Here’s How Much It Would Cost to Build Your Mother-in-Law Suite

Grandmother and granddaughter are cooking on kitchen.
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Ernie K-Doe once sang that his mother-in-law was “the worst person I know,” but for many, mothers-in-law are bastions of support and love. They offer insight, affection and wisdom gleaned from their lived experiences. Even when she’s driving you crazy, your mother-in-law knows best.

Mother-in-Law Day, which falls on Oct. 28 this year, is a chance for families to show their appreciation for their other mother. Another way to show appreciation is to invite your mother-in-law to live with you. What’s the best way to do that, while still keeping her at arm’s length? With a mother-in-law suite. Get started on a home renovation that will pay you back.

What Is a Mother-in-Law Suite?

The mother-in-law suite — sometimes called a granny flat — is a catch-all term that includes any living space with a bedroom, kitchen and bathroom and an entrance separate from the main house. Officially called “accessory dwelling units” (ADUs), in-law units can be converted garages, basements, attics, guesthouses or suites in the true sense of the word.

Related: 30 Home Upgrades That Won’t Blow Your Budget

How Much Does a Mother-in-Law Suite Cost?

Like families, building an in-law unit takes patience, planning and maybe sweat and tears. And also like families, in-law units cost money — anywhere from $40,000 to $125,000, according to Before you ask your mother-in-law to live with you, check your local zoning and building codes to make sure your future unit is within regulations. You might also want to consider separating the utilities between the unit and the primary residence to save money due to inactivity or if you want to rent out the unit later.

The Cost to Add a Mother-in-Law Suite to an Existing Home

You’re more likely to save money when you convert underused space instead of opting for a brand-new addition. All cost estimates are via

  • Garage: $20,000 to $50,000 (plus $15,000 to $25,000 for a bathroom): While insulation might be an issue to update, a garage already comes with framing and foundations and more than likely already affords occupants a degree of privacy. Wiring and piping for outlets and plumbing are essential for code compliance.
  • Basement: $10,000 to $27,000: A spacious basement should provide its own floorplan, especially since the piping and drains should already be there. It’s still wise to plan for add-ons and exterior doors in particular. They’ll provide a safe exit and keep the unit within regulations.
  • Attic: $41,400 to $90,000: Likely one of the more challenging spaces to convert, you’ll need to take into account the ceilings height, stairwell width, proper plumping extensions, sound-proofing measures and sources of light to make the space habitable.
  • Unused rooms: $2 to $4 per square foot for framing walls, plus $3,000 to $25,000 to add a bathroom: Since rooms should already have proper flooring, lighting and wiring, this renovation should be the easiest undertaking when adding an in-law unit to your home.
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The Cost to Build a Brand-New In-Law Suite

Building an addition can cost as much as six figures, with high-end additions approaching an average of $120,000, according to HomeAdvisor. Then again, they can also cost much less — it all depends on your in-law’s needs and your budget. Here are some estimates for some additions homeowners might undertake. Just remember to budget for the costly home repairs that will catch you by surprise. All cost estimates are via HomeAdvisor unless otherwise noted.

  • Additional Rooms: $80 to $200 per square foot: You can either build out or build up when it comes to adding a room. Building out adds a room to your home at ground level while building up requires adding a room to the second story of your home. Building out might be more complicated because it involves expanding your home’s physical footprint, but building up requires a whole new floor.
  • Granny Pod: $85,000 to $125,000: This portable abode is great for giving your in-laws privacy and proximity. It looks like a bungalow on the outside but has hospital-grade features on the inside. The unit hooks up to the main house’s sewer system and includes hand railings, lighted floorboards and a soft floor to lessen fall-related injuries.
  • Detached Additions: $15,000 to $25,000: These smaller units also exist on the property but are useful for guests or even office space. Simple shed-like rooms without amenities will run about $15,000, but also consider excavation costs ($70 per cubic yard).

Cost Factors

Once you’ve decided what sort of addition you want, it’s time to crunch the numbers. Here are some estimates to keep in mind as you’re planning your in-law’s perfect home-away-from-home-and-you:

  • Architectural services: Usually accounts for 10 to 17 percent of the total project’s cost
  • Support beams and roof trusses: $15-$30 per square foot
  • Drywall: About $9.80 for a 4-foot-by-8-foot sheet of drywall
  • Electrical wiring installation: Requires a licensed professional, who could charge $50-$100 per hour

A Unit Only a Mother-in-Law Could Love

An in-law unit or guesthouse should be seen not only as a financial investment but an emotional one. You can’t put a price on accommodating your family’s needs, and the addition can help you and your family grow together. It can also help your home’s equity grow. You can rent out the extra space in the short-term and homebuyers are also willing to pay extra for a property equipped with an in-law unit. So, bust out those blueprints the next time your mother-in-law hints she doesn’t see you nearly as often as she’d like.

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