each collected by someone different. The city takes permit and plan-check fees, plus development impact fees if it charges them. The water and sewer district — often not your city, with boundaries that ignore city lines — takes connection and capacity charges. The builder takes design, construction, and materials.
Most blown budgets are not construction overruns. They are the first two bills never being counted at all.
How much does the 750 sq ft exemption actually save?
State law waives development impact fees on any ADU of 750 sq ft or less. That is real money — but it covers only one of the three bills.
The very next paragraph of the same law, section 66311.5(c)(2), says an impact fee "does not include any connection fee or capacity charge." So a 400 sq ft detached ADU can owe zero impact fees and still owe a full sewer capacity charge. Two agencies, two bills.
What do the districts charge to connect?
This is the least-written-about bill and the one that swings hardest. Find out which district bills your address first, then read the table.
| District | Conversion (existing meter) | New detached (new service) |
|---|---|---|
| EBMUD water + wastewater (Oakland, Berkeley, Alameda...) | $0 | $19,527 + $3,768 per new meter |
| City of Hayward | — | $5,295 sewer + $7,846 water (each rises ~10% Sept 1) |
| Union Sanitary (Fremont, Newark, Union City) | $0 | $6.46/sq ft (rises to $6.60 Oct 1) |
EBMUD bills per new water meter, so no new meter means neither charge. Union Sanitary's own ordinance exempts ADUs inside existing living area or expanding under 150 sq ft. DSRSD (Dublin) and Central San price by meter size and discharge, not square footage — ask them directly.
Which cities waive impact fees?
Five East Bay cities go beyond the state floor and waive their own impact fees at any size. This is money most homeowners never know they are not paying.
| City | Local impact fees | What it means for you |
|---|---|---|
| Fremont | None at any size, per the city's ADU handout | About $33,700 saved on a 2-bedroom; get it in writing for detached |
| Oakland | Affordable housing, transportation, capital improvement fees waived at any size | Ordinance 13853, effective Sept 13, 2025 |
| Berkeley | Land Use Planning assesses none | School fees still apply over 500 sq ft |
| San Leandro | Park and street improvement fees waived at any size | Broader than state law |
| City of Alameda | $0 in the adopted schedule, plus the 1% Improvement Tax waived | About $17,300 + 1% of valuation saved |
The state floor: any ADU of 750 sq ft or less pays no impact fees (Gov. Code 66311.5(c)(1)). The cities above go further. Permit and plan-check fees are separate everywhere.
Where does the garage-conversion discount really come from?
Not from the construction quote. Three requirements attach only to new detached construction.
Solar: since January 1, 2026, a new detached ADU needs photovoltaic panels; a conversion counts as an addition and does not. A separate utility connection: a unit built inside existing space cannot be required to install one; new construction can. Capacity charges: no new meter means none of EBMUD's $23,295 — the largest single number in this article.
So before any drawings, answer one question: is the existing meter enough? That answer is worth about twenty thousand dollars.
What does construction cost per square foot?
Honestly: there is no reliable per-square-foot number. The only independent survey, from Berkeley's Terner Center in 2021, found a statewide median of $150,000, roughly $250 per square foot. It is five years old and predates the Bay Area's recent cost inflation.
Builder quotes for the same East Bay garage conversion run from $110,000 to $280,000. That spread is the finding: do not budget off a website's number, get three bids and let them argue with each other.
Leave room for soft costs too: design and structural engineering commonly add $20,000 to $30,000.
What other costs get missed?
The sewer lateral. In Oakland, Alameda, and other East Bay cities, construction valued over $100,000 triggers a lateral compliance certificate — $407 itself, but replacing an old lateral routinely runs five figures. Almost every new detached build crosses that line.
School fees. Space over 500 sq ft pays about $5.38 per square foot (the cap the State Allocation Board set in January 2026), billed by the school district; no city can waive it.
Permit and plan-check fees. Every city charges these, usually a few thousand dollars, separate from impact fees.
How much will an ADU raise my property taxes?
Only by the new part. The Alameda County Assessor is explicit: just the newly built portion is assessed at market value, and your existing home keeps its Proposition 13 base year value. Completion triggers a one-time supplemental bill, prorated from the finish date.
No assessor publishes a formula for what an ADU adds. Anyone quoting you a precise number is guessing.
Adam's takeaway
Ask for the three bills separately: the city's written fee list, the district's written connection quote, and three comparable construction bids. Only with all three papers on the table is your budget real.
The biggest savings decisions are not finishes. They are two early questions: does your city waive impact fees, and is your existing water meter enough? Get both answers before the first drawing.
Every number on this page carries a date, and two rates rise within two months. Verify anything you plan to spend against before breaking ground.
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Send me your address. I will reply with which water and sewer district actually bills you, whether your city's rules are current or overridden by state law, and the fees that would realistically apply. No obligation, and you will know more than most counters will tell you.
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Sources and Credits
- 政府法典第66311.5条影响费豁免，及接入费、容量费被排除在定义之外。
- EBMUD 费率表（2026年7月1日生效）系统容量费19,527美元、污水容量费3,768美元（1英寸及以下住宅水表）。
- Union Sanitary District 容量费ADU每平方英尺费率及既有空间豁免。
- 海沃德市供水与下水道接入费ADU专项下水道费率及9月1日上调。
- Terner Center ADU业主调查（2021）唯一独立的造价数据，注明其年份。
- 阿拉米达县估值官：新建重估只有新建部分被重新估值。
