How Much Does an In-Law Suite Cost?

Typical Range:

$40,000 - $130,000

Find out how much your project will cost.

Cost data is based on research by HomeAdvisor.

Updated September 21, 2022

Reviewed by Robert Tschudi, Expert Home Building and Remodeling Contributor.

Written by HomeAdvisor.

The cost to build an in-law suite addition ranges from $40,000 to $130,000, with the average hovering around $82,500. An in-law suite is an additional living space on an existing property, complete with a living area, bedroom, kitchen, and bathroom. It’s also called a mother-in-law house, a granny apartment, or an accessory dwelling unit (ADU) if detached and separate from the main home.

Average Cost to Build an In-Law Suite Addition

Average CostHigh CostLow Cost

In-Law Suite Addition Costs by Type

If you’re planning to have family members move into your home, you can convert existing rooms, including the basement or attic, into a functional suite. To add a rental property, you might consider turning your garage into an ADU or building a new structure.

Addition TypeAverage Cost
Converting a Finished Room$10,300 – $10,750
Extending a Room$20,600 – $54,800
Converting a Garage$20,000 – $60,000
Converting a Basement$10,000 – $30,000
Converting an Attic$40,000 – $100,000
Building an ADU$40,000 – $125,000

Converting a Finished Room

You can turn an existing, finished room in your home into an in-law suite for about $10,300 to $11,250. This includes the cost to add a bathroom or thecost to build a kitchen, both needed to convert an existing room into a fully functional in-law suite. Each of these additions costs about $5,000 minimum. Installing new walls costs $1,800 per wall on average.

Extending a Room

The cost to bump out a room for an in-law suite is $20,600 to $54,800. More than simply converting a room, an extension also involves expanding on the foundation at $75 per yard, excavating around the home’s perimeter at $2,600 on average, installing insulation and wiring, and matching exterior paint or siding. Extending a second-floor room costs even more than a first-floor room.

Converting a Garage, Basement, or Attic

If you aren’t ready to dig up precious space in the backyard to build an ADU, consider converting existing unused parts of your home into an in-law suite. Keep in mind that each type of space will need to be outfitted with drywall, insulation, flooring, HVAC, a water heater, plumbing, gas line or 220 outlet, private entrance, bathroom, and kitchenette to function as a separate dwelling.

  • Garage: With a foundation already in place, you can convert a garage to an in-law suite for $20,000–$60,000.

  • Basement: By tapping into the main home’s plumbing, the cost of a basement remodel for an in-law suite can be as little as $10,000–$30,000.

  • Attic: An attic conversion is more expensive at $40,000–$100,000 due to the need for reinforced floor joists and a safe stairway.

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Building an ADU

The average cost to build a guest house or an ADU on your property is about $82,500, but typically ranges from $40,000 to $125,000 or more depending on your design preferences and needs. In some regions, you could pay up to $400,000 for an average single-story detached ADU.

In-Law Suite Addition Costs by Size

You can expect to spend about $100 to $200 per square foot for the cost of an in-law suite, including materials and labor.

  • 400 square feet: A small, 400-square-foot suite costs $40,000–$80,000

  • 500 square feet: A 500-square-foot in-law suite costs $50,000–$100,000

  • 600 square feet: A 600-square-foot addition costs $60,000–$120,000

  • 750 square feet: In many municipalities, this is the maximum size allowed for an ADU. A 750-square-foot suite would cost $75,000 to $150,000.

In-Law Suite Addition Costs for Labor

To build an in-law suite, you’ll need to hire several professionals, including architects, electricians, HVAC/mechanical technicians, and plumbers.


Hiring local architectural services will cost about 10% to 20% of the project’s total cost. For example, for a $100,000 ADU, you’d spend an additional $10,000 to $20,000 for an architect. Hiring an architect will help you plan an efficient, functional in-law suite. If you’re moving in older family members with limited mobility, discussing universal, accessible design with the architects is important. 


A new in-law suite will need electrical work, which costs an average of $50 to $100 an hour. Work includes essential changes to your home’s current electrical system to bear the load of multiple families living under one roof. For safety, the unit needs lighting leading to an outdoor exit, as well as smoke alarms and carbon monoxide detectors connected to the main home.


Both the bathroom and kitchen of the suite will require new plumbing, and the bathroom must connect to the sewage system. Plumbers charge about $200 an hour for these intensive jobs.

  • Water main installation: $600–$2,500

  • Plumbing pipes: $300–$2,000

  • Sewer main installation: $1,500–$5,000


Depending on the size of the space, HVAC (heating, ventilation, and air conditioning) can be a relatively inexpensive extension of your existing systems, or require the installation of a new system. A new HVAC system costs around $7,500 on average.

“Many of our clients choose what is called a ‘mini-split’ system,” says Bob Tschudi, Expert Review Board member and Raleigh, NC-based general contractor. “These systems are great at heating and cooling smaller spaces and cost a fraction of a traditional HVAC system with ducts, vents, and air returns.”

General Contractors

You can hire local addition and remodeling contractors for many parts of the process, such as building the structure, painting the interior and exterior, laying floors, and installing windows. You’ll typically have a general contractor oversee the entire project, which costs 10% to 20% of the total. Additional workers who build, install, and paint cost about $100 to $300 per laborer, per day.

In-Law Suite Addition Material Prices

An in-law suite is similar to an apartment or tiny home, meaning you need many materials to build this structure inside and out. You’ll need everything from wall support beams to new floors to bathroom fixtures and kitchen stoves. The material costs to build an addition will vary depending on your design choices.

  • Support beams and roof trusses: For new construction, whether an extension of your home or a new structure on the property, you’ll need to build out the supportive frame. This will cost $15 to $30 per square foot.

  • Drywall and insulation: Adding drywall will cost $400–$600 for every 100 square feet, and insulation will cost $0.30–$1 per square foot.

  • Foundation materials: Building a new structure on your property requires a foundation or a load-bearing base level. This will cost $400–$700 for 100 square feet.

  • Bathroom plumbing: The in-law suite needs its own bathroom, whether built into an existing room or added to a new structure on your property. Plumbing materials, like pipes, will cost $1,500–$4,000 for 100 square feet.

  • Plumbing fixtures: New plumbing fixtures, like sink faucets, sinks, showerheads, bathtubs, and toilets, will vary widely in cost depending on your preferences. Expect to spend around $400 to $2,000 for each plumbing fixture.

  • Electric meter: You’ll need to install a separate electric meter for a detached property, especially if the person living there will be paying their own utilities. A new meter will cost $2,000–$5,000.

  • Water heater: A water heater is another necessity for a detached suite, and it will cost anywhere from $600–$3,000.

  • Paint: Adding new paint to an in-law suite will range from $5,000 to paint the interior and an additional $8,000 for the exterior.

  • Siding: Siding for an ADU costs about $10,000. The average is around $12 per sq. ft.

  • Windows: Expect to spend anywhere from $100 to $1,500 per window.

  • Doors: New doors for the interior and exterior will range from $500 to $5,000 each.

  • Floors: Adding new floors will cost around $5 to $20 per square foot, with options like hardwood or marble costing more, while laminate or vinyl will be less expensive.

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Benefits of Adding an In-Law Suite

An in-law suite is a flexible addition to your property that can offer many benefits. On the one hand, you can bring multiple generations of family under one roof, giving elder parents accessibility and independence. On the other hand, you can generate income with a rental property and increase your home’s value.

DIY In-Law Suite Addition vs. Hiring a Pro

There’s no way around it: you need to hire multiple professionals to build an in-law suite addition. This process requires permitting, excavations, construction, electrical work, HVAC, and plumbing, so you need to work with several experts to do the job. Start by finding the right local general contractor. They’ll help you manage the project and assemble the team you need.

If you convert an existing room or basement into an in-law suite, you might save several thousands of dollars on labor costs by taking on the project as a DIY. But even converting a room has risks—especially if you need to install a new bathroom for the resident. It’s crucial to invest in skilled professionals to add an in-law suite to your home or property.

Cost to Build an In-Law Suite Addition Yourself

Building an addition is a huge undertaking that requires construction, plumbing, HVAC, and electrical know-how. You’ll need to consult several contractors to complete this project; we do not recommend that you DIY this project. Even an experienced DIYer should leave this huge and dangerous job to professionals.

However, contractor labor costs make up about 30% to 50% of your total budget for a home addition project. You can take on certain tasks yourself (depending on your skill level) to cut some costs. For example, handy homeowners can lay flooring as a DIY for $0.50 to $20 per square foot. You can also do the interior painting for just $200 to $300.

If you’re interested in saving money on your project, be sure to do research in your area. Compare similar properties near you and aim to spend less on your build than on the projected increase in your home’s value, which can reach up to 30%.

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Frequently Asked Questions

Can I build an in-law suite in my backyard?

Before building an in-law suite on your property, you’ll need to check your local laws. Many communities do not allow accessory dwelling units at all, while other cities may require the suite to include specific features, such as separate water and sewer connections or an additional entrance that is separate from the house if you are building a suite connected to an existing home.

Will an in-law suite impact the value of my home?

An in-law suite can double as a rental property or simply add square footage to your home, making it attractive if you plan to sell your home in the future. You could see up to a 30% increase in your property value by investing in an in-law suite addition.

“We have had several clients request the construction of an ADU not for relatives, but for additional income, via short-term rental services, such as Airbnb or Vrbo,” says Tschudi. “Check with your municipality and your homeowners’ association before going that route, but a solid, well-run short-term rental can bring in $15,000 to $30,000 in income per year.”

How long does it take to build an in-law suite?

If you are converting a basement, attic, or existing room, expect to spend about two months on the project. For an in-law suite addition that requires building a new structure, the process takes about four to five months of construction. It could take one to two months to receive proper permitting before you begin construction.

How big is a typical in-law suite?

The typical in-law suite is anywhere from 300 to 1,000 square feet. Because ADUs typically add more value to a home than a repurposed room with an ensuite, it may be worth building a larger unit. Before you plan your project, check the restrictions in your city or municipality for limits on the size of in-law suites or ADUs in your local area.

Will zoning laws affect the construction of my in-law suite?

Local zoning laws will affect the construction of your in-law suite by defining the permitted features you can add. Most in-law suites are limited to a certain size and may require separate permits for aspects like a full kitchen or a unit separate from your main home. 

Common requirements for in-law suites include:

  • Full-time residence on the property.

  • An outdoor entrance that’s separate from the main house entrance.

  • Separate water and sewer connections.

  • Off-street parking.

Check with your city, municipality, or HOA for the specific zoning requirements in your area.