A 500 sq ft ADU costs $49,875–$187,500 all in at national prices, per our cost engine (estimate, standard finish): about $66,500 as a junior ADU, $91,500 as a garage conversion and $150,000 as a detached unit. It sits exactly on California's 500 sq ft junior ADU cap, so it is the one size where all five ADU types compete.
A 500 square foot ADU runs $49,875–$187,500 all in, nationally, at a standard finish. A junior ADU inside the house is the bottom at $66,500; a detached unit is around $150,000, and in California closer to $273,000.
Five hundred square feet is the size where the most options overlap. It is the largest junior ADU the law allows, a comfortable one bedroom, a roomy studio, and a size a two-car garage converts into without an addition.
500 sq ft ADU cost by type
Every build type at 500 square feet, with a $83,500 spread between the cheapest and the dearest. The junior ADU row is priced right at its cap.
| Build type | Base rate | Typical all-in | Range |
|---|---|---|---|
| Detached ADU | $300/sq ft | $150,000 | $112,500–$187,500 |
| Above-Garage ADU | $250/sq ft | $125,000 | $93,750–$156,250 |
| Attached ADU | $238/sq ft | $119,000 | $89,250–$148,750 |
| Garage Conversion ADU | $183/sq ft | $91,500 | $68,625–$114,375 |
| Junior ADU | $133/sq ft | $66,500 | $49,875–$83,125 |
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 500 Square Feet
This is the junior ADU ceiling. California defines a junior ADU as no more than 500 square feet of interior livable space, contained entirely within a single-family residence. So a 500 square foot unit inside your house can be a JADU, with no foundation and no new utility connection, and one square foot more cannot. That is a statutory cap, not a local one. Our junior ADU guide covers the rules that come with it.
Studio or one bedroom? At 500 square feet both work, and the drawings show the trade plainly. The studio keeps one big room; the one bedrooms spend a wall to get a door you can close:
| Layout | Footprint | Bed / bath | Living room | Bedroom |
|---|---|---|---|---|
| 500 Sq Ft: Studio, 1 Bath (Layout C) | 20 by 25 ft | Studio, 1 Bath | 340 sq ft | Shares the living space |
| 500 Sq Ft: 1 Bed, 1 Bath (Layout A) | 20 by 25 ft | 1 Bed, 1 Bath | 156 sq ft | 132 sq ft |
| 500 Sq Ft: 1 Bed, 1 Bath (Layout B) | 25 by 20 ft | 1 Bed, 1 Bath | 225 sq ft | 132 sq ft |
All three are on our 500 sq ft ADU floor plans page, drawn to scale with room dimensions. Could you fit two bedrooms? On paper, with two rooms near the 70 square foot habitable minimum, but I would not; our smallest two bedroom plan starts at 600 square feet.
Cost per square foot is still climbing. Our engine prices a 500 square foot detached unit at $150,000, or $300 a foot, with every line scaled to size. The kitchen, bath, utilities, permits and design are 27% of that budget and barely shrink with the unit; held at their 800 square foot cost, the same build comes to $174,300, or $349 a foot. Real bids land between the two.
Impact fees do not apply in California. Section 66311.5(c)(1) bars impact fees on an ADU of 750 square feet or less, so a 500 square foot unit owes none.
Prefab is competitive here. Our California cost guide's prefab ranges put a 500 square foot unit at about $167,000–$266,000 installed (interpolated between its 400 and 650 square foot rows). My breakdown of prefab ADU cost installed shows which part of that is the factory unit.
If you want to see what this size actually looks like laid out, we have 3 original 500 sq ft floor plans drawn to scale with room dimensions, each with a walkthrough of what works and what to watch.
500 sq ft ADU cost by city
The same 500 square feet through every city cost index we maintain, at a standard finish.
| City | Cost index | Garage conversion | Detached | Full range |
|---|---|---|---|---|
| SaratogaCalifornia | 2.54 | $232,410 | $381,000 | $126,683–$476,250 |
| Los AltosCalifornia | 2.48 | $226,920 | $372,000 | $123,690–$465,000 |
| Los GatosCalifornia | 2.48 | $226,920 | $372,000 | $123,690–$465,000 |
| CupertinoCalifornia | 2.31 | $211,365 | $346,500 | $115,211–$433,125 |
| San FranciscoCalifornia | 2.31 | $211,365 | $346,500 | $115,211–$433,125 |
| Santa ClaraCalifornia | 2.31 | $211,365 | $346,500 | $115,211–$433,125 |
| SunnyvaleCalifornia | 2.31 | $211,365 | $346,500 | $115,211–$433,125 |
| San JoseCalifornia | 2.19 | $200,385 | $328,500 | $109,226–$410,625 |
| BellevueWashington | 2.08 | $190,320 | $312,000 | $103,740–$390,000 |
| SeattleWashington | 2.02 | $184,830 | $303,000 | $100,748–$378,750 |
| CampbellCalifornia | 1.99 | $182,085 | $298,500 | $99,251–$373,125 |
| BoulderColorado | 1.90 | $173,850 | $285,000 | $94,763–$356,250 |
| OaklandCalifornia | 1.85 | $169,275 | $277,500 | $92,269–$346,875 |
| DenverColorado | 1.79 | $163,785 | $268,500 | $89,276–$335,625 |
| San DiegoCalifornia | 1.73 | $158,295 | $259,500 | $86,284–$324,375 |
| Long BeachCalifornia | 1.67 | $152,805 | $250,500 | $83,291–$313,125 |
| AnaheimCalifornia | 1.56 | $142,740 | $234,000 | $77,805–$292,500 |
| Los AngelesCalifornia | 1.56 | $142,740 | $234,000 | $77,805–$292,500 |
| PortlandOregon | 1.56 | $142,740 | $234,000 | $77,805–$292,500 |
| EugeneOregon | 1.44 | $131,760 | $216,000 | $71,820–$270,000 |
| RiversideCalifornia | 1.44 | $131,760 | $216,000 | $71,820–$270,000 |
| SacramentoCalifornia | 1.44 | $131,760 | $216,000 | $71,820–$270,000 |
| BendOregon | 1.36 | $124,440 | $204,000 | $67,830–$255,000 |
| FresnoCalifornia | 1.27 | $116,205 | $190,500 | $63,341–$238,125 |
| TacomaWashington | 1.27 | $116,205 | $190,500 | $63,341–$238,125 |
| RenoNevada | 1.24 | $113,460 | $186,000 | $61,845–$232,500 |
| SalemOregon | 1.24 | $113,460 | $186,000 | $61,845–$232,500 |
| Colorado SpringsColorado | 1.18 | $107,970 | $177,000 | $58,853–$221,250 |
| HendersonNevada | 0.92 | $84,180 | $138,000 | $45,885–$172,500 |
| SpokaneWashington | 0.92 | $84,180 | $138,000 | $45,885–$172,500 |
| Las VegasNevada | 0.89 | $81,435 | $133,500 | $44,389–$166,875 |
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 moves a 500 square foot detached budget by $63,000 from bottom to top, most of it in the kitchen and bath.
| Finish level | Multiplier | Detached ADU | Above-Garage ADU | Attached ADU | Garage Conversion ADU | Junior ADU |
|---|---|---|---|---|---|---|
| Budget | 0.86x | $129,000 | $107,500 | $102,340 | $78,690 | $57,190 |
| Standard | 1.00x | $150,000 | $125,000 | $119,000 | $91,500 | $66,500 |
| High-end | 1.28x | $192,000 | $160,000 | $152,320 | $117,120 | $85,120 |
National point estimates at 500 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
Here is where $150,000 goes on a 500 square foot detached build at a standard finish, nationally, split across the engine's detached cost profile.
| Line item | Share | Amount |
|---|---|---|
| Site work and excavation | 8% | $12,000 |
| Foundation | 9% | $13,500 |
| Framing and structural shell | 15% | $22,500 |
| Roofing, siding, and windows | 11% | $16,500 |
| Mechanical, electrical, and plumbing | 13% | $19,500 |
| Utility connections | 7% | $10,500 |
| Interior finishes | 14% | $21,000 |
| Kitchen and bath | 9% | $13,500 |
| Design and engineering | 6% | $9,000 |
| Permits and fees | 5% | $7,500 |
| Contingency | 3% | $4,500 |
| Total | 100% | $150,000 |
Detached ADU, 500 sq ft, standard finish, national. $300 per sq ft all in, with a planning range of $112,500–$187,500.
A budget is only half the question. For how homeowners actually cover a number like this, with the honest math on home equity, renovation and construction loans and the 2026 rental-income rules, see our guide to financing an ADU.
Frequently asked questions
How Much Does a 500 Sq Ft ADU Cost?
Nationally, $49,875–$187,500 all in at a standard finish: about $66,500 as a junior ADU, $91,500 as a garage conversion and $150,000 as a detached unit, per our cost engine. In California a detached 500 square foot unit prices around $273,000.
Can a 500 Sq Ft ADU Be a Junior ADU?
Yes, if it is built entirely inside the existing house. California caps a junior ADU at 500 square feet of interior livable space under Government Code section 66313, so 500 is exactly the limit. A detached 500 square foot unit is a full ADU, not a JADU.
Can a 500 Sq Ft ADU Have 2 Bedrooms?
Technically, if each bedroom meets the 70 square foot habitable-room minimum, but the living room and kitchen pay for it. I would draw a one bedroom at 500 square feet and go to 600 or more for two bedrooms.
Is 500 Square Feet a Tiny House?
Not by the building code definition. IRC Appendix Q treats 400 square feet or less as a tiny house, so 500 square feet is a small house built under the ordinary residential code. Permitted on a foundation, it is simply an ADU.
If I had a spare bedroom and bath inside the house, I would build the 500 square foot junior ADU first; it is the cheapest legal unit there is. If I were building new, I would price 500 against what 600 square feet costs, because the extra hundred feet usually buy a better one bedroom for not much more.
Price a different size
The same five build types and the same engine, priced at every size we cover.