Building an ADU costs about $80,000 to $270,000 in 2026. A 600 sq ft detached unit averages about $175,000 including design, permits and utility hookups, while a garage conversion can come in under $100,000. Enter your plans below for an itemized estimate and a side-by-side comparison of ADU types.
ADU Construction Cost Calculator
Cost breakdown
| Item | Cost |
|---|---|
| Total | $0 |
Compare ADU types
| Type | Total | Per sq ft |
|---|
On a septic system? An ADU with its own bathroom and kitchen usually needs more septic capacity, and some counties require a separate system. Price a larger tank or drain field with the septic system cost calculator. If you mainly need more room attached to the house, compare with the garage addition cost calculator.
How Much Does an ADU Cost in 2026?
An accessory dwelling unit (ADU) costs about $80,000 to $270,000 in 2026, depending mostly on the type of ADU and its size. A 600 sq ft detached ADU averages about $175,000 all-in, or roughly $290 per square foot, including design, permits and utility connections. Converting an existing garage or basement is the cheapest route, usually $70,000 to $135,000.
| ADU type | 400 sq ft studio | 600 sq ft, 1 bedroom | 800 sq ft, 2 bedrooms |
|---|---|---|---|
| Detached new build | $112k – $165k | $148k – $218k | $184k – $271k |
| Attached addition | $100k – $148k | $132k – $194k | $163k – $240k |
| New garage with apartment above | $122k – $179k | $162k – $238k | $202k – $298k |
| Garage conversion | $66k – $97k | $82k – $121k | $99k – $145k |
| Basement conversion | $73k – $108k | $91k – $135k | $110k – $161k |
| Prefab or modular | $97k – $143k | $136k – $201k | $176k – $258k |
Smaller ADUs cost more per square foot. A 400 sq ft detached studio runs about $330 per square foot, while an 800 sq ft unit drops to about $270, because the kitchen, bathroom, permits and utility work cost about the same at any size.
ADU Cost Breakdown
Here’s where the money goes on a typical 600 sq ft, one-bedroom detached ADU at national average prices:
| Item | Cost | Share |
|---|---|---|
| Construction: foundation, framing, roof, systems and finishes | $105,000 | 60% |
| Kitchen | $18,000 | 10% |
| Bathroom | $15,000 | 9% |
| Bedroom walls, doors and closet | $4,000 | 2% |
| Site prep and grading | $4,000 | 2% |
| Utility connections | $8,000 | 5% |
| Architect, engineering and design | $12,300 | 7% |
| Permits, impact and utility fees | $8,000 | 5% |
| Total | $174,300 |
Costs That Are Easy to Miss
- Design and engineering: plans, structural engineering, surveys and energy reports usually add 6% to 10% of construction cost.
- Permits and impact fees: often $5,000 to $15,000, and more in some cities. Many states have capped or waived impact fees for small ADUs, so ask your city.
- Utilities: tying into the main house’s water, sewer and power runs about $5,000 to $8,000. Separate meters can cost $14,000 to $18,000, and an electrical panel or service upgrade may be needed.
- Septic: homes on septic often need a larger tank or drain field for the extra bedroom. Estimate that with our septic system cost calculator.
- Sloped or tight sites: excavation, retaining walls and limited access can add $10,000 or more.
Which Type of ADU Is Right for You?
- Garage conversion: the lowest cost because the walls, roof and slab already exist. See our garage conversion cost calculator for room-by-room pricing.
- Basement conversion: also affordable, but needs enough ceiling height, egress windows and usually a separate entrance.
- Attached addition: shares a wall and often utilities with the house, so it costs less than a detached unit but offers less privacy.
- Detached new build: the most privacy and rental appeal, and the most flexibility in design.
- Garage with apartment above: adds parking and living space on one footprint. See our guide to the cost to build a garage with an apartment and browse garage apartment plans.
- Prefab or modular: built in a factory and set by crane, often in 4 to 7 months instead of 7 to 12. The savings depend heavily on site work and delivery costs.
Compare Local ADU Builders
Get at least three bids, and make sure each one says whether design, permits, utility connections and site work are included. Those items explain most of the gap between low and high quotes.
ADU Costs by Region
| Region | 600 sq ft detached ADU |
|---|---|
| Rural and low-cost areas | About $148,000 |
| South and Midwest | About $160,000 |
| National average | About $174,000 |
| Mid-Atlantic and Mountain West | About $192,000 |
| Northeast and Pacific Northwest | About $218,000 |
| California and high-cost metros | About $270,000 |
How to Pay for an ADU
Most homeowners use a home equity loan, HELOC, cash-out refinance or construction loan. At October 2026 rates, a $174,000 ADU financed with a 20-year home equity loan at 8.8% costs about $1,550 a month. If the unit rents for more than that in your area, it can pay for itself. Compare options with our loan calculator, and check whether your state or city offers ADU grants, low-interest loans or fee waivers.
Ways to Save on an ADU
- Convert before you build. A garage or basement conversion costs about half as much as a detached unit.
- Use pre-approved plans. Many cities offer free or pre-approved ADU plans that cut design costs and speed up permits.
- Keep it simple. A rectangular footprint, a simple roof and one bathroom keep costs down.
- Share utilities. Tying into the main house is usually far cheaper than separate meters.
- Choose durable materials. See our guide to building materials that last 30+ years to avoid paying twice.
Frequently Asked Questions
What is the cheapest way to build an ADU?
Converting an existing garage or basement is usually cheapest, at about $70,000 to $135,000 for a one-bedroom unit, because the structure already exists. Among new builds, a simple attached addition or a prefab unit on an easy site tends to cost least.
How much does a 600 sq ft ADU cost?
About $148,000 to $218,000 for a detached one-bedroom unit at national average prices, including design, permits and utilities. In California and other high-cost markets, expect $230,000 to $340,000.
Is prefab cheaper than building on site?
Sometimes. The unit itself can be priced competitively and installed faster, but delivery, crane, foundation and utility work add significantly to the total. Get a full turnkey quote before comparing.
How long does it take to build an ADU?
Design and permitting typically take 2 to 6 months. Construction takes about 4 to 7 months for conversions and prefab units and 7 to 12 months for site-built detached ADUs.
Does an ADU add value to my home?
A permitted ADU usually adds value, especially in areas with strong rental demand. It can also provide rental income or housing for family. Unpermitted units can hurt resale, so always build with permits.
Pricing last updated October 2026 using national-average construction costs. ADU rules and fees vary widely by city; check local requirements and get written bids before you build.
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