The cost to build an ADU in 2026 is $110,000 to $390,000. Most people spend $150,000 to $250,000. That is about $185 to $410 per square foot. Garage and basement changes start at around $60,000.
Not sure if your budget is right? Outsource preliminary estimating services as they give you a real price from your early plans before you hire anyone.
All prices here are full prices. They include design, permits, site work, utility lines, and finishes. They are not just the builder’s framing bid. That is where most ADU budgets go wrong.
What Is the Average Cost to Build an ADU in 2026?
The average ADU cost is close to $180,000. Half of all ADU jobs land between $120,000 and $260,000.
Three things change the price most:
- The type: A garage conversion costs half as much as building new.
- The city: The same 600 sq ft unit runs about $135,000 in Atlanta and $340,000 in Seattle.
- Your yard: A sloped yard, bad soil, long pipe runs, and tight access increase cost.
Size matters less than you think. A kitchen and bath cost about the same in a small unit as a big one. That is why small ADUs cost more per square foot.
Why Are ADU Prices Higher than You Remember?
Building costs have increased a lot since 2021. California’s building cost index rose 44% from January 2021 to December 2025. Snap ADU, a San Diego builder, put it in plain terms. A unit that cost $300,000 in early 2021 now costs about $430,000.
Using an old 2022 price guide? Add 30% to 40% to it.
How Much Does an ADU Cost Per Square Foot?
A new backyard unit costs $185 to $410 per square foot. Garage and basement jobs cost $100 to $260. Pricey coastal cities run $350 to $650.
| Type of build | Price per sq ft (full price) | Total for 600 sq ft |
| Basement change | $100 – $220 | $60,000 – $130,000 |
| Garage change | $120 – $260 | $70,000 – $155,000 |
| Attached unit (new space) | $165 – $350 | $100,000 – $210,000 |
| Prefab, dropped and set up | $180 – $400 | $108,000 – $240,000 |
| New detached unit | $185 – $410 | $110,000 – $245,000 |
| Unit above a garage | $210 – $450 | $125,000 – $270,000 |
These ADU cost per square foot 2026 numbers are full prices. Many builder bids show only the building itself. That is about 80% to 85% of your real cost. Ask each builder in writing if permits, pipe trenching, and site prep are in the bid. Many will say no.
Watch out for the above-garage plan. It looks cheap because the base is already there. It rarely is. Old garage slabs and walls seldom carry a second floor without reinforcement. They need work first. Stairs also take floor space.
How Much Does an ADU Cost by Size?
A 400 sq ft studio costs $80,000 to $170,000. A 1,200 sq ft three-bedroom costs $205,000 to $440,000.
Here is the ADU construction cost by size for new detached units in 2026:
| Size | Layout | Price per sq ft | Total price |
| 400 sq ft | Studio | $200 – $425 | $80,000 – $170,000 |
| 600 sq ft | 1 bed, 1 bath | $185 – $410 | $110,000 – $245,000 |
| 800 sq ft | 2 bed, 1 bath | $180 – $390 | $145,000 – $310,000 |
| 1,000 sq ft | 2 bed, 2 bath | $175 – $375 | $175,000 – $375,000 |
| 1,200 sq ft | 3 bed, 2 bath | $170 – $365 | $205,000 – $440,000 |
The price per square foot gets lower as the ADU gets bigger. The costly parts stay the same. Kitchen, bath, foundation, pipes, and permits do not change much. You just spread them over more floor space.
Here is the smart move. Going from 400 to 600 sq ft adds only $30,000 to $50,000. But it adds a real bedroom. That is the best deal in most ADU jobs. Past 600 sq ft, the savings flatten out. Every extra foot costs close to full price.
What Does the Full Cost Breakdown Look Like?
Soft costs take 12% to 20% of your budget. The building takes the rest.
Here is a full accessory dwelling unit cost breakdown for a 600 sq ft detached ADU.
Soft Costs
| Item | Price |
| House plans from an architect | $3,500 – $12,000 |
| Engineering and soil test | $2,000 – $6,000 |
| Land survey | $800 – $2,500 |
| Building permit and plan check | $2,000 – $12,000 |
| Impact, school, and hookup fees | $0 – $15,000 |
| Soft cost total | $8,300 – $47,500 |
Hard Costs
| Item | Price |
| Site prep, grading, digging | $4,000 – $12,000 |
| Foundation | $7,000 – $18,000 |
| Framing, roof, windows, siding | $32,000 – $50,000 |
| Pipe trenching and hookups | $8,000 – $18,000 |
| Plumbing and fixtures | $7,000 – $13,000 |
| Wiring and panel upgrade | $5,000 – $11,000 |
| Heating and cooling (mini-split) | $3,500 – $7,000 |
| Insulation and drywall | $5,000 – $9,000 |
| Kitchen | $6,000 – $14,000 |
| Bathroom | $5,000 – $10,000 |
| Floors, paint, trim, doors | $5,000 – $9,000 |
| Walkway and yard work | $2,500 – $6,000 |
| Hard cost total | $90,000 – $177,000 |
Add 10% for surprises. Your total lands at $110,000 on the low end and roughly $245,000 on the high end.
Where the Money Really Goes
Framing and the shell take the biggest bite. That is close to one third of the building cost.
Pipes and wires are the sneaky ones. Running water, sewer, and power 60 feet across a yard costs far more than running it 15 feet. Digging under a driveway costs even more.
ADU foundation cost changes more than any other item. A flat yard with good soil takes a simple slab. That is $12 to $20 per square foot. A sloped yard needs a raised base, walls, or deep piers. That jumps to $35 to $60 per square foot. Get a soil test before you set your budget, not after.
This is why residential estimating services matter. A full item-by-item takeoff from your drawings gives you a real number. Then you can check if a $210,000 bid is fair or not.
What Is the ADU Permit and Fees Rule Most People Miss?
California drops impact fees for ADUs under 750 square feet. Build a 749 sq ft unit instead of a 760 sq ft one. You can save five figures with that one choice. Check your own state and city for the same kind of size rule before you lock in a floor plan.
The rules keep getting better for owners. HUD’s research office is funding studies on how new state ADU laws change housing supply and cost. Their published HUD data on accessory dwelling unit policy shows which rules actually work. Other states are copying them.
What Factors Affect ADU Cost in 2026?
ADU construction cost depends on the following factors:
- Size: A larger ADU requires more materials and labor.
- Site conditions: Your land also affects your cost. A steep yard, bad soil, or a hard-to-reach backyard adds extra digging and equipment. This increases the cost before the build even starts.
- Finish level: Basic cabinets and counters cost much less than custom finishing.
- Location: Labor and permit prices change a lot by city and state.
- Design: A simple square or rectangle shape is cheaper to build.
How Long Does It Take to Build an ADU?
Plan on 9 to 18 months from first drawing to move-in day. The building part is the shorter half.
| Step | Time |
| Design and engineering | 1 – 3 months |
| Permit review | 2 – 6 months |
| Site work and foundation | 3 – 6 weeks |
| Framing through rough-in | 6 – 10 weeks |
| Finishes and inspections | 6 – 12 weeks |
Permits take the most time. California now makes cities approve or deny a full ADU plan within 60 days. New 2026 rules also force a completeness check in 15 business days. Cities without deadlines can take six months just on plan review.
Time is money here. Every month of delay adds loan interest and higher material prices.
How Do People Pay for an ADU?
Most people use home equity. Five paths cover almost every project:
- HELOC: Take money out as you need it. Pay interest only on what you use. Good for slow, staged builds.
- Cash-out refinance: You get a bigger loan. Use the extra money for your ADU. Bad idea if your current rate is low.
- Construction loan: Money comes out in stages as work gets done. Then it turns into a normal mortgage.
- Renovation loan: It lends against your home’s value after the ADU is done. Good if you lack equity now.
- Cash or builder financing: Fastest way. No appraisal risk at all.
The Rent Rule That Changed Everything
The FHA put out Mortgagee Letter 2023-17. It lets lenders count future ADU rent when they review your loan. HUD’s news release confirms lenders can count income from units inside, attached to, or on the same lot as your main home.
Is ADU Demand Still Growing in 2026?
Local numbers show the trend accelerating. UC San Diego’s housing research center found that finished ADUs in San Diego County rose 480% from 2020 to 2024. Permits more than tripled in the same years.
The national baseline comes from Freddie Mac’s 2020 analysis of 600 million home listings. They found 1.4 million U.S. homes with ADUs. Sales of homes with ADUs jumped from 8,000 in 2000 to 70,000 in 2019.
Final Thoughts
The cost to build an ADU comes down to three things: the type of ADU, the city, and the shape your yard is in. Answer those three honestly and your budget will hold. Skip them and you will pay the difference in change orders.
Start with a soil test and a feasibility check. Then get a real item-by-item estimate before you talk price with builders. Then bid three builders on one full permit set. That order is what keeps a job on budget instead of stopping at 60% done.
