What does an ADU cost?

What’s the short answer?

Most finished ADUs in the US cost between about $60,000 and $285,000, and the national average is about $180,000 (Angi, 2026). Angi is a contractor-referral site with its own cost data, so treat these as planning ranges, not a quote.

Those national figures mix conversions with new construction. The examples below cover different building types, places and years; they do not establish a cost range for a new ADU on a Finger Lakes lot.

• A midrange 384-square-foot suite addition, with a bathroom but no kitchen, averaged $170,517 nationally, about $444 per square foot (Zonda Cost vs Value, 2025).
• Boxabl advertises a $150,000 turnkey starting price for its 361-square-foot studio, about $415 per square foot (Boxabl, 2026).
• In a California survey, detached ADUs permitted in 2018 and 2019 had a median cost of $180,000, or $300 per square foot (UC Berkeley Center for Community Innovation, 2021).

Use these figures to compare what a price includes, not to set a local budget. Get an itemized estimate for your site and scope before committing to a project.

Where you land depends on whether you convert or build new, how far the unit sits from utilities, and how small it is. Prices are still climbing: small builders reported a median 9.1% rise in material costs in the year to July 2026 (NAHB, 2026).

New York’s own grant program hints that projects here run high. Its Plus One ADU grants top out at $125,000 per unit outside New York City, and the state says homeowners will need more than that in most or all cases (NYS Homes and Community Renewal, 2026).

Why does “price per square foot” mislead for small homes?

A small cottage needs most of what a big house needs: a kitchen, a bathroom, a water heater, heat, an electrical panel, utility lines, drawings and permits. Those costs barely shrink with the floor, so spread over fewer feet they push the per-foot price up.

Angi’s garage numbers show the pattern. A one-car garage conversion (about 240 square feet) runs $36,000 to $96,000, and a two-car conversion (about 360 square feet) runs $45,000 to $120,000 (Angi, 2026). The bigger space costs more in total but less per foot, partly because running utilities costs the same whatever the garage size (same source).

The examples above cover different scopes, so their per-foot figures are not a single comparable range. Angi’s published national range is $150 to $300 per square foot (Angi, 2026). Use per-foot figures to compare similar buildings, not to price yours.

What do different kinds of ADU cost?

These are published national ranges. Angi’s figures by type leave out permits and fees (Angi, 2026). For what each type is, see ADU types and how ADUs are built.

Angi estimates factory building can save up to 20% compared with custom work (Angi, 2026). Any saving shows only when you compare installed prices.

Watch the words “in-law suite.” Angi prices a garage-to-in-law-suite job at $20,000–$60,000 (Angi, 2026), but a garage-to-ADU job at $60,000–$150,000 (Angi, 2026). Our reading: a legal second home usually needs a full kitchen, its own entrance, fire separation and its own utilities; a guest suite often doesn’t.

Where does the money go?

An installed ADU is a stack of separate costs, and the building is only one. The low ends of these national ranges often assume an easy site.

For a standard septic system, Angi’s page gives two ranges: $3,590–$12,463 in the text and $3,051–$9,887 in its summary box (Angi, 2026).

Early budgets often miss local fees, such as impact fees (one-time charges some towns put on new homes), and putting the yard back. For a full new house in 2024, the permit averaged $7,640, impact fees $6,367 and water and sewer fees $6,260 (NAHB, 2025). ADU fees vary by town, so ask yours.

What pushes cost up or down on a Finger Lakes lot?

We found no published ADU cost survey for Rochester or the Finger Lakes, so here is what moves the number on local lots. Our site and utilities guide goes deeper.

• Septic instead of sewer. New York sizes septic systems by the number of bedrooms (NYS Department of Health, 2016). An ADU bedroom can mean a bigger tank or a bigger leach field, the buried pipes that spread treated water into the soil.
• A private well. If the house runs on a well, the ADU will probably share it. Yield and water-quality tests show whether it can serve two homes.
• Long trenches. Every foot between the ADU and the sewer, the water line and the electric panel costs money. Bringing power to a new building runs $25–$50 a foot (Angi, 2026), and new underground lines to a detached garage add 20%–30% to a garage conversion (Angi, 2026). On a big lot, the spot with the best view can be the priciest.
• Frost depth. Footings, the concrete a building rests on, must go below the frost line or be insulated so the ground under them can’t freeze and lift them. Water lines go below frost too. Deeper digging means more labor and concrete. See winter-ready ADUs.
• Rock, slope and wet ground. Shallow rock slows digging, can rule out a basement and limits where a septic field can go. Slopes and trees add grading and clearing, and a tight driveway can make a crane lift harder.

What pulls cost down:

• Converting space you already have, especially an attached garage close to the house’s plumbing, wiring and heat (Angi, 2026).
• Ready-made plans. New York City launched eleven pre-reviewed ADU designs in March 2026 (NYC Mayor’s Office, 2026). Ask your town whether it has anything similar.

How do you read a quote?

The same building can be advertised at one price and cost more than twice that in your yard. Before you compare bids, find out which of these four prices you’re looking at.

Boxabl’s studio is a clear example. It lists $60,000 for the unit alone and a $150,000 turnkey starting price, and the turnkey package includes a site survey, engineering, a foundation, shipping and crane, installation, water, sewer and electric hookups, and a final inspection (Boxabl, 2026). The advertised unit price is 40% of the advertised turnkey starting price (our arithmetic); that comparison is not an installed quote for your lot. Mighty Small Homes, which sells shell kits, says the total project usually ends up two to three times the kit price (Mighty Small Homes, undated). Angi warns that land and site prep for a modular home can add $10,000–$200,000 (Angi, 2026).

Site-built quotes need the same care. Ask each builder, in writing, whether the price includes:

• design, engineering and a survey
• permits, plan review and any impact or utility fees
• site work, and how many feet of trench it assumes
• septic work or an electrical panel upgrade
• heating and cooling, the water heater and appliances
• steps, decks, gutters, driveway and landscaping
• delivery, crane and road permits, for prefab
• sales tax and a contingency

Get at least two itemized quotes for the same scope, then turn each into an installed, move-in-ready price with the same items. A low number that leaves out the trench, the septic and the foundation isn’t a low price. And a quote isn’t a permit: your town still has to approve the project (permits and process).

What’s different in New York?

A few New York rules change the budget, not just the paperwork. Our New York ADU law page covers the legal side.

In New York: Angi lists typical ADU costs for New York at $135,000–$325,000 in 2026 (Angi, 2026) but doesn’t say whether that blends New York City with upstate. The state’s Plus One ADU grant of up to $125,000 per unit outside the city also has to cover the local administrator’s fee of up to 10% (NYS Homes and Community Renewal, 2026). When we checked, the state listed no local Plus One program in Monroe, Ontario, Wayne, Seneca, Yates or Steuben counties (NYS Homes and Community Renewal, 2026). See paying for an ADU.

In New York: New York enacted restrictions for covered new construction, with exceptions for alterations, additions and changes of use to buildings existing before the applicable prohibition (NYS Energy Law §11-104). As checked October 9, 2026, the Department of State says the adopted fossil-fuel provisions remain suspended by court order; other 2025 code provisions remain in effect (DOS notice). We have not confirmed when the suspension will end. Ask your code office which current rules apply to your project before choosing equipment.

When two homes share a wall or floor, as a basement or attached ADU does, the model residential code calls for walls and floors rated to resist fire for at least one hour (International Code Council, 2021). That adds material and labor a detached cottage doesn’t need. Ask your code office how New York’s current code applies to your plan.

In New York: an ADU property tax break exists, but only where local governments adopt it. Real Property Tax Law §421-p, added in 2024, lets each county, city, town, village and school district decide for itself (NY Senate, 2024). Where adopted, it covers ADU construction that starts after adoption, costs more than $3,000 and sits on a property with no more than two families. It exempts up to $200,000 of the value the ADU adds: 100% for five years, then less each year until it reaches zero by year ten (same source). Kingston was the first city to adopt it, in August 2024, and its version leaves out short-term rentals (City of Kingston, 2024). A separate home improvement exemption, §421-f, covers up to $80,000 of added value on homes at least five years old and phases out over eight years; state guidance doesn’t say whether a new detached ADU qualifies (NYS Department of Taxation and Finance, 2023). Ask your assessor which, if any, your town, county and school district have adopted, and ask a tax preparer how an ADU affects your own taxes.

What this means for you

• Start with your path, not a number. Converting space you already have usually costs less than building a new detached cottage (Angi, 2026).
• Walk the utility path before you pick a spot. Distance to sewer or septic, water and power costs money, and a septic system or well should be tested before you pay for plans.
• Compare installed, move-in-ready prices only.
• Hold back 10% to 20% for surprises, the band federal renovation loans use (HUD, 2024), and lean toward the high end for an older house or a conversion.
• In New York, ask four questions early: does the all-electric law cover your project, can your septic take another bedroom, how does fire separation apply, and has your town and school district adopted the §421-p or §421-f exemption?
• Bring in the right people. These published comparisons and labelled calculations are not a price for your property. An engineer, a septic designer, a lender and a tax preparer each answer questions this page can’t. To see what your town allows, start with the property checker.

General information, not legal, tax or financial advice. Your municipality decides what can be built, and a professional should review decisions about money and contracts.