ADU Construction Cost 2026: Detached, Attached & Conversion

Quick answer

Building an accessory dwelling unit (ADU) is a small-house project, and it is priced like one. Angi’s ADU guide (updated September 29, 2026) puts the national average at about $180,000, with most homeowners spending $60,000 to $285,000, or roughly $150 to $300 per square foot. Its type table does not include permits and other fees. Conversions of existing space usually cost less than new construction, and detached units usually cost the most because they need their own foundation, roof and utility runs.

This guide covers ADUs on single-family lots in the United States: detached backyard units, attached additions, above-garage units and conversions. It focuses on what drives the total and what a quote may leave out. For a garage you want to turn into a room rather than a separate dwelling, see our garage conversion cost guide.

Detached, attached or prefab: what you are choosing

An ADU is a complete second home on the same lot. Fannie Mae’s Selling Guide describes it as an additional living area that may be added to, created within or detached from a one-unit home. For lending purposes it must have its own living, sleeping, cooking and bathroom facilities and a separate means of entry. A second kitchen alone does not make a space an ADU. That definition explains why ADU budgets look like small-house budgets.

Detached ADU: a separate backyard building. It offers the most privacy and flexibility, but it needs its own foundation, roof, exterior walls and the longest utility runs. Angi: $110,000–$285,000.
Attached ADU: an addition sharing at least one wall with the house. The foundation and roof tie into the existing structure, and utilities are usually closer. Angi: $100,000–$216,000. Tying into an older house can reveal structural or code issues that a detached build avoids.
Prefab or modular ADU: built in a factory and set on a site-built foundation. The factory price covers only part of the job; site work, foundation, delivery, crane, hookups and permits remain. HomeGuide lists about $80–$160 per square foot (2023 data).

ADU cost by type

ADU type	Angi range (excludes permits and fees)	HomeGuide per sq ft
Basement conversion	$60,000–$150,000	$50–$100+
Garage conversion	$60,000–$150,000	$50–$200
Attached new construction	$100,000–$216,000	$125–$225
Above-garage construction	$128,000–$225,000	$200–$500+
Detached new construction	$110,000–$285,000	$150–$250
Prefab / modular ADU	Not broken out	$80–$160

Sources: Angi, “How Much Does an ADU Cost to Build? [2026 Data]” (updated Sept. 29, 2026); HomeGuide, “How Much Does It Cost to Build an ADU? (2026)” (page titled 2026; last updated November 2023, so treat its figures as older). The garage-conversion ADU figures here are for a full dwelling with a kitchen and bathroom. That is why they are far higher than converting a garage into a single room.

ADU cost by size

Angi’s per-square-foot range applied to common sizes:

ADU size	Angi range
400 sq ft	$60,000–$120,000
600 sq ft	$90,000–$180,000
750 sq ft	$112,500–$225,000
1,000 sq ft	$150,000–$300,000
1,200 sq ft	$180,000–$360,000

Angi notes that some builds exceed $600 per square foot depending on materials and finishes. Small units often cost more per square foot because a kitchen, bathroom, utility connections and permits cost about the same regardless of floor area.

Where the money goes

Beyond foundation, utilities and construction (covered below), Angi breaks out these components. Use them to read a bid, not as a checklist of add-ons:

Component	Angi figure
Design work	About 10% to 15% of the project (Angi: $12,000–$18,000 on a $120,000 project)
Building permits	$1,350–$9,000 (national average about $1,350)
Post-construction cleanup	$0.10–$0.50 per sq ft ($60–$600 for a standard ADU)
Waste disposal	$100–$600; a dumpster rental is $300–$800 per week
General contractor fee	10%–20% of the total project cost

HomeGuide’s planning list adds house plans ($1,200–$5,000), a land survey ($400–$1,800) and site excavation ($1,500–$5,000). Its figures are from 2023.

Foundations, utilities and construction: the three big cost blocks

Foundation and site work

A new detached or attached ADU needs a foundation designed for its soil and local code. Angi lists concrete foundations at about $5–$40 per square foot, or roughly $2,400–$30,000 for most 600–1,200 sq ft ADUs. Land leveling and grading add about $1,000–$5,800. Sloped lots, poor soil, tree removal or limited access for equipment push these numbers up. They are also where early surprises show up, so ask whether a soil report or engineer’s design is included.

Utilities

Angi puts the electrical, plumbing and HVAC systems inside an ADU at about $3,400–$10,400 on average. Its examples include running electricity at $1,000–$3,800 and about $6,000 for a small furnace and AC unit. Those figures describe systems inside the unit. Getting water, sewer or septic, power and sometimes gas to a detached unit is a separate trenching and connection scope, and local fees may apply. Ask the utility and building department early whether your electrical service and sewer or septic capacity can carry a second dwelling.

Construction and finishes

Angi lists framing at about $25,000 on average and finishing at $10–$60 per square foot, rising past $150 per square foot with high-end materials. A general contractor’s fee is typically 10%–20% of the project. Kitchens and bathrooms concentrate cost, which is why a small ADU is not proportionally cheaper than a large one.

Garage conversion vs. a new detached ADU: different projects

These are often compared, but they are not the same scope. Our garage conversion cost guide covers turning a garage into a room, such as a bedroom, office or gym. Angi’s national range for that is $5,986–$27,428. That room relies on the main house for its kitchen, and often for its bathroom and utilities.

An ADU must function as an independent dwelling with its own kitchen, bathroom, sleeping area and entrance. Converting a garage into an ADU therefore adds plumbing, a kitchen, a bathroom and sometimes separate utilities. That is why Angi prices garage-to-ADU conversions at $60,000–$150,000. A new detached ADU goes further again: it adds a foundation, structure and roof that a garage already provides, and utility runs across the yard ($110,000–$285,000 per Angi).

Decision guide:

If you need one extra room for the household, a garage conversion is the smaller project.
If you need a self-contained home for a relative or tenant, and the garage is structurally sound, close to plumbing and allowed by local rules, a garage ADU conversion can cost less than building new.
If you need parking, the garage is in poor condition, or the lot has space and utilities can reach it, a new detached ADU may be the better long-term choice despite the higher price.

What is often excluded

Utility connections. Running water, sewer, power and sometimes gas to a detached unit means trenching and possibly new meters or a larger electrical service. Ask whether the quote stops at the building’s footprint. Related guides: electrical panel upgrade cost, sewer line cost and, on septic properties, septic system cost.
Local fees beyond the building permit. Impact, school, utility connection or capacity fees may apply depending on where you live.
Design, engineering and surveys. Some design-build firms include them; many general contractors do not.
Appliances, landscaping, fencing, paths and parking.
Contingency. Excavation and old-house tie-ins can uncover problems. Plan a reserve; our renovation budget guide explains how.

Rules and fees: check locally before you design

ADU rules are set by states and local governments. Ask your planning department about allowed ADU types and sizes, setbacks, height, parking, owner-occupancy and rental rules, and which fees apply.

California is a useful example of how much state law can change the bill. Under Government Code section 66311.5 (effective January 1, 2026), local agencies, special districts and water corporations may not impose impact fees on an ADU with 750 square feet of interior livable space or less. Larger ADUs may be charged only proportionately to the primary home. Connection fees and capacity charges are treated separately and must be proportionate where they apply. Other states and cities differ, so do not assume these rules apply where you live.

Worked example: a 600-square-foot detached ADU

This is a hypothetical planning exercise using Angi’s published ranges only. It is not a quote.

Construction at $150–$300 per sq ft: 600 × $150 = $90,000; 600 × $300 = $180,000 → $90,000–$180,000
Design at 10%–15%: 10% × $90,000 = $9,000; 15% × $180,000 = $27,000 → $9,000–$27,000
Building permit: $1,350–$9,000

Subtotal: $90,000 + $9,000 + $1,350 = $100,350 at the low end; $180,000 + $27,000 + $9,000 = $216,000 at the high end. Add any utility connections, local impact or school fees, appliances, landscaping and a contingency that your contractor’s quote does not already include.

Check with each bidder whether design, site preparation and the foundation are inside the per-square-foot price. Count each item only once. To total your own line items with a contingency, use the project budget calculator.

Financing an ADU

Most homeowners pay for an ADU with savings, home equity, a cash-out refinance, a renovation or construction loan, or a combination. Rates and terms vary by lender and credit profile, so get written quotes rather than relying on averages.

One detail affects qualifying: rental income. Fannie Mae updated its Selling Guide (announcement SEL-2025-08, October 8, 2025) to let lenders count rental income from an ADU on a one-unit principal residence, within limits:

Purchase or limited cash-out refinance transactions only.
Income from one existing ADU only.
No more than 30% of the borrower’s total qualifying income.

For a unit that has not been built yet, do not assume future rent will help you qualify. Ask the lender how it treats a planned ADU before you commit to a design. Some states and cities run ADU assistance programs; check your state housing agency for current availability.

National figures vs. local quotes

Location changes the total substantially. Angi’s city table runs from about $110,000–$285,000 in Boston and $110,000–$290,000 in Atlanta to $150,000–$375,000 in Los Angeles. Labor rates, permit fees, soil, lot access and how far utilities must run all vary by site. Get at least three written bids on the same plans and specifications.

Prefab and modular ADU quotes

If you are pricing a factory-built ADU, ask whether the quote is factory-only, delivered, set on the foundation or turnkey. Then add whatever is missing: foundation, crane, utility hookups, permits and finish work. Our Prefab Homes hub and modular home cost guide explain how those quote types differ.

How to compare ADU bids

Use the same plans, square footage and finish specifications for every bidder.
Get it in writing who handles design, engineering, the survey and permit applications.
Ask where the scope starts and stops: site work, foundation, utility trenching and connections, and meters.
Ask for the HVAC approach (for example, a ductless system; see our mini-split cost guide) and the electrical service plan.
Compare allowances for fixtures, cabinets and appliances.
Compare the payment schedule, change-order process, timeline and warranty.

FAQ

How much does it cost to build an ADU in 2026?

Angi reports a national average of about $180,000, with most projects between $60,000 and $285,000, or roughly $150 to $300 per square foot, not counting permits and other fees.

Is a detached ADU more expensive than an attached one?

Usually. Angi lists attached new construction at $100,000–$216,000 and detached new construction at $110,000–$285,000. Detached units need their own foundation, roof and separate utility runs.

Is converting a garage cheaper than building a new ADU?

Often, because the structure exists. Angi lists garage and basement ADU conversions at $60,000–$150,000. A garage converted into a single room without a kitchen costs much less; see our garage conversion guide.

Do ADU permits cost a lot?

Angi lists building permits at $1,350–$9,000. Separate impact, school and utility fees may apply, and some states limit them.

Can planned ADU rent help me qualify for a loan?

Not automatically. Fannie Mae’s current policy counts rental income from one existing ADU, capped at 30% of qualifying income, on purchase or limited cash-out refinance loans. Ask your lender how it handles an ADU you have not built yet.

Bottom line

Plan on a six-figure budget for most new ADUs, confirm local rules and fees before design, and make every bid show where site work and utility connections begin and end.

Reviewed by the Home Cost Desk editorial team on November 2, 2026. Prices are national planning ranges from dated sources; local rules and quotes govern.

ADU Construction Cost 2026: Detached, Attached & Conversion Prices