An in-law suite is an additional living space, complete with a living area, bedroom, kitchen, and bathroom, that is on an existing property. This type of housing is also known as a mother-in-law suite, a granny apartment, or an accessory dwelling unit (ADU) if it is detached and separate from the main home. The average cost to build an in-law suite addition ranges from $40,000 to $125,000, with the average price hovering around $82,500.

On This Page

1. Average Cost to Build an In-Law Suite Addition
2. In-Law Suite Addition Material Prices
3. In-Law Suite Addition Costs for Labor
4. In-Law Suite Addition Costs by Size
5. In-Law Suite Addition Cost by Type
6. Cost to Build an In-Law Suite Addition Yourself
7. DIY vs. Hiring a Pro
8. FAQs

Average Cost to Build an In-Law Suite Addition

The average cost of an in-law suite addition is $82,500 for materials and labor. The cost ranges from $40,000 to $125,000. Keep in mind that in-law suites include bathrooms and kitchens, which can drive up the cost.

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 it’s 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 will cost about $10,000. The average is around $12 per square foot.
• 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.

In-Law Suite Addition Costs for Labor

To build an in-law suite, you’ll need to hire several professionals, including architects, electricians, and plumbers, for various stages of the project.

Architect

Hiring an architect will help you plan an efficient, functional in-law suite. If you are moving in older family members with limited mobility, it’s important to discuss universal, accessible design with the architects, too. 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 to hire an architect.

Electrician

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

Plumber

Both the bathroom and kitchen of the suite will require new plumbing, and the bathroom will need to be connected 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

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 one general contractor overseeing the entire project, which will cost 10% to 20% of the project’s cost. Additional workers who are building, installing, and painting will cost about $100 to $300 per laborer, per day.

In-Law Suite Addition Costs by Size

You can expect to spend about $100 to $200 per square foot for a new 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: For 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 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 Room

You can turn an existing, finished room in your home into an in-law suite for about $2 to $5 per square foot, plus the cost to add a bathroom is up to $30,000.

Extending a Room

Expanding a room will cost more for second-floor rooms. No matter what floor, this process will involve expanding on the foundation and matching the exterior paint or siding, so it will cost more than converting an existing room. You’ll spend about $80 to $200 per square foot to extend an existing 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.

• Garage: Outfit your garage with everything from drywall to plumbing to electrical work for $20,000–$60,000.
• Basement: A basement conversion will typically cost about $10,000–$30,000.
• Attic: An attic conversion averages from $40,000–$100,000.

Building an ADU

The average cost to build an ADU on your property is about $82,500 but ranges from $40,000 to $125,000 or more, depending on your design preferences and needs.

Cost to Build an In-Law Suite Addition Yourself

Building an addition is a huge undertaking that requires construction, plumbing, and electrical know-how. You’ll need to consult with several contractors to complete this project, and it isn’t recommended that you try to do it yourself. Even an experienced DIYer needs to leave this huge and dangerous job to professionals.

DIY 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, and plumbing, so you need to work with several experts to do the job.

If you are converting an existing room or basement into an in-law suite, you might save several thousand dollars on labor costs. 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.

FAQs

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

You’ll need to check your local laws before building an in-law suite on your property. 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.

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 2 months on the project. For an in-law suite addition that requires building a new structure, the process takes about 4 to 5 months of construction. It could take 1 to 2 months to receive proper permitting before you begin construction.