How Much Does an Accessory Dwelling Unit (ADU) Cost to Build? [2026 Data]

In 2026, the national average cost to build an accessory dwelling unit (ADU) is $180,000, with most homeowners spending between $60,000 and $285,000. This breaks down to $150 to $300 per square foot. Whether you call it an in-law suite, granny flat, or accessory apartment, an accessory dwelling unit is an excellent way to create extra living space with enough privacy for family members or rental tenants. This guide will help you budget for the job.

How Much Does an ADU Cost by Type?

The type of ADU you choose is one of the biggest factors shaping your total project cost, with options ranging from basement and garage conversions ($60,000 to $150,000) to fully detached new construction ($110,000 to $285,000). Conversion projects tend to cost less because they use existing structure, while new construction—especially detached builds—typically sits at the higher end of the range. Here's a breakdown of average costs by ADU type, not including permits and other fees:

How Much Does an ADU Cost by Square Foot?

An ADU costs $150 to $300 per square foot on average, though some builds exceed $600 per square foot depending on materials and finishes. Most ADUs fall between 600 and 1,200 square feet, which is the typical range for a comfortable, code-compliant living space. Larger or more complex builds naturally push costs higher, while smaller footprints can help keep the project within a tighter budget. Here's how ADU costs break down by square footage:

How Much Does an ADU Pro Cost?

Hiring a general contractor is a necessary part of building an ADU, and general contractors charge between 10% and 20% of the total project cost. That translates to $50 to $150 per hour or $300 to $500 per day, though most general contractors don't bill hourly. Instead, they typically apply a percentage-based fee that covers sourcing materials, managing subcontractors, and overseeing the entire build from start to finish.

Designing and constructing an ADU or home addition is a significant project that requires a wealth of professional expertise. Your general contractor isn't just responsible for getting the job done. They'll also need to find competent subcontractors, source materials, and manage the team.

Additional Pros to Consider Hiring 

Labor accounts for around 40% of your total ADU project cost, with specialized trades making up the bulk of that figure. Hiring an architect, plumber, and electrician typically accounts for about 20% to 30% of labor costs, while your general contractor covers the remaining 10% to 20%. Here's a breakdown of fees per contractor:

• Architect fees: $125–$250 per hour

• Structural engineer fees: $70–$250 per hour

• Subcontractors: $150–$250 a day per additional helper

• Electrician fees: $50–$100 per hour

• Plumber fees: $45–$200 per hour

ADU Costs by Location: Where Prices Vary Most

Where you build your ADU directly impacts cost, primarily because labor rates rise with the local cost of living. That said, some states and cities offer financial incentives to offset those costs—for example, the California House Finance Agency (CalHFA) provides a limited number of grants of up to $40,000 to help cover predevelopment and nonrecurring closing costs. New York's Plus One ADU Program similarly helps homeowners build or improve ADUs on their property. Research your specific area before finalizing your budget to uncover any available aid or incentives. Refer to the table below for average ADU costs in a few locations:

ADU Additional Costs: What to Budget For

Building an ADU involves several costs beyond the base construction price, including land leveling and foundation work, as well as plumbing and electrical installation. Each phase of the project adds to your overall budget, and overlooking any one of them can lead to unexpected expenses. Planning these additional line items up front gives you a more accurate picture of what your ADU will actually cost.

Design Costs

Design fees alone will run 10% to 15% of your total ADU project cost. On a $120,000 project, that means setting aside $12,000 to $18,000 just for design work. Factoring this in early prevents sticker shock when you receive your first contractor quotes.

Materials

Materials are the single largest cost category in an ADU build, typically accounting for 45% to 50% of the total project cost. This includes lumber, sheathing, joists, roofing, windows, doors, trim, and more. Choosing durable, mid-range materials from the start can help you avoid costly replacements down the road.

Site Prep

Site preparation is a required first step before any ADU foundation can be built, and land leveling and grading to clear landscaping and trees costs $1,000 to $5,800 depending on the size of the area. Larger or more heavily wooded lots will push costs toward the higher end of that range. Getting a site assessment early helps you anticipate grading costs before breaking ground.

Foundation

A concrete foundation costs $5 to $40 per square foot, with most standard 600- to 1,200-square-foot ADUs running between $2,400 and $30,000. Garage and basement conversions often skip this expense entirely since they use the existing structure. Above-garage ADUs, however, frequently require additional structural support, which can add to your foundation budget.

Framing

Framing an ADU costs around $25,000 on average, with 1,000 board feet of lumber running $350 to $500. Metal framing is a durable alternative, but expect to pay roughly 10% more than standard wood framing. If longevity and resistance to moisture or pests are priorities, the added cost of metal framing can be worth the investment.

Finishing

Finishing your ADU typically runs $10 to $60 per square foot, but high-end materials and quality appliances can push that figure to over $150 per square foot. The cost to finish a basement converted into an ADU ranges from $3,500 to $34,500, depending on the scope of work. Insulation, drywall, flooring, ceilings, stairs, fixtures, and appliances all factor into your finishing budget.

Systems

Every ADU must include electrical, plumbing, and HVAC systems to meet code, and these three systems cost $3,400 to $10,400 on average. Local and state regulations may impose additional requirements—for example, California mandates solar panels on new ADUs. Here's the breakdown of each system:

• Running electricity cost: $1,000–$3,800

• HVAC system cost: $6,000 for a small furnace and AC unit 

• Plumbing system cost: $900

• Solar panel system cost: $2,500–$3,500 for a standard 2.5 kW solar panel (optional, depending on where you live)

Permits

Building permits average $1,350 nationally, but your location can significantly change that figure. In California, ADU permits are priced at $10 to $12 per square foot, which means a 750-square-foot ADU will cost between $7,500 and $9,000 in permits alone. Researching your local permit requirements before finalizing your budget helps you avoid a costly surprise mid-project.

Post-Construction Cleanup

Post-construction cleanup costs $0.10 to $0.50 per square foot, which translates to $60 to $600 for a standard-sized ADU. Larger or more complex builds tend to generate more debris, pushing costs toward the higher end of that range. Budget for this line item early so it doesn't catch you off guard at the end of the project.

Waste Disposal

Waste disposal after an ADU build runs $100 to $600, with full truckloads pushing costs to the top of that range. Renting a dumpster costs $300 to $800 per week and may be an additional expense if you need to store post-construction waste before hauling it off. Factor both cleanup and disposal into your budget as separate line items to get an accurate final cost.

Prefab ADU vs. Custom

Prefab ADUs come fully constructed or in partially constructed kits, similar to modular homes, container homes, or yurts. The main benefit is the cost—you can save up to 20% on the project cost if you go with a prefab ADU. In addition, prefabricated home retailers have clear-cut price estimates and still allow for some customization, helping you limit surprise expenses while creating a place that feels like your own.

Why You Should Hire an ADU Building Pro

If you're building a custom ADU from scratch, it's best to hire a local home addition builder or general contractor who builds ADUs. Handy homeowners may be able to DIY a prefab ADU, but you may still need to hire pros to run utilities so they're code-compliant.

Here’s why it’s ideal to hire an ADU building pro

Hiring a pro can save you time and frustration—here’s why:

• Building an ADU can be as complex as building a house, but on a smaller scale, and pros have experience with the process from beginning to end.

• A licensed builder has the equipment and tools needed for excavation and building.

• Builders have relationships with subcontractors and can streamline the building process and coordinate with everyone involved.

• A builder will ensure your ADU complies with all local regulations and codes. 

• Builders can complete the job efficiently, saving time and keeping things on schedule if unexpected challenges arise.

If you want to DIY building an ADU

If you’re confident in your DIY skills, here are some things you can do to be involved in the process:

• Build a prefab ADU from a kit

• Install doors, windows, fixtures, flooring, and other features after the ADU is framed and built

• Landscape around the ADU and install outdoor lighting

• Build a porch, deck, or patio for the finished ADU

3 Ways to Save on ADU Building Costs

With the high investment involved in building an ADU, you might want to find ways to save on the cost of your project. Here are three ways you can save:

1. Choose a prefabricated design instead of a custom design. 

2. Pick long-lasting materials like metal framing and good insulation for a strong and durable structure. 

3. Avoid unnecessary add-ons, like upgraded appliances and crown molding, when finishing your ADU.

Does Adding an ADU Increase Home Value? 

Since ADUs create more livable space on your property, they add value to your home like an addition. A study from the National Association of Realtors® found that ADUs add an average of 35% to a home’s value, but it depends on the area.

According to the study, ADUs held the most value in large cities, and some areas even saw homes with ADUs listed for 200% more than average. However, ADUs with homes were listed for less than average in some areas, so be sure to research your area.

If you're planning to build an ADU, it’s a good idea to talk to a realtor to get a sense of what’s desirable in your specific area.

How Angi Gets Its Cost Data

Home is the most important place on earth, which is why Angi has helped more than 150 million homeowners transform their houses into homes they adore. To help homeowners with their next project, Angi provides readers with the most accurate cost data and upholds strict editorial standards. We extensively research project costs to develop the pricing data you see, so you can make the best decisions for you and your home. We rely on reputable sources, including the U.S. Bureau of Labor Statistics, academic journals, market studies, and interviews with industry experts—all to ensure our prices reflect real-world projects. 

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