How Much Does an In-Law Suite Cost?

Average Cost to Build an In-Law Suite Addition

The average cost to build an in-law suite addition ranges from $25,000 to $265,000, with most homeowners spending around $82,750. 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 an accessory dwelling unit (ADU) if it’s detached and separate from the main home. 

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.

Converting a Finished Room

You can turn an existing, finished room in your home into an in-law suite for $10,150 to $65,625. This pricing includes the cost to add a bathroom or the cost to build a kitchen, both of which are 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. You can expect excavating around the home’s perimeter to cost $2,600 on average, including installing insulation and wiring, and matching exterior paint or siding. Extending a second-floor room costs 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 to $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 low as $10,000 to $30,000.

• Attic: An attic conversion is more expensive at $25,000 to $65,000 due to the need for reinforced floor joists and a safe stairway.

In-Law Suite Addition Costs by Size

The cost to build an in-law suite ranges from $100 to $200 per square foot, including materials and labor. Here's what you can expect based on size:

Labor Cost for an In-Law Suite Addition 

Building an in-law suite involves coordinating various professionals. Here are the primary professionals involved, their costs, and common tasks:

Electrician

A new in-law suite will need electrical work, which costs an average of $50 to $100 per 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.

Plumber

Both the bathroom and kitchen of the suite will require new plumbing, and the bathroom must connect to the sewage system. Plumbers charge about $150 to $275 an hour for these intensive jobs:

• Rough-in plumbing: $6,500 

• Installing a sink: $450

• Water main installation: $1,400

• Sewer main installation: $3,500

HVAC

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 $5,000 to $12,500 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 $300 to $500 per laborer per day.

Material Prices for an In-Law Suite Addition 

An in-law suite is similar to an apartment or tiny home, meaning you need many materials to build this structure inside and out. The material costs to build an addition will vary depending on your design choices.

• Support beams and roof trusses: $15 to $30 per square foot

• Drywall and insulation: $4 to $6 per square foot

• Insulation: $0.30–$1 per square foot

• Foundation materials: $4 to $16 per square foot

• Bathroom plumbing: $15 to $40 per square foot

• Plumbing fixtures: $400 to $2,000 per fixture

• Siding: $12 per square foot

• Windows: $100 to $1,500 per window

• Doors: $500 to $5,000 each

• Floors: $3.50 to $21 per square foot

Benefits of Adding an In-Law Suite

Adding an in-law suite offers numerous advantages:

• Multi-Generational Living: Provide a comfortable and independent living space for elderly parents or adult children, keeping family close while respecting privacy.

• Increased Property Value: An in-law suite can boost your home's value by up to 30%, making it a smart investment.

• Rental Income: If permitted in your area, renting out the suite can generate additional income, helping to offset the construction costs.

• Flexibility: Use the space as a guest house, home office, or even a private retreat.

DIY vs. Hiring a Pro

While it might be tempting to tackle the project yourself, building an in-law suite is a complex undertaking that involves permits, excavation, construction, electrical work, HVAC, and plumbing. It's essential to hire experienced contractor professionals to ensure the job is done safely and correctly. Starting with a trusted local general contractor can streamline the process, as they can coordinate the project and assemble a skilled team on your behalf.

How HomeAdvisor Gets Its Cost Data

No place is more important than your home, which is why HomeAdvisor connects homeowners with local pros to transform their houses into homes they love. To help homeowners prepare for their next project, HomeAdvisor provides readers with accurate cost data and follows strict editorial guidelines. After a project is complete, we survey real customers about the costs to develop the pricing data you see, so you can make the best decisions for you and your home. We pair this data with research from 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.