How much does it cost to build an ADU?
Ashburn, VA

How much does it cost to build an ADU?

Ashburn, VA

How much does it cost to build an ADU?

$60,000 – $180,000 average total cost (600 SF)

Get free estimates for your project or view our cost guide below:

$60,000 – $180,000 average total cost (600 SF)


Get free estimates for your project or view our cost guide below:
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Tamatha Hazen
Written by
Tamatha Hazen
Edited by
Kristen Cramer
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Tara Farmer

Average cost to build an accessory dwelling unit

An accessory dwelling unit (ADU) costs $100 to $300 per square foot for an attached, detached, or converted secondary living space. A 500-square-foot ADU costs $50,000 to $150,000. A larger 1,200-square-foot ADU costs $120,000 to $360,000. Prefab ADUs cost less and take less time to build.

Accessory Dwelling Unit (ADU) cost by size - Chart
Accessory Dwelling Unit (ADU) cost by size - Chart
ADU cost by size
ADU size (square feet) Average cost
400 $40,000 – $120,000
500 $50,000 – $150,000
600 $60,000 – $180,000
750 $75,000 – $225,000
1,000 $100,000 – $300,000
1,200 $120,000 – $360,000

A small cottage ADU built in a home's backyard
A small cottage ADU built in a home's backyard
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Cost to build an ADU by type

An Accessory Dwelling Unit (ADU), also called a granny flat, in-law suite, or accessory apartment, costs $100 to $300 per square foot on average to build. The self-contained living space is located on the same property as the main residence and is equipped with a kitchen and bathroom for permanent residential use.

ADU costs vary depending on whether the unit is attached to your existing home, a detached freestanding unit, or converted from an existing garage or basement space. Prefab units are factory built, which saves money over using traditional construction methods.

Cost to build an ADU by construction type
Type Average cost per SF Description
Prefab modular $80 – $160 Separate structure built in a factory and assembled onsite
Attached $125 – $225 Connected to the primary residence, often through a shared wall or extension
Detached $150 – $250 Separate structure from the main house, typically located in the backyard but not physically attached to the main house
Above garage $200 – $500+ Built over an existing garage by adding a new second-story structure on top
Basement conversion $50 – $100+ Located in the basement of the main house either by converting the existing space or excavating to create a basement ADU
Garage conversion $50 – $200 Built by repurposing an existing garage space into a living space, requiring modifications to make it habitable

Prefab ADU

A prefabricated ADU costs $80 to $160 per square foot or $50,000 to $100,000 for a 600 square foot unit. Prefab units cost less because they are built more efficiently in a factory setting, without delays caused by weather, material deliveries, or labor scheduling issues.

Attached ADU

An attached ADU costs $125 to $225 per square foot or $75,000 to $135,000 for a 600 square foot for a home addition that includes a bedroom, living room, full kitchen, and bath for long-term living.

Attached ADUs cost more than a garage or basement conversions because the construction requires pouring a foundation extension and new framing. They cost less than a detached ADU since they have easier access to the existing HVAC, plumbing, and electrical systems.

Detached ADU

A detached ADU averages $150 to $250 per square foot or $90,000 to $150,000 for a 600 square foot freestanding unit. Costs can easily reach $250,000 or more when including high-end finishes. Detached units require extra labor and materials to deliver electricity, plumbing, and HVAC to the building.

Detached units are frequently called granny flats or granny cottages because they are constructed away from the main house and serve as full-time living quarters for older family members. These cottages are popular because they are close enough to keep an eye on aging parents while allowing them privacy.

A small green ADU and guest house in a home's backyard with a flagstone pathway
A small green ADU and guest house in a home's backyard with a flagstone pathway

Above garage ADU

Building an ADU above an existing garage costs $200 to $500 per square foot to add a second story and finish the space for full-time living. An above-garage ADU costs $120,000 to $300,000 for a 600 square foot complete living unit.

Some benefits of installing the ADU above your garage include:

  • The second story apartment doesn’t take up any valuable lawn area.

  • The proximity to wiring and piping from the main house saves you money on construction costs.

  • The added living space increases your home’s square footage and property value.

  • The separate living quarters are an excellent option for multigenerational living.

Basement conversion ADU

Finishing a basement costs $50 to $100 per square foot to create a basement ADU with a living area, bedroom, bathroom, and kitchen. On average, a 600 square foot basement ADU costs $30,000 to $60,000, depending on the finishes you select.

  • You’ll spend less if you already have a finished basement that just needs a kitchen and bathroom to make it a complete living unit.

  • You’ll spend more if you need to excavate to dig and build an entire basement addition.

Garage conversion ADU

Converting a garage costs $50 to $200 per square foot to turn an unused garage into a functional secondary dwelling unit. A full conversion costs $20,000 to $140,000, depending on the size of your garage and the fixtures and finishes you include.

A garage conversion ADU project takes 2 to 12 months and involves adding new floors, walls, windows, insulation, drywall, electrical, plumbing, and HVAC. A garage conversion may decrease the home value in regions where garage parking space is highly valued.

A garage converted into an accessory dwelling unit (ADU)
A garage converted into an accessory dwelling unit (ADU)

ADU cost factors

The cost of building an accessory dwelling unit (ADU) can vary depending on a number of factors, including:

  • Size – Larger ADUs use more materials and take longer to build, increasing labor costs.

  • Zoning – Some areas have specific ADU regulations, which may include permitting fees, impact fees, and other costs to meet local requirements.

  • Location – Building an ADU closer to existing utility connections is less expensive than locating it further away, where additional infrastructure is needed.

  • Prefab vs. custom build – A prefab or modular ADU is cheaper and faster to install since it is delivered with the structural shell fully assembled and many of the interior and exterior finishes already complete.

  • Amenities – The more amenities you include, the higher your cost but the more desirable the home will be to renters or long-term guests. Common amenities include:

    • Adding large windows, French doors, or skylights create a spacious feel.

    • Adding a dishwasher and laundry facilities makes the home more convenient.

    • Adding a fireplace and HVAC keeps guests comfortable year-round.

  • Interior finishes – High-end fixtures and finishes, such as granite countertops, stainless steel appliances, and hardwood floors will increase the cost of the project.

  • Permits and construction delays – Unexpected holdups due to permits, inclement weather, or delays in getting materials can lead to increased labor costs.

Planning and site prep

Before you can build an ADU on your property, you’ll need to spend some time and money planning your ADU design and preparing the home site for construction. The following table details common costs.

ADU planning and site prep costs
Factor Average cost Details
House plans $1,200 – $5,000 Includes drafting fees plus input from an engineer or architect
Land survey $400 – $1,800 Includes marking foundations, utilities, boundaries, and topography services before building
Home site excavation $1,500 – $5,000 Includes digging a foundation pad for the new ADU site, with prices depending on the dirt type, depth, terrain, and amount of earth moved
Grading and leveling $1,000 – $5,000 Includes regrading the backyard for an ADU addition
Foundation $2,400 – $10,500 Includes costs for pouring a concrete slab foundation for the ADU
Building permits $500 – $2,000 Permitting costs vary by location and ADU size.

Labor costs

Labor alone makes up 40% to 60% of your ADU project costs. A custom-built ADU uses more professional labor, including designers, architects, engineers, general contractors, electricians, and plumbers. A prefab ADU uses less professional labor, but a general contractor is recommended to manage the installation.

ADU alternatives

Building a guest house costs $150 to $250 per square foot for a custom built space or $80 to $160 per square foot for a modular prefab guest house. Unlike an ADU designed for long-term occupancy, a guest house is designed to accommodate short-term guests or visitors.

Because a guest house is for temporary stays, it doesn’t require a full kitchen or bath. However, many people add a bathroom, living area, and kitchenette for guest comfort and convenience. Some common ADU alternatives are detailed in the table below.

ADU alternative costs
Options Average cost per SF Description
Modular guest house cost $80 – $160 Manufacturers construct the individual sections, or modules, in a factory then transport and assemble on your property.
Modular home addition cost $90 – $210 Modular sections built off-site are attached to the side, rear, or upper level of an existing home.
Tiny house cost $150 – $450 A custom-built or prefabricated house or studio shed that is usually 400 square feet or smaller
Shipping container guest house cost $150 – $300 A factory or custom-built guest quarters using one or more repurposed metal shipping containers
Prefab or modular log cabin cost $100 – $250 Built in a factory and assembled on site in one or two fully finished pieces
Metal building conversion cost $25 – $80 Creates a finished living space inside an existing metal shed or building
Pole barn conversion cost $20 – $60 Creates a finished interior living space inside an existing pole barn structure
Yurt cost $25 – $110 A round, tent-like structure with fabric walls, typically 20’ to 30' in diameter

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A prefab tiny home guest house or ADU
A prefab tiny home guest house or ADU

Accessory dwelling unit FAQs

What is an ADU?

An ADU, or Accessory Dwelling Unit, is a secondary, self-contained living unit located on the same property as the primary home. The unit is designed for full-time, permanent residential living and must have a full kitchen and bathroom.

Is an ADU the same as a guest house?

An ADU and guest house are not the same thing. Both are additional housing structures built on a residential property, but an ADU is for permanent, long-term occupancy. To make the space livable for family members or renters, the unit must include a full bathroom and full kitchen.

In contrast, a guest house is designed for short-term guests or visitors. A kitchen and bath are not required since the guests can use the facilities inside the main home during their short stay.

Can I build an ADU on my property?

Local zoning regulations, building codes, and property-specific conditions dictate whether you can build an ADU on your property. Consulting with local architects or home builders experienced in ADU construction can be helpful in navigating the process.

How much does an ADU increase property value?

In many cases, a detached ADU will increase the value of your home. In areas with a high demand for housing, an ADU can be highly desirable for renters or potential buyers. A well-constructed ADU with modern amenities is more likely to have a positive impact on property value than a low-quality unit.

How to build an ADU for cheap?

To save money on building an ADU for your property, consider the following suggestions:

  • Choose a simple design – Stick with a straightforward, functional design with limited architectural features that increase construction costs.

  • Size matters – Smaller units are cheaper to build, so consider building the smallest unit that meets your needs and local regulations.

  • DIY when possible – Do as much of the work as you can to save money on labor costs. Simple tasks like painting, landscaping, and interior finishing are costly when using professional labor.

  • Order a prefab ADU – A prefabricated or ADU kit is more cost-effective and quicker to assemble than traditional construction.

  • Choose affordable materials – Opt for cost-effective building materials and finishes when possible.

  • Share utilities – Building the ADU near your primary home and sharing utilities like water, gas, and electricity from the main house saves money.

Getting estimates from an ADU contractor

If you are considering adding an ADU to your property, first check to see if your neighborhood and city zoning regulations allow it. Then, follow these suggestions to get the best ADU contractor near you for your project:

  • Decide whether a prefab or custom-built ADU is best for your needs and budget.

  • Get several estimates with the same ADU size and features to compare.

  • Check out their recent reviews on HomeGuide and Google.

  • Ask to visit a job site to see their work in progress.

  • Pick a contractor experienced in ADU construction.

  • Request an itemized contract in writing in case of disputes.

  • Ask your builder about their preferred lenders for easier financing.

  • Avoid large upfront payments. Never pay in full or in cash.

Questions to ask a prospective builder

Ask these important questions to narrow down the list of builders near you for your guest house or ADU:

  • How many years have you been involved in the ADU construction industry?

  • How many guest houses or ADUs have you built within the past year?

  • Do you offer design assistance for creating a floor plan?

  • Can you share a list of references along with their contact information?

  • May I review your portfolio showcasing completed ADU projects?

  • Could you provide information on the locations of the homes you are currently building?

  • Do you hold the necessary licenses, insurance, and bonding?

  • What is your procedure for addressing any job-related damage?

  • Who will be designated as my project coordinator, and do you use subcontractors?

  • What are the working hours for you and your construction team on-site?

  • How long will the ADU construction take?