How Much Does an Accessory Dwelling Unit (ADU) Cost to Build? [2025 Data]

The average accessory dwelling unit (ADU) costs $180,000, but most homeowners spend between $40,000 and $360,000 building their unit. The average cost by size is $150 to $300 per square foot. Prices range widely due to different types of construction, including converting a basement or garage into an apartment or building a free-standing structure on your property.

Whether you call it an in-law suite, granny flat, or accessory apartment, ADUs are an excellent way to create an extra living space with enough privacy for family members or rental tenants. This guide will help you budget for the job.

Building an accessory dwelling unit on your property requires many decisions, each of which can increase or decrease the project's total price. Below is a breakdown of some of the most important cost factors for building an ADU.

ADU Costs by Type 

There are various types of ADUs to choose from, all of which impact the total cost of your project. See how much you might spend to build an ADU on your property, not including permits and other fees, for some of the most common ADUs.

Size

An ADU costs about $150 to $300 per square foot on average, but some ADUs cost over $600 per square foot. The average-sized ADU is between 600 and 1,200 square feet, but your building’s size may vary with personal preferences. 

General Contractor Costs 

When building an ADU, you’ll need to hire a general contractor. General contractors charge between 10% and 20% of the total project. This equates to $50 to $150 per hour or $300 to $500 per day, though most general contractors don’t charge hourly.

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 alone costs around 40% of your ADU project’s cost. Hiring an architect, plumber, and electrician will account for about 20% to 30% of this cost, while hiring a general contractor accounts for the other 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

Location 

You’ll pay more for an ADU in areas with a higher cost of living, mainly because of labor costs. That said, certain areas may have incentives and grants to build ADUs.

For example, the California House Finance Agency (CalHFA) offers a limited number of grants of up to $40,000 to help with predevelopment and nonrecurring closing costs associated with ADU construction. Similarly, New York has a Plus One ADU Program that helps homeowners build or improve ADUs on their property.

Research your specific area to learn about any potential aid or incentives.

Additional ADU Costs to Consider 

Building an ADU is a big undertaking that comes with several additional costs. Everything from leveling the land and building the foundation to installing the plumbing and electricity will add to the overall cost. Here are some things to consider when planning your budget.

Design Costs

Simply designing your ADU will cost 10% to 15% of the total cost of your project. In other words, if your project costs $120,000, then $12,000 to $18,000 goes to designing the ADU. 

Materials

The materials for building an ADU will account for about 45% to 50% of the total cost of your project and include lumber, sheathing, joists, roofing, windows, doors, trim, and more.  

Site Prep

Like with any home addition, you’ll need to do site prep before building a foundation for your ADU. You’ll need land leveling and grading to clear away landscaping and trees, which costs $1,000 to $5,800, depending on the size of the area. 

Foundation

The concrete foundation costs between $5 and $40 per square foot, with most standard 600- to 1,200-square-foot ADUs coming in between $2,400 and $30,000. Garage conversion ADUs and basement ADUs might not need an additional foundation, so you can avoid these expenses. However, above-garage ADUs often require additional structural support for a stable foundation.

Framing

Framing your ADU costs around $25,000 on average, with 1,000 board feet of lumber costing anywhere from $350 to $500 on average. If you choose metal framing, anticipate spending around 10% more on the cost of the framing.

Finishing

Finishing your ADU ranges between $10 and $60 per square foot, but can increase to over $150 per square foot for high-end materials and high-quality appliances. The cost also depends on the type of ADU you’re building and how much you’re willing to invest. For example, the cost to finish a basement converted to an ADU is between $3,500 and $34,500.

Insulation, drywall, floors, walls, ceilings, stairs, appliances, fixtures, and other finishing touches can all increase your ADU pricing.

Systems

Your ADU will need three main systems to be up to code: electrical, plumbing, and HVAC. These systems cost $3,400 to $10,400 on average. You’ll also want to check your local and state regulations for other requirements. For example, California requires ADUs to have solar panels.

Here’s the breakdown of each system:

• Running electricity cost: Running electricity to an ADU costs $1,000–$3,800.

• HVAC system cost: A small furnace and AC unit cost a combined average of $6,000. 

• Plumbing system cost: The average cost to add plumbing to an addition is $900.

• Solar panel system cost (optional, depending on where you live): A standard 2.5 kW solar panel adds $2,500 to $3,500 to your project. 

Permits

Permits can throw a giant curveball at your project’s cost if you’re unprepared. The average cost of a building permit is $1,350, but where you live plays a major role in this expense. For example, the average cost of a building permit for ADUs in California is between $10 and $12 per square foot. So, an average-sized 750-square-foot building will cost between $7,500 and $9,000 in California.

Post-Construction Cleanup

The post-construction cleanup costs between $0.10 and $0.50 per square foot. For standard-sized ADUs, this will cost between $60 and $600. The bigger the mess, the more you’ll pay to have a cleaning crew take care of the job.

Waste Disposal

After the cleanup, the cost to dispose of the waste can range from $100 to $600. If the waste takes up an entire truckload, expect the cost to increase to the higher end of the price range. Renting a dumpster costs $300 to $800 per week and may be an additional expense if you need to store your post-construction waste before hauling it off. 

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.

Cost to Build an ADU Yourself vs. Hiring a Pro 

You could save between $24,000 and $144,000 if you decide to complete an ADU project yourself, which is roughly 40% of your project’s total cost. While this might seem like the perfect way to save on costs, building an ADU is a laborious process best left to a home additions contractor near you.

We don’t recommend trying to DIY an ADU unless you have extensive experience managing projects of this size and carpentry skills to match. Otherwise, you’ll risk building things that don’t comply with local codes and may cause injury. Your best bet is to hire a contractor who builds home additions or ADUs to ensure your addition is built safely and to code.

If you have the necessary experience, your best bet is to go with a prefabricated ADU, which tends to cost less. Prefab ADUs come either fully or partially constructed, making them easier to install than building a unit from scratch.

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. 

Want to help us improve our cost data? Send us a recent project quote to [email protected]. Quotes and personal information will not be shared publicly.