How much does it cost to build a mother-in-law suite?
How much does it cost to build a mother-in-law suite?
$100 – $350 average cost per square foot
$5,000 – $100,000 average total cost (convert existing space)
$30,000 – $280,000 average total cost (add or build new structure)
Average cost to build an in-law suite
The average cost to build a mother-in-law suite is $100 to $350 per square foot, depending on the size and type of project. Converting an existing space to an in-law suite costs anywhere from $5,000 to $100,000+, whereas building an entirely new addition or detached structure costs $30,000 to $280,000.
| Square footage | Average total cost* |
|---|---|
| 300 | $30,000 – $105,000 |
| 400 | $40,000 – $140,000 |
| 500 | $50,000 – $175,000 |
| 600 | $60,000 – $210,000 |
| 700 | $70,000 – $245,000 |
| 800 | $80,000 – $280,000 |
*For building a home addition or detached structure
What is a mother-in-law suite?
A mother-in-law suite is a private space in your home or on your property created for an in-law or relative to live in independently. The specific size and interior vary, but mother-in-law suites usually have most or all of these features:
Full bathroom
Private entrance
Heating and cooling
Kitchen or kitchenette
Water heater
Cost to build a mother-in-law suite by type
There are a few different way to build an in-law suite. The type of addition or construction project you choose will influence the cost. You can adapt an existing space in your home, extend your house, or create a separate structure.
| Project type | Average cost |
|---|---|
| Extending or adding a room | $5,000 – $240,000 |
| Detached structure | $45,000 – $280,000 |
| Converting a room | $5,000 – $100,000+ |
Extending or adding a room
Adding or extending a room is one way to create an in-law suite that is attached to your existing home. This project may require excavation and expansion of your foundation, insulation, wiring, and exterior siding. Some options include:
Bathroom: Adding a bathroom costs $5,000 to $75,000. Sometimes all a basement or living space needs to be considered a primary suite is a bathroom.
Bump-out: A bump-out addition costs $85 to $200 per square feet. This extends your home and increases the size of an existing room, enough to add a bathroom or kitchen area.
Second story: Adding a second story costs $200 to $500 per square foot and is the least common way to add a mother-in-law suite because of its high cost and complexity.
Detached structure
There are several different options for detached in-law suites:
An accessory dwelling unit or ADU costs $100 to $300 per square foot, or $30,000 to $240,000 total for 300 to 800 square feet. The materials and features you choose will have a significant impact on the total cost.
Building a garage with an apartment costs $200 to $350 per square foot or $60,000 to $280,000 on average, depending on the size. A prefabricated detached garage kit with space for an apartment costs $35,000 to $100,000, but you'll still have to add flooring, HVAC, plumbing, and furnishings.
Building a guest house costs $150 to $250 per square foot, or $45,000 to $200,000 total for 300 to 800 square feet. Prefabricated tiny homes or modular guest houses cost $80 to $250 per square foot.
When considering building an apartment above a garage as an in-law suite, keep in mind that the residents may face mobility issues and difficulty climbing stairs as they get older.
Converting a room
Converting an existing space in your home into a mother-in-law suite costs anywhere from $5,000 to $100,000+. Converting can be convenient since you already have the framing and foundation in place, but it can be tricky and expensive to get materials in and out of an existing home.
Garage: Converting a garage costs $5,000 to $50,000+ when turning an existing garage into a living space.
Basement: Remodeling a basement and converting it to a mother-in-law suite costs $7,500 to $52,000. Finishing a basement costs $30 to $50 per square foot if your basement doesn't have drywall, floors, or electricity.
Attic: The cost to finish an attic and turn it into a mother-in-law suite with a bedroom and bathroom is $50,000 to $100,000+.
As of 2018, roughly 64 million people in the U.S. live in multigenerational homes or in-law suites. This type of home addition is popular and potentially profitable.
Mother-in-law suite cost factors
Your actual costs will depend mostly on whether you're working with an already-existing space in your home and whether it's finished or not. Building a structure from scratch will usually cost more than remodeling or finishing a room.
| Factor | Average cost per square foot |
|---|---|
| Drywall cost | $1.50 – $3.50 |
| Insulation cost | $1.00 – $4.50 |
| Flooring cost | $4 – $15 |
| Plumbing cost | $4 – $5 |
| Electrical wiring cost | $4 – $9 |
| Interior painting cost | $1 – $3 |
| Countertops cost | $50 – $150 |
| HVAC system cost | $3 – $6 |
| Cabinets cost | $150 – $500 per linear foot |
| Appliances cost | $2,500 – $6,800 per kitchen |
| Toilets cost | $350 – $800 each |
| Light fixture cost | $70 – $300 each |
| Exterior door cost | $300 – $1,000 each |
These factors also affect construction costs:
Material quality: Higher-quality building materials like steel framing or marble countertops cost more than basic, builder-grade materials.
Existing structure: If your basement or garage already has walls, flooring, HVAC, and electricity, you won't have to do as much work as you would if you're building from scratch.
Utility meter: If you're going to rent out your in-law suite and want the tenants to pay for their own utilities, you’ll have to install an electrical meter. Adding a meter costs about $2,000 to $5,000.
Egress window: To qualify as an independent living space, the mother-in-law suite must have an egress window or a safe exterior entrance. Egress windows cost $3,000 to $7,000 with installation.
Extras: Some mother-in-law suites include enough space for a living room, office, or laundry room, while others only have a sleeping area and bathroom.
Labor
In order to get the job done safely and effectively, you’ll need several different professionals, who each charge different labor rates:
A general contractor charges $50 to $150 per hour.
Electrician services cost $50 to $130 per hour.
Plumber services cost $75 to $150 per hour.
FAQs about building a mother-in-law suite
Does a mother-in-law suite add value to a home?
A mother-in-law suite can raise your home's value by up to 30%, so it's absolutely a smart investment. Adding an in-law suite also offers a few other advantages:
It provides an independent living space for parents or adult children, which means you can keep family close while respecting their privacy.
If your area allows it, you can rent out the suite and generate additional income. This will help offset the construction costs.
Your space will be flexible since you can use it as a guest house, office, or private retreat.
Can I build a mother-in-law suite on my property?
Yes, you can build a mother-in-law suite in your home or on your land as long as you follow local zoning laws and building codes. There may be restrictions regarding size, and you'll need a permit before starting any construction projects.
How long does it take to build an in-law suite?
Building an in-law suite generally takes 2 to 7 months, depending on the construction type. Converting a basement, attic, or other room in your home will take less time than building a new structure.
Can you rent out a mother-in-law suite?
You can rent out an ADU or mother-in-law suite, just as you would any other piece of property. However, be aware that some counties prohibit it or require a license first. Be sure to properly vet any prospective tenants and research reasonable rental terms and conditions beforehand.
Even if you're renting your space to a friend or family member, have them sign a contract or lease in order to protect yourself just in case there's damage or disputes.
Getting estimates from home remodelers
While hiring home addition contractors or home remodelers near you, remember to follow these steps:
Get multiple estimates from local contractors, preferably 3 or more.
Look for licensed contractors with relevant experience.
Explore their customer reviews on HomeGuide and Google.
Pick a licensed, insured, bonded, and reputable company.
Avoid the lowest quotes, which can indicate low-quality work.
Ask for a written, detailed estimate and never pay in full before they finish the job.
Questions to ask home addition contractors
Ask these questions to find the best available contractors:
How long have you been building mother-in-law suites?
Do you have any relevant certifications?
Will you be hiring any subcontractors to help with this project?
Do you recommend building a detached structure or remodeling an existing space in my house?
What would be a reasonable size for this project?
What are my options for materials and appliances?
How long will this take to build?
Can you match my siding and roof?
What kind of coverage does the warranty offer?
Using our proprietary cost database, in-depth research, and collaboration with industry experts, we deliver accurate, up-to-date pricing and insights you can trust, every time.