Originally Posted On: Why Build a Mother-in-Law Suite for Your In-Laws? – Acton ADU


These days, there’s no such thing as an average household. And the truth is, that’s okay–it’s an opportunity for your family to come together. In times of crisis, whether you’re staring down the rising cost of living in California or trying to care for an aging parent, the best thing you can do is find support and solace in your loved ones.

With a mother-in-law suite, you can keep your family even closer, offering the support your parents need and the ease of access that you never had before. If you’re thinking of building a mother-in-law suite, here’s what you need to know before you kick off the project.

What is a Mother-in-Law Suite?

A mother-in-law suite, as the name implies, is an additional living space within your property or home designed for use by your in-laws. Put another way, it’s an additional, fully functional living space added onto a single family home.

While the term might sound like a sardonic Don Draper reference, many homeowners are now turning to mother-in-law apartments as the perfect solution for those who want a bit of extra space to support their parents during difficult times.

After all, when the going gets hard, the best thing you can do is keep your family together.

Other Names

Chances are, if you live in California, you’ve heard of mother-in-law suites before, though you may not have heard them referenced by that name.

They’re more commonly referred to (on this site and in much literature on the subject) as accessory dwelling units, or ADUs, which is their legal and regulatory name. You may have also heard of them under other names, including:

  • Accessory apartment
  • Basement apartment
  • Ancillary unit
  • Backyard cottage
  • Backyard apartment
  • Carriage house
  • Garage apartment
  • Garden cottage
  • Granny flat
  • Home-within-a-home
  • Junior accessory dwelling unit
  • Laneway house
  • Mother-daughter home
  • Multigenerational home
  • Secondary unit
  • Tiny house

All of these names refer to the same thing: a secondary living unit within a single family property. This is not to be confused with a guest room, and while tiny houses can sometimes be ADUs, they don’t always qualify.

That’s because mother-in-law suites must be fully functional living spaces, complete with a bathroom, kitchen, sleeping area, plumbing, and foundations (which is why not all tiny houses qualify).

However, while the unit itself is fully functional and independent, it cannot be treated as its own independent property. It is instead part of the original single-family unit that it was built into, and must be sold with that property.

Types of Mother-in-Law Suites

Whatever name you call them, there are four types of mother-in-law suites:

  1. Detached
  2. Attached
  3. Interior
  4. Garage conversion

Detached units are the most independent. These are freestanding units with their own plumbing lines and utility hookups, often with their own entrance. Another person could live a completely independent life, almost as if they were living in a separate house.

An attached unit is a bit closer to the family, in that the unit shares a wall with the primary residence. It still has its own foundations and utility hookups, and there is no internal entrance from the main house into the unit.

Interior units are the most integrated into the original home. These are often basement or attic apartments–fully functional living spaces that a casual observer would not notice from the street. Nonetheless, this is not a guest room. It is an apartment in every sense of the word, with everything the resident would need to function independently.

A garage conversion can be any of the previous three types, but as before, it must be a fully functional living space.

Benefits of a Mother-in-Law Suite

So, why would you want to build a mother-in-law suite? In times like these, between an uncertain economy, the California housing crisis, and global health concerns, now is the time to keep your family close to home.

Putting Family First

The foremost benefit of a mother-in-law suite is simple: putting your family first.

The California housing crisis has hit everyone hard, but for older people, the situation is especially dire. With limited income and a narrower span of time to make up the money they need, many seniors fear eviction, homelessness, and isolation in the face of rents they simply cannot afford.

With so much fear on people’s minds, this is the time when you need your family to take care of you–especially if you’re an older person hit hard by difficult times. And if you’re an adult watching your parents struggle, it’s hard to watch the situation unfold without wanting to do something to help.

Mother-in-law suites allow you to provide the safety net your parents need. It’s an increasingly common trend–64 million Americans aged 55 to 64, roughly 24% of that population group, now live in multigenerational housing with their loved ones, and 21% of those aged 65 and older are in similar situations.

Personal Touch

More than that, mother-in-law suites offer you the opportunity to provide housing with a family touch.

You could help your parents try to find a home on their own, but with housing prices so high, they may need to be several hours away before they can find something within the constraints of their budget. And if they needed your help, getting to them would be an undertaking.

That’s assuming you can find a housing option that suits their needs–and with demand as high as it is, that’s a huge if.

With a mother-in-law suite, you can offer them a housing option that works for them. They can find something in their budget (assuming you even charge them rent) and you get to keep the whole family closer together, a win-win for everyone.

Care Close to Home

If your parents or in-laws are in need of additional support as they get older, mother-in-law suites offer you the opportunity to be the care and support they need.

If housing in California is expensive, supported living isn’t much better. The national median hovers around $4,000 per month, and if you’re like most families, you don’t have a spare $4,000 on hand to spend every month even if your in-laws need the extra care.

You could try a nursing home, but for an aging parent who just needs support and not medical care, this can be especially dispiriting. After all, they’ve lived independently for decades, and it’s difficult for many people to adjust to changing support needs after being independent for so long. It’s even worse if they know they don’t need as much support as they’re getting.

Of course, as an adult with your own children, pets, job, responsibilities, and all-around busy life, taking care of your parents in their own home may not be on the table either. You just can’t find the extra time to dart to their house every day, especially if they live further away.

A mother-in-law suite is a solution that works for everyone. Now, your parents or in-laws are just a backyard away, easily accessible anytime they need support.


The joy of mother-in-law suites is that they also open up a whole new window of opportunities for families to find a housing solution that works for them.

With housing demand astronomically high, the reality is that many seniors may not be able to find the kind of housing they actually need. And if they can find a place that fits their budget, it may require extensive remodeling to be usable, which throws it back out of their budget again (assuming the landlord will allow those modifications).

When you build your own mother-in-law suite, you can create a unit that’s designed to work for your parents for many years to come, a unit that will meet their needs even if their mobility starts to decline or they have additional needs. You can create a unit that fits exactly what your parents need, no matter what those needs are.


Plus, there’s something to be said for the convenience of mother-in-law suites–for everyone involved.

With your parents or in-laws in a mother-in-law suite, both sides have access they never would have had otherwise. You can dart across the backyard to take care of your parents, and your parents, too, can come to the house to help out with dinner, sit with the kids to do homework, run an errand, or just keep you company.

It’s a chance for your family to spend time together in ways that never would have been possible before, a bonding opportunity that joins everyone.

Added Comfort During the Holidays

Even if your parents or in-laws don’t take up full residence in the mother-in-law suite, they can still make use of the unit when they do come to visit.

It doesn’t matter how close you are with your parents or in-laws–when you get two distinct family units that are used to functioning separately, there are going to be a few kinks to iron out. They’re used to being independent, you’re used to being independent, and you’re both used to being independent on a different schedule.

Mother-in-law suites allow you just enough space during holidays to enjoy each other’s company and have a bit of breathing space at the end of the night. You can disengage if you need some space or jump right into the middle of the hubbub–either way, you’ll have the comfort of keeping your own routines without disturbing each other.

Added Support

Last but not least, mother-in-law suites are a great chance to get extra support for your parents and for your own family.

If you have kids and your parents or in-laws live farther away, both sides would love to spend more time together, even though the distance keeps getting in the way. Mother-in-law suites allow your family to do what family does best: support each other.

Your parents or in-laws get to spend more time with the grandkids, and you get another person to help out when you need it. It’s a win-win scenario for everyone involved, and it still gives you a sustainable housing option for years to come, no matter what your needs look like in the future.

Building a Mother-in-Law Suite

Is a mother-in-law suite the right fit for you? Chances are, if your parents or in-laws need extra support, it may just be the answer you’ve been looking for.

However, you should know what you’re signing up for. Here are a few factors to keep in mind before building a mother-in-law suite for your in-laws.

The Cost

With housing prices as steep as they are, the cost of building a mother-in-law suite is a significant consideration, especially for a middle class family.

First, it’s important to understand that mother-in-law suites are more expensive per square foot than a regular single family house, though there are several reasons why that is. For one thing, there are fixed costs associated with permitting and fees, and you’ll have to pay for a new foundation, utility hookups, plumbing, etc.

Plus, while the square footage of a mother-in-law suite is significantly smaller than the average single family home, keep in mind that a significant part of the square footage is expensive square footage–kitchen and bathroom. Plus, because there are no extra bedrooms and hallways, the cost per square footage isn’t offset by extra rooms.

That said, you are packing a lot more value into a much smaller space.

So, how much does it cost to build a mother-in-law suite? That depends on the suite.

As with any other construction, the more complicated the project, the more expensive it will be to build. So if you’re doing a detached suite that requires pouring a whole new foundation and getting new plumbing hookups, it’s going to be more expensive than a basement apartment and plumbing modifications.

However, you can significantly offset the costs with shortcuts offered in your area. In San Jose, for example, there are pre-approved master plans which significantly lower the cost of permitting and approval.

For most people in San Jose, this comes out to a sticker price between $400 to $600 per square foot, which sounds like a lot until you realize that the cost per square foot for most single family homes in San Jose is higher than this.

Choosing the Right Building Option

Keep in mind, however, that not all price options will be within your control.

For example, you may want to build a basement unit to save on costs, rather than pouring new foundations and paying for new utility hookups in a freestanding unit. However, this depends on your basement. If your house’s plumbing and utilities are not compatible with the requirements for a mother-in-law suite, you may not be able to go ahead with a basement unit.

Detached and attached units have similar limitations. You have to position a unit in the right place, which means working with your contractor to figure out where a new unit might be positioned in relation to your existing home. They can also help you determine what options are workable for your current budget.

If you’re going to have your parents or in-laws moving directly into the suite, it may be a good idea to get their input with the contractor. This will help ensure you end up with a suite that makes everyone happy (and meets their needs as well as your building constraints).

The Building Process

Once you know what kind of mother-in-law suite is the right fit for you, the building process is relatively straightforward. You just have to know what you’re doing, how to allocate your resources, and how to simplify the process.

One of the best things you can do before you begin building is to get organized. Here’s a quick guide to essential issues you’ll need to address before the building process can begin in earnest.

Where You Can Build an ADU

The first question is knowing where you can build an ADU, which, as we’ve noted, is a bit more complicated than it sounds.

The big issue is zoning–you have to know whether or not your property is zoned in a way that qualifies for mother-in-law suite construction.

According to San Jose’s Universal ADU Checklist, the first thing to ask is whether your property is in a residential zone beginning with R-1, R-2, R-M, or PD. Or, regardless of zoning, your property must meet one of the following General Plan designations:

  • Residential neighborhood, mixed use neighborhood, or mixed-use commercial
  • Urban residential, transit residential, or rural residential
  • Downtown or urban village

If not, your property cannot have an ADU.

If yes and you have a single family property, your property may have one ADU (subject to standards).

If yes and you have a duplex or multifamily property, two detached ADUs may be allowed. Duplexes may be allowed one attached ADU. Multifamily units may have a number of attached ADUs equal to 25% of existing units.

Keep an eye on current ADU ordinances for the most recept updates to ADU rules and regulations in San Jose. Your contractor can also help you make sense of ADU regulations as they apply to your property.

Fire Requirements

One of the important areas to watch is fire requirements. You can find all of this information in the San Jose Universal Checklist, but we’ll do a quick review of the key points.

Your suite plans will be reviewed for compliance with the California Fire Code. San Jose will ask six questions:

  1. Is the primary residence protected by fire sprinklers? If yes, the ADU must also have fire sprinklers.
  2. Is the project an attached ADU greater than 500 feet AND does the overall gross floor area of the main unit exceed 3,600 square feet? If yes, the ADU and main unit must be protected by fire sprinklers.
  3. Is the address of the ADU visible from the street? If not, your project may require a Fire Variance with additional safety measures.
  4. Is the distance from the street curb of the lot to all portions of the proposed ADU no greater than 200 feet as measured along a minimum 3-foot clear path to all sides of the ADU? If no, your project may require a Fire Variance with additional safety measures.
  5. Are all exterior walls of the ADU within 600 feet of a fire hydrant? If no, your project may require a Fire Variance with additional safety measures.
  6. Is a minimum flow of 1,000 gpm at 20 psi available from the closest hydrant? You are required to submit a letter from your water company detailing water flow data when you submit your building permit application

If any of these requirements are not met, or neglected in your application, your application will not be approved.

Parking Requirements

Last but not least are parking requirements.

As a rule, in San Jose, one parking space is required unless the property qualifies for an exemption. Units that meet any of the following criteria are exempt:

  • The unit is located in a historic district listed in San Jose’s Historic Resources Inventory
  • The unit is part of or within the primary residence (such as a basement apartment)
  • The unit is located within .5 miles of a bus or rail station AND the path of travel is publicly accessible
  • The unit is on a street that requires on-street parking permits, but the permit has not been offered to the occupant of the accessory unit
  • The unit is located within one block of an area where a motor vehicle provides daily or hourly services as part of a regional fleet run by a public agency or publicly-leased motor-sharing organization

Otherwise, your property is required to have one parking space. If your construction eliminates that parking (for example, if you convert your garage) you will have to replace that parking in some other way.

Ready to Build Your Mother-in-Law Suite?

We know that while a mother-in-law suite is a small space, it’s no small undertaking for you and your loved ones. After all, you’re navigating city and state laws to build a home that will support your loved ones for years to come. To do that, you need builders who are ready to support you.

That’s where we come in.

We’re ADU experts, specializing in designing, navigating, and building personalized mother-in-law suites. More importantly, we’ve served families just like yours for over 30 years–we know what it takes to create a unit you love, and we’re ready to make it happen.

Let’s make your housing dream a reality. Get in touch today to start the conversation about your project.