Supplementary living spaces aren’t anything new, but they’re rapidly growing in popularity. With the rise of suburbs in the post-war era came increased zoning restrictions that disallowed extra buildings on single lots. However, with the current housing crisis and fewer people being able to buy their own homes, cities are starting to lessen those zoning restrictions. Now people have more opportunities to add square footage onto their existing lots by building accessory dwelling units (ADUs).  This post describes different types of ADUs and their costs.

What Is an ADU?

An ADU is an added living space placed on a lot of an existing single-family home. To be classified as an ADU, the structure needs to be self-sufficient. It should have room for sanitation, cooking, and sleeping. These additions are usually relatively small and work perfectly for adding rental value or a place for a family member to stay.

With housing prices through the roof, more families are willing to live together on the same property. An ADU would work well for older children who haven’t saved enough to move away or elderly parents who want to be close to family while maintaining their independence.

Types of ADUs

ADUs come in many shapes and sizes, but they can be boiled down into three main categories:

  • Detached ADUs

A detached ADU doesn’t connect to the single-family home in any way. In most cases, it needs its own utility hookups. Examples of detached ADUs are poolhouses, guest cottages, and detached rental units.

These dwellings offer their tenants the most privacy out of any ADU options. This type of ADU is often the most expensive type to build because it is essentially a tiny house. You need to construct the entire thing from the ground up.

  • Attached ADUs

Attached ADU

These ADUs come in the form of an addition to the existing home. Expansion options could include building a second story over your garage or adding to the main floor. Attached ADUs share wall space with your home.

These units will often have their own entrance, so they still offer some privacy, but less than if the unit were detached. The cost to construct an attached ADU is typically cheaper than detached ADUs because you’re building off of an existing structure and can tap into the utilities already there.

  • Interior Conversion ADUs

Interior Design ADU

Interior conversion ADUs transform a pre-existing room in your home into an individual living space. Typical conversions involve the basement, attic, and garage, and are often used for rental income or in-law suites.

Interior conversions offer the least amount of privacy. They sometimes have separate entrances, but that isn’t always the case. On the plus side, these are the cheapest to build because the main structure is already there. Constructing an interior ADU is more like a renovation than a new build.

How Much Does an ADU Cost?

Even though ADUs are relatively small units, the costs are quite high. In general, detached ADUs are the most expensive and interior conversions are the least expensive. Prices also vary depending on the size of your ADU and your location.

A 2021 survey in California found that the median cost of building an ADU was $250 per square foot, with 71% costing under $200,000. Even within California, there were price discrepancies. Areas with higher costs of living usually have higher building expenses as well.

When estimating the price to build your ADU, you also need to consider the types of materials you’ll need for your unique build. Detached and attached ADUs will need a roof and exterior walls. Detached ADUs also require separate utilities. Interior conversions are less expensive because the structure is there.

You’ll probably need to run existing electric and plumbing into the space. Also, to keep heat from escaping, you’ll want to insulate the area thoroughly, especially when converting garages.

What to Know About ADUs

If you’re ready to move forward with building your own ADU, these are some things you should keep in mind:

  • Your Property Taxes Will Increase

Many people are interested in building ADUs for rental income. While it’s certainly possible to make money from these types of additions, you need to consider your other expenses. When you add on to your property, it increases the value, thereby raising the property taxes. Also, factor in building costs like materials, labor, and permits.

These added expenses may mean your rental property is not profitable, at least for a while. You may not see any extra income until you pay off the building costs. Again, internal conversions will raise your taxes the least because, in most cases, you aren’t adding as much square footage but instead merely renovating what’s already there.

  • ADUs Can’t Be Sold Separately

Once you build an ADU on your property, it cannot be sold away from the existing home. It’s an accessory, not a central living area. For sale purposes, it’s not much different than a detached garage or she-shed.

This is definitely something to consider if you’re building an ADU for older children. If they move in with their family and someday want to own the home on their own, they’d need to purchase the whole property with both units. They can’t sell the house and then live in the ADU or vice-versa.

  • Building a JADU Could Save You More

Junior accessory dwelling units (JADUs) are a smaller version of an interior conversion ADU. They’re usually required to be under 500 square feet but come with relaxed regulations for kitchen and bathroom requirements.

The kitchen needs to have counter space, a sink, and a way to cook food which doesn’t have to be a stove – it could include a microwave or other small electronic heating methods. A JADU can also share a bathroom with the main living unit. Since bathrooms and kitchens are two of the highest expenses in new builds and renovations, you can cut costs with the lower requirements for JADUs.

Which One Is Right for You?

Choosing the right type among different types of ADUs comes down to your needs for the space. Are you looking to get rental income or build a space for the family to live in? There are often zoning laws that regulate the type, size, and location of an ADU, so check with your local authorities for restrictions on the kinds of ADUs allowed where you live.


In case you have architectural, structural, and MEP design requirements for making your building or different types of ADUs, feel free to contact us.  We provide you with the full permit set design + T24.


Author’s Bio:

Rose is the managing editor of Renovated. She’s most interested in sharing home projects and inspiration for the most novice of DIY-ers, values she developed growing up in a family of contractors.