An 800 square foot ADU runs $109,800–$300,000 all in, nationally, at a standard finish. Detached, which is what most 800 square foot projects are, prices around $240,000. In California that same build is closer to $436,800.
800 is the most requested size I see, and it is also the first size where California quietly starts charging you again. The construction cost of the fifty square feet between 750 and 800 is $15,000. The fee cost of those same fifty square feet can be several times that.
So this page prices the build, and then it prices the threshold, because at this specific size the second one is the decision.
800 sq ft ADU cost by type
Five ways to build 800 square feet, with a $93,600 spread between the cheapest and the dearest. Read the garage conversion row with one caveat, which I get into below.
| Build type | Base rate | Typical all-in | Range |
|---|---|---|---|
| Detached ADU | $300/sq ft | $240,000 | $180,000–$300,000 |
| Above-Garage ADU | $250/sq ft | $200,000 | $150,000–$250,000 |
| Attached ADU | $238/sq ft | $190,400 | $142,800–$238,000 |
| Garage Conversion ADUBeyond its typical size | $183/sq ft | $146,400 | $109,800–$183,000 |
| Junior ADU | Not available at this size. Capped at 500 sq ft of interior livable space by state law, so it cannot be built at this size. | ||
National, standard finish, before any city adjustment. Typical all-in is the point estimate; the range is the engine's residual spread for site conditions, utilities and contractor.
What Changes at 800 Square Feet
You crossed the 750 square foot fee line. Government Code section 66311.5(c)(1) exempts an ADU of 750 square feet or less of interior livable space from every impact fee. Subdivision (c)(2) covers what happens above it: any impact fee charged on an ADU over 750 square feet is charged proportionately in relation to the square footage of the primary dwelling. So an 800 square foot ADU next to a 2,000 square foot house pays 40 percent of the fee, not none of it.
Fifty square feet of construction is $15,000 on a detached build at a standard finish. Whether those fifty square feet are worth it depends entirely on your city's fee schedule, and this is the one number I would go and look up before you draw anything.
Campbell publishes its threshold cleanly enough to show the shape of the problem. Campbell's park in-lieu fee is $0 under 750 sq ft and jumps to $9,328 at 750 sq ft or larger. That single line is the difference between roughly $7,700 and $17,100 in combined city fees, so it is a design decision rather than a surprise bill. City of Campbell FY2026-27 Master Fee Schedule, effective July 1, 2026. Set that fee against the $15,000 of construction that the fifty square feet actually cost and the decision makes itself. Well, in Campbell it does. Your city's park fee may be smaller, or absent entirely, which is exactly why this is a question to ask rather than a rule to follow. Our Campbell cost page has the rest of that fee stack.
Cupertino did the same thing in its own code rather than leaving it to the state. Cupertino Municipal Code 19.112.020(C) charges no impact fees on an ADU or JADU under 750 sq ft, and charges fees proportionally to the primary dwelling at or above 750 sq ft. That restates the statewide Government Code Section 66324 exemption. We found no Cupertino-specific waiver beyond what state law already requires. Cupertino Municipal Code Chapter 19.112, as adopted by the September 2025 ordinance
800 is the size the state guarantees you. Section 66321(b)(3) requires local zoning standards to permit an ADU of at least 800 square feet of interior livable space with four foot side and rear setbacks. So the irony of this size is that it is the one California promises you can build and the first one it lets your city charge you for.
A garage conversion at 800 square feet is not a normal garage conversion. Our garage conversion guide publishes 300 to 650 square feet as the typical band, because that is a one or two car garage. Getting to 800 means a three car garage, or a conversion plus an addition, and an addition prices like the addition it is. Treat the $146,400 on the table above as the floor for a best-case existing structure.
If you want to see what this size actually looks like laid out, we have 4 original 800 sq ft floor plans drawn to scale with room dimensions, each with a walkthrough of what works and what to watch.
800 sq ft ADU cost by city
800 square feet through every city cost index we maintain, at a standard finish. Anything above about 1.8 on the index column is a market where the fee question above matters less than the construction one, simply because construction is so dominant.
| City | Cost index | Garage conversion | Detached | Full range |
|---|---|---|---|---|
| SaratogaCalifornia | 2.54 | $371,856 | $609,600 | $278,892–$762,000 |
| Los AltosCalifornia | 2.48 | $363,072 | $595,200 | $272,304–$744,000 |
| Los GatosCalifornia | 2.48 | $363,072 | $595,200 | $272,304–$744,000 |
| CupertinoCalifornia | 2.31 | $338,184 | $554,400 | $253,638–$693,000 |
| San FranciscoCalifornia | 2.31 | $338,184 | $554,400 | $253,638–$693,000 |
| Santa ClaraCalifornia | 2.31 | $338,184 | $554,400 | $253,638–$693,000 |
| SunnyvaleCalifornia | 2.31 | $338,184 | $554,400 | $253,638–$693,000 |
| San JoseCalifornia | 2.19 | $320,616 | $525,600 | $240,462–$657,000 |
| BellevueWashington | 2.08 | $304,512 | $499,200 | $228,384–$624,000 |
| SeattleWashington | 2.02 | $295,728 | $484,800 | $221,796–$606,000 |
| CampbellCalifornia | 1.99 | $291,336 | $477,600 | $218,502–$597,000 |
| BoulderColorado | 1.90 | $278,160 | $456,000 | $208,620–$570,000 |
| OaklandCalifornia | 1.85 | $270,840 | $444,000 | $203,130–$555,000 |
| DenverColorado | 1.79 | $262,056 | $429,600 | $196,542–$537,000 |
| San DiegoCalifornia | 1.73 | $253,272 | $415,200 | $189,954–$519,000 |
| Long BeachCalifornia | 1.67 | $244,488 | $400,800 | $183,366–$501,000 |
| AnaheimCalifornia | 1.56 | $228,384 | $374,400 | $171,288–$468,000 |
| Los AngelesCalifornia | 1.56 | $228,384 | $374,400 | $171,288–$468,000 |
| PortlandOregon | 1.56 | $228,384 | $374,400 | $171,288–$468,000 |
| EugeneOregon | 1.44 | $210,816 | $345,600 | $158,112–$432,000 |
| RiversideCalifornia | 1.44 | $210,816 | $345,600 | $158,112–$432,000 |
| SacramentoCalifornia | 1.44 | $210,816 | $345,600 | $158,112–$432,000 |
| BendOregon | 1.36 | $199,104 | $326,400 | $149,328–$408,000 |
| FresnoCalifornia | 1.27 | $185,928 | $304,800 | $139,446–$381,000 |
| TacomaWashington | 1.27 | $185,928 | $304,800 | $139,446–$381,000 |
| RenoNevada | 1.24 | $181,536 | $297,600 | $136,152–$372,000 |
| SalemOregon | 1.24 | $181,536 | $297,600 | $136,152–$372,000 |
| Colorado SpringsColorado | 1.18 | $172,752 | $283,200 | $129,564–$354,000 |
| HendersonNevada | 0.92 | $134,688 | $220,800 | $101,016–$276,000 |
| SpokaneWashington | 0.92 | $134,688 | $220,800 | $101,016–$276,000 |
| Las VegasNevada | 0.89 | $130,296 | $213,600 | $97,722–$267,000 |
Standard finish. The cost index is that city's published average against our national reference, so these columns move whenever a city cost page does. Full range spans every eligible build type at this size.
What finish level does to it
Finish level swings an 800 square foot detached budget by $100,800. Worth holding next to the fee conversation: your finish decision moves more money than the threshold does in most cities.
| Finish level | Multiplier | Detached ADU | Above-Garage ADU | Attached ADU | Garage Conversion ADU |
|---|---|---|---|---|---|
| Budget | 0.86x | $206,400 | $172,000 | $163,744 | $125,904 |
| Standard | 1.00x | $240,000 | $200,000 | $190,400 | $146,400 |
| High-end | 1.28x | $307,200 | $256,000 | $243,712 | $187,392 |
National point estimates at 800 sq ft. The junior ADU column, where it appears, is priced at its 500 sq ft statutory cap rather than at this page's size.
A worked budget
Where $240,000 goes on an 800 square foot detached build at a standard finish, nationally.
| Line item | Share | Amount |
|---|---|---|
| Site work and excavation | 8% | $19,200 |
| Foundation | 9% | $21,600 |
| Framing and structural shell | 15% | $36,000 |
| Roofing, siding, and windows | 11% | $26,400 |
| Mechanical, electrical, and plumbing | 13% | $31,200 |
| Utility connections | 7% | $16,800 |
| Interior finishes | 14% | $33,600 |
| Kitchen and bath | 9% | $21,600 |
| Design and engineering | 6% | $14,400 |
| Permits and fees | 5% | $12,000 |
| Contingency | 3% | $7,200 |
| Total | 100% | $240,000 |
Detached ADU, 800 sq ft, standard finish, national. $300 per sq ft all in, with a planning range of $180,000–$300,000.
Frequently asked questions
How Much Does an 800 Sq Ft ADU Cost?
Nationally, $109,800–$300,000 all in at a standard finish across the build types, with a detached unit at around $240,000. In California the same detached build prices near $436,800, and in the Bay Area markets on the city table it runs meaningfully higher. Those are planning ranges, not bids.
Do I Pay Impact Fees on an 800 Sq Ft ADU in California?
Yes, but only proportionally. Government Code section 66311.5(c)(1) exempts ADUs of 750 square feet or less, and subdivision (c)(2) requires that any impact fee on a larger unit be charged proportionately in relation to the square footage of the primary dwelling. An 800 square foot ADU beside a 2,000 square foot house therefore pays about 40 percent of the fee rather than the whole thing.
Is It Worth Dropping to 750 Sq Ft to Avoid the Fees?
Sometimes, and the math is local. The fifty square feet cost $15,000 to build on a detached unit at a standard finish, so the question is whether your city's impact fees at 800 exceed that. In a city with a large park in-lieu fee the answer is often yes; in a city that charges little or exempts ADUs outright it is no. Ask for both numbers before the footprint is fixed.
Can My City Refuse an 800 Sq Ft ADU?
Not in California. Government Code section 66321(b)(3) requires local development standards to permit an ADU with at least 800 square feet of interior livable space and four foot side and rear yard setbacks, so 800 is a floor a local ordinance cannot go below. Height, design and lot coverage rules still apply around it.
How Many Bedrooms Fit in 800 Square Feet?
Two, comfortably, which is why this size is so heavily searched. 800 square feet supports two real bedrooms and one bath, or one bedroom with a genuinely large living space. Our two bedroom ADU cost page prices the whole 700 to 1,000 square foot band that two bedroom layouts fall into.
If I were designing at 800 today in California, I would ask the planning counter for the impact fee number at 800 and at 750 before I let the architect settle the footprint. Not because 750 is always right, but because that is a $15,000 construction decision carrying a fee decision that nobody tells you about.
If the fee comes back small, build the 800. It rents better. If it comes back like Campbell's, look hard at what 600 square feet costs and at whether the layout really needs the extra room.
Price a different size
The same five build types and the same engine, priced at every size we cover.