How Much Does an ADU Cost in 2026? Real Prices by Type and Size

How much does an ADU cost? Most U.S. homeowners spend between $100,000 and $300,000 to build a legal accessory dwelling unit. Garage conversions can come in around $80,000. A new detached unit in a high-cost metro can push past $400,000. The number that matters for your project depends on three things: what type of ADU you build, how big it is, and where you live.

That range is wide on purpose — because an ADU isn't one product. A garage conversion and a detached backyard build are as different as a kitchen remodel and a new house. Once you know your type, size, and region, your range narrows fast.

We built this guide to do what most ADU cost pages don't: give you numbers organized enough to actually budget with, explain why online estimates contradict each other, and show you where the real surprises hide — so you can move forward with confidence instead of sticker shock.

National ADU Cost Overview

Here's the national picture at a glance:

Why You Keep Seeing Wildly Different ADU Prices Online

If you've spent 20 minutes searching ADU costs, you've probably noticed something frustrating: the numbers don't agree. One site says $30,000. Another says $400,000. A third says "it depends" and never tells you on what.

There's a reason for the chaos, and understanding it will save you from bad decisions.

Different sources quote different things. Some show construction cost only — no permits, no design, no site work, no utilities. Others show all-in cost for one specific metro. Some reference prefab kit prices without the $50,000+ in site prep, foundation, and hookups required to make it livable. And a few are quoting data from 2019 or earlier, before material costs climbed significantly. (For context, the California Construction Cost Index rose 44% from January 2021 to December 2025, per SnapADU's March 2026 pricing data. Other regions saw smaller but still meaningful increases.)

Here's the mental model that cuts through the noise:

How Much Does an ADU Cost by Type?

This is the most important section on this page. The type of ADU you build is the single biggest factor in your total cost. Find your type below and you'll have a realistic budget range in 30 seconds.

Garage Conversion ADU: $80,000–$175,000

A garage conversion is the most budget-friendly path to a legal ADU for one simple reason: the structure already exists. You've got a foundation, walls, and a roof. What you're paying for is everything that turns a cold garage into a livable home — insulation, drywall, plumbing, electrical, HVAC, a kitchen, a bathroom, flooring, and replacing the garage door with an insulated wall and windows.

When this makes sense: You have a garage you don't use regularly, you want the lowest entry cost, and your local zoning allows the conversion. Some jurisdictions require replacement parking; others don't. Check with your city's planning department before assuming.

Basement Conversion ADU: $50,000–$150,000

If you have an existing basement, converting it into an ADU leverages the most expensive components you've already paid for — foundation and structure.

Key costs: Egress windows (required by building code for bedrooms in most jurisdictions), waterproofing, HVAC, plumbing, insulation, finishing, and a separate entrance.

When costs climb: If you need excavation to deepen the ceiling height, or if waterproofing reveals serious moisture or structural problems. A basement that needs a dig-out can cost $100,000+ more than one that's already at livable height.

When this makes sense: You have an unfinished or underused basement with adequate ceiling height (typically 7 feet minimum per IRC Section R305.1) and your local codes allow basement ADUs.

Attached ADU: $150,000–$300,000

An attached ADU shares at least one wall with your primary home. It's essentially a home addition designed to function as an independent living space with its own entrance, kitchen, and bathroom.

Because it can leverage existing foundation, roofline, and utility connections, attached ADUs generally cost less than a detached unit of the same size — though the savings depend on your specific property and how much existing infrastructure can be reused.

Typical size: 400–800 sq ft.

When this makes sense: You have limited backyard space but room to extend your home's footprint, or you want the efficiency of shared infrastructure without giving up a separate living space.

The trade-off: Less privacy than a detached unit, which can affect rental desirability in some markets.

Above-Garage ADU: $200,000–$350,000

Building a living space above an existing garage requires structural reinforcement (most standard garages aren't built to support a second story), staircase access, and potentially raising the roofline to meet ceiling height codes.

When this makes sense: You want to preserve your parking, don't have yard space for a detached unit, and your garage structure is in good condition.

Prefab / Modular ADU: $150,000–$300,000

Prefab ADUs are factory-built and assembled on site. The unit cost (structure, finishes, fixtures) is generally lower than equivalent custom construction, and the timeline is shorter.

But here's what the ads don't always mention: the prefab unit price is not the all-in price. You still need site preparation, a foundation, utility hookups, permits, and potentially crane access for delivery. Those costs don't go away because the walls were built in a factory. As Maxable (an ADU planning service) has noted, prefab advertising often excludes transportation, site prep, foundation, utilities, and crane costs — and when you add those back in, the all-in price can be comparable to a traditional build. (Source: Maxable, maxablespace.com.)

When prefab makes sense: Your lot has good access, is relatively flat, and your local jurisdiction accepts prefab construction.

When it doesn't: If your lot has a steep slope, irregular shape, or tight access, the site work to accommodate a pre-sized prefab unit can cost more than building custom. Always get quotes for both.

Detached ADU — New Construction: $200,000–$400,000+

A detached ADU is a standalone structure built from the ground up. New foundation, new framing, new roof, new siding, all new utility connections. It's the most expensive option — and it's also the one that commands the highest rents, adds the most flexibility, and provides the most privacy.

Source: Compiled from Angi 2026 data, DYM Builders national estimates, and regional builder pricing.

When this makes sense: You want maximum rental income, full independence from the main house, multigenerational privacy, or the strongest long-term property value impact.

The biggest cost driver: Everything is new. Foundation, utility trenching, and separate systems are the fixed costs that make even small detached ADUs expensive on a per-square-foot basis.

How Much Does an ADU Cost by Square Footage?

After ADU type, size is the second biggest factor. Here's what to expect at each common size tier:

If budget allows, building toward the upper end of what your lot permits often makes the best financial sense. The marginal cost of adding square footage is lower than the base cost of the kitchen, bath, utilities, and permits you're already paying for — and the additional space translates directly into higher rental income.

ADU Cost by Number of Bedrooms

Many homeowners think in terms of bedrooms rather than square footage. Here's how bedroom count typically maps to cost:

One-bedroom units between 400–600 square feet tend to hit the sweet spot for most homeowners: manageable construction cost, strong rental demand, and enough space to feel like a real home.

ADU Cost Per Square Foot: Useful Shortcut or Misleading Metric?

Cost per square foot is the most commonly searched ADU pricing metric. It's also the most commonly misunderstood.

National range: $150–$400+ per square foot in 2026. Garage conversions sit at the low end. Detached new builds in coastal metros sit at the high end. Some complex or high-end projects exceed $500 per square foot. (Sources: Angi 2026; Casita Coalition estimates $250–$500 per square foot for most projects.)

Cost per square foot also hides important variables: site conditions, design complexity, local labor markets, finish level, and utility distance. Two ADUs of identical size can differ by $100,000+ based on factors that have nothing to do with square footage.

When $/sq ft is useful: Comparing quotes from different contractors for the same project. Sanity-checking whether a bid is in the normal range for your region.

When it isn't: Comparing different ADU types, different sizes, or different locations.

How Much Does an ADU Cost by Region?

Where you live changes everything. A detached ADU that costs $180,000 in a mid-size Southern city can cost $350,000+ in the Bay Area — same size, same finish level, same ADU type. The difference is labor rates, material costs, permit fees, and local code requirements.

If you're in a high-cost market and the numbers feel steep, remember: high-cost markets also have the highest rental income potential and the strongest ADU property value premiums. A $350,000 ADU in Los Angeles that generates $3,000+/month in rental income tells a very different financial story than the construction cost alone suggests.

Where Every Dollar Goes: The Full ADU Budget Breakdown

Most people think of "ADU cost" as one number. In reality, it's dozens of line items in two categories: soft costs (everything before construction starts) and hard costs (the actual building).

Understanding this breakdown helps you budget accurately and evaluate contractor bids. If a quote seems low, check which of these items are missing.

Soft Costs: 10–20% of Your Total Budget

• Architectural design and plans: $5,000–$30,000. Pre-approved or stock plans adapted to your site can bring this to $3,000–$8,000. Custom designs with complex layouts cost more. (Sources: Angi 2026; RenoFi estimates $8,000–$13,000 for custom plans.)

• Structural engineering: $2,000–$5,000. Required for most new construction and many conversions.

• Permits and review fees: $3,000–$25,000 depending on your city. In California, permits can run $10–$12 per square foot per the Angi 2026 cost guide — a 750 sq ft ADU could cost $7,500–$9,000 in permit fees alone. Smaller cities and lower-cost states may charge as little as $3,000.

• Impact and development fees: $0–$15,000+. Fee structures vary widely by jurisdiction. In California, ADUs under 750 sq ft are exempt from impact fees under state law (California Government Code §66341), and larger ADUs can only be charged proportionally. Other states handle impact fees differently — check your local fee schedule early.

• Surveys, soil reports, geotechnical studies: $1,500–$5,000. Not always required, but common for detached builds, sloped lots, or areas with known soil issues.

Hard Costs: 80–90% of Your Total Budget

• Site preparation (grading, excavation, demolition): $5,000–$20,000. More for sloped lots, tight access, or tree removal.

• Foundation: $5,000–$20,000 for a concrete slab. Frost footings in cold climates or hillside lots requiring caissons can push this significantly higher.

• Framing and structural: $20,000–$50,000.

• Roofing: $5,000–$15,000.

• Exterior (siding, windows, doors, trim): $10,000–$30,000.

• Plumbing: $5,000–$15,000. Higher if running new lines a long distance from existing connections.

• Electrical: $3,000–$10,000. Panel upgrades for the main house may add $2,000–$5,000.

• HVAC: $3,000–$10,000. Ductless mini-split systems are the most common choice for ADUs.

• Insulation and drywall: $5,000–$15,000.

• Interior finishes (flooring, cabinets, countertops, fixtures, paint): $15,000–$50,000. This is where finish level creates the biggest swing — basic versus premium can differ by $20,000+.

• Appliances: $3,000–$8,000.

• General contractor fee: Typically 10–20% of hard costs for project management, scheduling, and subcontractor coordination. (Source: Angi 2026.)

What a Real ADU Budget Looks Like

Here's a worked example for a 700 sq ft detached ADU with mid-range finishes in a moderate-cost market:

The Hidden Costs That Surprise Homeowners

This is the section we wish every ADU cost guide led with. These are the costs that don't appear in headline numbers — and they're the reason many projects exceed initial estimates.

What Makes One Property a $90,000 Project and Another a $350,000 Project?

You now know the cost ranges. Here's what pushes a project to the low end or the high end — so you can see where yours might land.

The Cheapest Way to Build an ADU (Legally)

Not every homeowner has $300,000 to spend. If budget is your primary constraint, here are the paths that keep costs lowest — ranked from least expensive to most:

1. 1

   Interior conversion ($50,000–$150,000)

   Converting a bedroom, attached garage area, or storage space within your existing home. Minimal structural work. Lowest cost, but smallest unit and limited rental income potential. (In California, this may qualify as a JADU under state law.)

2. 2

   Garage conversion ($80,000–$175,000)

   Reuses an existing garage. No new foundation or framing needed. Best cost-to-livable-space ratio for most homeowners.

3. 3

   Basement conversion ($50,000–$150,000)

   If your basement has adequate ceiling height and isn't prone to moisture, this is one of the cheapest paths. But if you need to excavate or waterproof extensively, costs escalate quickly.

4. 4

   Attached ADU with pre-approved plans ($150,000–$250,000)

   Sharing a wall with the main house saves on foundation and utilities. Using pre-approved plans cuts design costs and permitting time.

5. 5

   Prefab or modular ADU on a straightforward lot ($150,000–$250,000)

   If your lot is flat with good access, prefab can deliver real savings and speed. Get quotes for both prefab and custom before assuming either is cheaper.

Other ways to reduce cost without cutting corners

• Keep the floor plan rectangular. Every corner, bump-out, and roof angle adds cost. A clean rectangle isn't boring — it's efficient. Good interior design makes a simple layout feel thoughtful.

• Choose durable mid-range finishes. Luxury vinyl plank, quartz counters, and porcelain tile look great, last long, and cost significantly less than hardwood, marble, and natural stone. For a rental unit, mid-range finishes are the sweet spot.

• Get at least three bids. Provide identical project scope to each contractor. Ask for itemized bids, not lump-sum quotes.

• Check for grants and incentives. California's CalHFA ADU Grant Program offers up to $40,000 toward pre-development costs. As of March 2026, the most recent funding round has been fully allocated — monitor calhfa.ca.gov for future rounds. New York's Plus One ADU Program offers capital loans and construction financing grants through NY Homes and Community Renewal. Other states and cities may offer local incentive programs — check with your planning department.

Let's Talk About Sticker Shock — and Why the Investment Works

Here's the honest thing most ADU cost guides won't say directly: building an ADU is expensive relative to expectations.

An ADU is a complete home. It has a kitchen, a bathroom, HVAC, electrical, plumbing, and must meet the same building codes as any other legal dwelling. The expensive components — a full kitchen, a code-compliant bathroom, utility systems, permits — don't get cheaper just because the building is 600 square feet instead of 2,000.

That's the real reason a "small backyard cottage" costs $200,000+. It's not that contractors are overcharging. It's that building a small legal home costs nearly as much as building the core of a bigger one.

But here's what balances the equation:

ADUs can meaningfully increase property value. When a property includes an ADU, appraisers are required to describe the unit and analyze its contribution to overall value and marketability (source: Fannie Mae Selling Guide, Section B4-1.3-05). The actual impact depends on your local market, the quality of the ADU, and buyer demand — but in markets with strong rental demand, ADUs are increasingly recognized as a significant value-add by both buyers and appraisers.

Rental income changes the math. Depending on your market, an ADU can generate $1,500–$3,500+ per month in rental income. Over time, that income can recover a substantial portion — or all — of the construction cost. (Rental income projections are estimates based on market data. Actual income depends on local rental demand, ADU size and condition, and market conditions.)

A Financial Framework (Not a Guarantee — a Starting Point)

The flexibility is permanent. An ADU can house aging parents today, generate rental income tomorrow, serve as a home office next year, and increase your property's appeal for decades. That kind of optionality doesn't show up in a simple payback calculation, but it's real — and it's the reason homeowners who build ADUs overwhelmingly say they'd do it again.

How Long Does It Take to Build an ADU?

Cost and timeline go hand in hand — a longer project means more carrying costs, more contractor overhead, and more disruption to your daily life.

Prefab and modular ADUs can compress the construction phase to several weeks of on-site work, since the unit is factory-built while permits are being processed. But permitting and site prep timelines remain the same.

Can You Build an ADU on Your Property?

ADU regulations vary not just by state but by city, county, and sometimes neighborhood. A full legal guide for every jurisdiction would fill a book. Here's what you need to know to get started.

A growing number of states have passed ADU-specific legislation that overrides restrictive local zoning. States with established ADU-friendly laws include California (Government Code §66310–66342), Oregon (ORS 197.312), Colorado (HB24-1152, effective 2025), Maine (LD 2003), and Massachusetts (Affordable Homes Act, 2024). Other states including New York, Connecticut, Hawaii, and Washington have varying levels of state-level support — verify current status with your state's housing agency or legislature.

Many other states leave ADU regulation entirely to local governments, which means your city or county may allow them, restrict them, or prohibit them. Some cities in otherwise restrictive states — like Austin in Texas — have progressive ADU ordinances.

The key local questions that determine feasibility:

• Does your zoning allow ADUs?
• What are the setback requirements?
• What's the maximum allowed ADU size?
• Is owner occupancy required? (Rules vary significantly — verify locally.)
• Are there parking requirements?
• What are the utility connection requirements?
• Are there HOA or deed restrictions?

Fannie Mae's current Selling Guide defines ADUs as secondary dwelling units on the same parcel as a primary residence, including over-garage, basement, and manufactured-home configurations — confirming that major mortgage institutions recognize ADUs as legitimate property features. (Source: Fannie Mae Selling Guide, Section B2-3-04.)

ADU vs. Home Addition vs. Buying a Second Property

If you're weighing options, here's how they stack up:

ADUs and home additions solve different problems. An ADU is an independent dwelling — it has its own kitchen, bathroom, entrance, and can generate rental income. An addition extends your primary home. Compare them on independence, rental potential, privacy, and total scope — not just a single metric.

Compared to buying a second property, an ADU avoids a second mortgage, second set of property taxes, and the complexity of managing a property across town. You build on land you already own.

Financing an ADU: Common Paths Homeowners Explore

Building an ADU is a significant investment, and most homeowners don't pay cash. Here are the most common financing approaches — framed as education, not recommendations. (The Dwelling Index is not a lender or financial advisor. Consult a qualified financial professional for advice specific to your situation.)

Fannie Mae currently allows rental income from an existing, occupied ADU on a one-unit principal residence to be considered in certain qualifying scenarios, subject to documentation requirements and a cap of 30% of total qualifying income. (Source: Fannie Mae Selling Guide, Section B3-3.8-01.)

How to Budget and Plan Your ADU Project: Step by Step

If you've read this far, you have a solid understanding of ADU costs. Here's how to turn that knowledge into action:

1. 1

   Verify your property's eligibility.

   Contact your local planning department or use our eligibility tool to confirm zoning, size limits, setbacks, and utility requirements.

2. 2

   Decide on ADU type.

   Based on your property, budget, and goals (rental income? family housing? flexibility?).

3. 3

   Explore financing early.

   Talk to a lender before you hire a designer. Knowing your budget ceiling prevents wasted design fees.

4. 4

   Choose a design path.

   Pre-approved plans (fastest, cheapest), stock plans modified for your site (moderate), or full custom design (most flexible, most expensive).

5. 5

   Get 3+ contractor bids.

   Provide identical scope to each. Demand itemized bids. Compare line-by-line.

6. 6

   Budget with 15–20% contingency.

   This is non-negotiable.

7. 7

   Submit for permits.

   Your designer or contractor typically handles this.

8. 8

   Build.

   Maintain regular communication with your general contractor.

9. 9

   Final inspection and certificate of occupancy.

   Don't skip this — an unpermitted ADU creates legal, insurance, and resale risk.

10. 10

    Move in, rent out, or enjoy.

Methodology: How We Estimate ADU Costs

Frequently Asked Questions