Homeowner Advice

What Is a Mother-in-Law Suite?

April 12, 2025

24 min read

A mother-in-law suite—also called a granny flat or accessory dwelling unit (ADU)—is a self-contained living space within or adjacent to your home. Ideal for housing aging parents, accommodating guests, or generating rental income, these versatile units offer practical solutions to multigenerational living and rising housing costs. Read on to learn more about their benefits and how adding one can enhance your property’s value and usability.

What Is a Mother-in-Law Suite - Better Place Design & Build

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Daniel Zakheim is a design expert who creates custom ADUs that integrate seamlessly with existing homes, delivering high-ROI results for San Diego homeowners.

Key Takeaways

While a homeowner may originally build a mother-in-law suite on their property to house elderly parents, these living spaces are far more flexible than that. Depending on the family and stage of life they’re in, in-law suites can also be used as an apartment for adult children, a source of rental income, or a guest suite.

Because of this flexibility, building a mother-in-law suite on your property, whether attached to your original home or a detached accessory dwelling unit, can significantly increase the overall value of your property. Not only do in-law suites provide valuable extra space for growing families, but they can accommodate the nearly 1 in 10 families in the United States who enjoy multigenerational living.

Better Place Design & Build ADU landscaping in San Diego - Key Takeaways _ TLDR section

Maximize ROI by designing ADUs with privacy-focused landscaping and architectural harmony—essential for long-term flexibility and market appeal.

What is the Definition of a Mother-in-Law Suite?

Mother-in-law suites, in-law apartments, granny flats—all of these words describe the same thing: a fully independent, separate living space located within or adjacent to a single-family home. Since most mother-in-law suites include their own bathroom and kitchen space and have a separate entrance, they would also be considered an accessory dwelling unit or ADU. If the in-law suite is built within the boundaries of the existing home (think: a basement suite or attic apartment), it would be considered a junior accessory dwelling unit or JADU, so long as it was also under 500 square feet.

Floor plans and square footage of mother-in-law suites can vary widely, depending on the size of the lot, the construction budget, and the unique needs of the homeowner, but generally, in-law suites contain 1-2 bedrooms. A separate master bedroom gives these backyard apartments the feel of a true single-family home, and any extra bedrooms can be used as an office, a yoga studio, art studio, or a space for overnight guests.

In-progress ADU construction by Better Place Design & Build - What is the Definition of a Mother-in-Law Suite section

Building an ADU from the ground up allows for full customization—critical for aligning structure layout with multigenerational needs and maximizing space efficiency.

Why Are Mother-in-Law Suites So Popular?

Even if they aren’t used to house elderly parents, mother-in-law suites are incredibly popular, especially in California, where the housing market is so competitive. They fulfill the increasing demand for multi-generational living, provide a source of potential income, and increase property value.

Increasing Demand for Multi-Generational Living

In 2022, nearly 8% of households in the United States were multigenerational. That means that almost 5 million households across the country have 3 or more generations living together.

Finding a new house that can support this kind of family structure can be difficult, especially in states like California experiencing a housing shortage. This is where building a backyard cottage or detached mother-in-law suite can make a lot of sense. Creating this additional space allows families to keep older generations close without the cost and hassle of moving.

Increasing Demand for Multi-Generational Living - aerial view of San Diego suburban neighborhood

In competitive markets like San Diego, building a backyard suite is often the only viable way to support multigenerational living without relocating.

Mother-in-Law Suites’ Income Potential

Mother-in-law suites don’t have to be used to house aging parents. In fact, many homeowners we work with build detached suites as a source of additional income. While California prohibits most short-term rentals, accessory apartments can be used for long-term rentals, and with features like a master suite, in-unit laundry, and built-in storage, they can generate thousands of dollars a month in rental income.

Flexibility and Convenience

Secondary units like mother-in-law suites may be used to house elderly family members in the short-term, but over the years, they can also serve as a home office, guest house, hobby space, or private retreat. They provide property owners with tremendous flexibility and convenience, allowing them to get the most out of their property for years to come.

Increased Property Value

Because of their flexibility and wide range of potential uses, mother-in-law suites are incredibly attractive to prospective buyers. Having one on your property can increase its value by as much as 35%. This figure may be a little lower for attached units or attic space conversions, but even these types of ADUs have ample return on investment when compared to their construction costs.

Guest Accommodation

A separate, private living space is a fantastic way to host guests—especially for extended periods—without disrupting life in the main home.

Why Are Mother-in-Law Suites So Popular - modern kitchen interior in detached ADU

Kitchens make or break functionality. A fully independent suite should include high-efficiency appliances and smart storage to attract long-term tenants or comfortably house family.

Types of Mother-in-Law Suites

Common types of mother-in-law suites include attached and detached units, garage conversions, basement conversions, and junior ADUs.

Attached Suites

Attached in-law suites are private living spaces that are built alongside and share a wall with an existing single-family home. While the shared wall doesn’t provide quite the same level of privacy as a fully detached unit, attached units offer all the same functionality. They can be a smart solution for homeowners with limited lot space or budget. Attached mother-in-law suites can be as large as half the size of the existing property but can’t exceed the state maximum of 1200 square feet.

Detached Units

Detached units are essentially small backyard homes built behind a single-family home. While they are typically the most expensive type of ADU to build, they also offer the most privacy, functionality, and return on investment. Detached units can be as small as 300-400 square feet or as large as 1200 square feet, depending on the lot size, budget, intended use, and local zoning laws.

Garage Conversions

For some families, the garage is an underutilized space. Converting an unused garage space into a garage apartment can offer the same benefits as a newly built detached unit at a significantly lower cost. This is because the foundation work and exterior structure have already been built, saving homeowners tens of thousands of dollars on their construction budget.

Basement Conversions

While basements aren’t as common in California as in other parts of the country, they do exist, and for many families, they can be converted into a separate living space for family members. When it comes to basement units, it’s important to consider lighting—especially natural lighting—and how to accommodate a private entrance.

Junior ADUs (JADUs)

Junior ADUs or JADUs are secondary living spaces built within the existing boundaries of the home (think: attached garage, unused guest bedroom, etc). Unlike standard ADUs, JADUs don’t require a separate entrance or bathroom, though they do include a kitchenette. JADUs are most often used as a semi-private space for extended family and not a rental.

Types of Mother-in-Law Suites - exterior ADU with patio and garden integration

Integrating your ADU with outdoor living spaces boosts both livability and value. Think pergolas, patios, and privacy landscaping when designing detached units.

How Much Does It Cost to Build a Mother-in-Law Suite?

Building a mother-in-law suite on your property typically costs between $150,000 and $400,000+, depending on the type and size of structure you’re building, as well as local building costs.

Cost Breakdown

Garage conversions and JADUs are generally the most cost-effective options for mother-in-law suites, costing between $150,000 and $200,000. Attached units are next, costing between $250,000 and $350,000, while new, detached units are the most expensive, costing around $350,000 to $450,000.

Garage conversions save homeowners money on foundation work and framing, as the exterior structure is already standing. With attached units, homeowners typically save on utilities, as these structures can often be connected to the main home’s existing electrical and plumbing systems. With detached units, this work all needs to be done, hence the extra cost.

Factors That Influence Costs

Along with the type of unit you’re building (e.g., attached or detached), the size of the unit definitely influences the overall cost. Larger units cost more because they require more building materials. Additional bathrooms, plumbing, and fixtures can also increase the budget, as can local labor costs—many homeowners don’t realize that labor accounts for as much as 50% of their project’s budget!

Lastly, the finishes and customizations you choose for your home, such as exposed beams, tile work, and built-in storage, can all increase your project budget. To get an idea of how much your ADU project might cost, use our cost calculator or schedule a free consultation.

Financing Options for Mother-in-Law Suites

Most homeowners we work with don’t have the funds to pay for their ADU builds in cash. Luckily, there are many financing options available to you, from home equity loans to construction loans. You can learn more about ADU financing in our separate blog post here.

Cost to Build a Mother-in-Law Suite - attached ADU under construction

Attached ADUs can be cost-effective by leveraging shared walls and utilities—perfect for tight budgets without compromising on livability.

FAQ About Mother-in-Law Suites

Mother-in-law suites go by many names, including in-law apartments and granny flats.

There’s no one best layout for all mother-in-law suites. Typically, though, we advise homeowners to consider at least a 1-bedroom layout, as this provides a much more homey feel than a studio.

If just being used to house a single parent, mother-in-law suites can be both functional and comfortable with as little as 400-440 square feet. For 2 parents, we typically advise considering a unit that’s 500-750 square feet.

Building any kind of accessory dwelling unit on your property can increase your property tax, but these costs are generally mitigated by an overall increase in property value. To learn how property taxes are assessed in California, read our post here (hint: the answer will probably surprise you!).

It can be! So long as it has a separate entrance, kitchen, and bathroom, your mother-in-law suite is considered an ADU.

Ready to Add a Mother-in-Law Suite? Better Place Design & Build Can Help

If you want to get the most out of your mother-in-law suite when it comes to comfort, durability, and ROI, a custom build is the way to go. And when you build with Better Place Design & Build, your project will go from pipe dream to move-in-ready in as little as 8 months.

Our experienced team of ADU builders can get you started at absolutely no cost. Schedule a free consultation, and we’ll come to survey your property, discuss your goals, and provide you with a free, detailed estimate.