How Much Does an ADU Cost to Build? (2026)

ADU Cost Summary: What to Expect in 2026

The average ADU costs between $100,000 and $300,000 to build in 2026, but that range is nearly meaningless on its own. A garage conversion in Texas costs nothing like a new detached ADU in San Francisco.

The cost of your ADU depends on four things: what type you build, how large it is, where you live, and the finish level you choose.

This guide breaks down ADU costs by each of those factors so you can estimate what your specific project might cost.

ADU Cost by Type

The type of ADU you build is the single biggest driver of cost.

Garage Conversion ADU: $40,000 – $150,000

Converting an existing attached or detached garage into a living space is the most affordable ADU path. The structure already exists — you're just finishing the interior, adding insulation, running utilities, and meeting building code requirements.

What drives cost up:

• Adding a bathroom (plumbing rough-in can cost $15,000–$25,000 alone)
• Installing a kitchen (cabinets, appliances, exhaust fan)
• Upgrading electrical to support a full living unit
• Adding HVAC
• Insulating and finishing previously unfinished walls and ceilings

What keeps cost down:

• The foundation and roof already exist
• No site work required
• Shorter permitting timeline in most cities

Best for: Homeowners with an existing garage who want the most cost-effective entry into ADU ownership.

Junior ADU (JADU): $25,000 – $80,000

A JADU is created within the existing square footage of your home — converting an unused bedroom, basement, or bonus room into a self-contained unit capped at 500 sq ft. In California and many other states, JADUs are specifically defined in law and have streamlined permitting.

Since you're not adding square footage, a JADU is typically the least expensive option. The main costs are adding a separate entrance (if required), a kitchenette, and meeting egress requirements.

Best for: Homeowners who want rental income or a multigenerational living space with minimal construction disruption.

Attached ADU: $100,000 – $250,000

An attached ADU shares at least one wall with your primary home and is typically added as a home addition. It requires new foundation work (unless you're converting existing space), full framing, and complete MEP (mechanical, electrical, plumbing) systems.

Attached ADUs cost less than fully detached units because they share walls and sometimes utility connections, but significantly more than conversions.

Best for: Homeowners who want a larger unit than their existing home allows and have lot space on one side of the house.

Detached ADU: $120,000 – $400,000+

A detached ADU is a standalone structure in the backyard — the most flexible option and typically the most valuable addition to a property, but also the most expensive.

Every system must be built from scratch: foundation, framing, roofing, electrical, plumbing, HVAC, insulation, windows, finishes. You also need a separate utility connection (or sub-panel), which adds $10,000–$40,000 depending on your city.

Best for: Homeowners who want maximum privacy for tenants, plan to use the unit for vacation rentals, or have adult children or aging parents who need an independent space.

Prefab/Modular ADU: $80,000 – $250,000 (all-in)

Prefabricated ADUs are built off-site and delivered to your property. They've become increasingly popular because they offer predictable pricing and faster timelines than traditional construction.

Note: the unit cost (often quoted by manufacturers as $50,000–$150,000) doesn't include site work, foundation, utility connections, or permits — which add $30,000–$80,000. Always ask for an all-in quote.

Best for: Homeowners who want design predictability, a faster timeline, and minimal on-site construction disruption.

ADU Cost by Size

Size has a direct and significant impact on cost.

Size	Typical Cost Range
Under 400 sq ft (studio)	$60,000 – $160,000
400–600 sq ft (1 bed)	$90,000 – $220,000
600–800 sq ft (1–2 bed)	$130,000 – $280,000
800–1,000 sq ft (2 bed)	$160,000 – $350,000
1,000–1,200 sq ft (2–3 bed)	$200,000 – $400,000+

Smaller units cost less in absolute terms but more per square foot — fixed costs like foundation work, utility connections, and permits are the same regardless of size.

ADU Cost by State

Geography is a major cost driver, primarily due to labor costs and local construction market conditions.

State	Detached ADU Cost Range
California (Bay Area/LA)	$200,000 – $450,000
California (other markets)	$150,000 – $300,000
Washington	$140,000 – $320,000
Oregon	$130,000 – $280,000
Colorado	$130,000 – $280,000
Massachusetts	$140,000 – $350,000
New York (NYC metro)	$200,000 – $450,000
New Jersey	$130,000 – $300,000
Virginia (Northern)	$130,000 – $280,000
Minnesota	$100,000 – $250,000
Illinois (Chicago)	$110,000 – $280,000
Florida	$90,000 – $210,000
Texas	$100,000 – $250,000
Arizona	$90,000 – $220,000
Nevada	$95,000 – $220,000
Georgia	$85,000 – $210,000
North Carolina	$90,000 – $220,000
Montana	$90,000 – $220,000

The Hidden Costs Most Homeowners Forget

The number your contractor quotes you is not your all-in cost. These additional expenses surprise a lot of first-time ADU owners.

Soft costs: Architectural design ($6,000–$40,000), engineering ($3,000–$10,000), permit fees ($2,000–$25,000+), and project management.

Site work: Grading, excavation, drainage improvements. Highly variable — a flat lot costs much less than a sloped one. Budget $5,000–$30,000.

Utility connections: Electrical sub-panel or new meter ($5,000–$15,000), new water service ($5,000–$20,000), sewer lateral ($5,000–$25,000). These costs vary enormously by city and lot.

Impact fees: Many cities charge transportation, water, sewer, and school impact fees for new dwelling units. In California, units under 750 sq ft are exempt. In other states, fees can be $5,000–$30,000.

Contingency: Build in 15–20% above your contractor's bid. ADU projects almost always encounter unexpected conditions — old plumbing, soil issues, city correction requests.

What Increases ADU Cost the Most

Knowing what inflates cost helps you make trade-offs:

High ceilings: 10-foot ceilings add roughly 15% to framing cost compared to 9-foot ceilings.

Vaulted or complex rooflines: Stick with a simple gable or shed roof to keep framing costs manageable.

Separate utility meter: Required by some cities; adds $10,000–$30,000 compared to a sub-panel from the main house.

Sloped lots: Foundation costs on a hillside can be 2–3x a flat lot. Soils engineering adds $3,000–$8,000.

Custom windows and doors: Standard windows are fine. Custom sizing or premium brands add quickly.

High-end finishes: Countertops, appliances, and flooring for a rental ADU don't need to be luxury grade — mid-range finishes perform just as well and cost 40–60% less.

How to Get an Accurate Estimate for Your Project

Online calculators (including ours) give ballpark numbers — they're not quotes. To get an accurate cost for your specific ADU, you need bids from licensed general contractors.

Steps to get reliable numbers:

1. Get your property's zoning confirmed and understand what you're allowed to build
2. Have a designer or ADU design firm create a schematic design
3. Solicit bids from at least three licensed GCs with ADU experience
4. Compare bids line by line — a cheap bid that excludes utility connections or permits isn't cheap

Use our ADU Cost Calculator to get a rough starting estimate based on your state, size, and type, then use that as a baseline for contractor conversations.

Is an ADU Worth the Cost?

For most homeowners, yes — especially if you plan to rent the unit. A detached ADU in a desirable rental market can generate $1,500–$4,000/month in rental income. At $2,000/month, a $200,000 ADU pays for itself in about 8–9 years — and then generates cash flow indefinitely.

Use our ROI Calculator to model the return on your specific project based on estimated rental income, construction cost, and financing assumptions.

See your state's guide for more specific cost data and permitting information:

• California ADU Costs
• Texas ADU Costs
• Florida ADU Costs
• Washington ADU Costs
• Colorado ADU Costs