Accessory apartments make money and increase the housing supply. Here’s how to build one.

In this generation, more people are staying single longer, are having fewer children, and frequently prioritize walkable urban neighborhoods over suburban ones. Those cultural shifts, along with the high cost of living in the Washington region, have forced politicians and residents to rethink where and how new housing should be accommodated. Accessory apartments (also known as Additional Dwelling Units, Accessory Dwelling Units, or ADUs) help meet this need while minimally impacting infrastructure in cities.

In general, a greater diversity of housing types allows for different types of households to live in a city. Accessory apartments are far from the typical suburban layout of large properties spread in acres of land—rather, they allow families to live closer together and closer to amenities.

Accessory apartments help create a virtuous circle: they supply new and usually more affordable housing (often near public transportation), increase the value of existing properties, help existing homeowners to stay in their houses longer, and allow new residents to live in a neighborhood they might not otherwise be able to afford. Cities benefit because these residents can share existing services like roads and parks.

What's an “accessory apartment” exactly?

An accessory apartment is a residence built within the limits of an existing property. It works completely independently from the main dwelling, but remains legally retained by a single owner. A homeowner cannot sell an accessory apartment separately from the main property, but it can be leased. All access, utilities, and uses between the dwellings are independent from each other.

Some examples of accessory apartments include granny flats or basement apartments. In the Washington region, the most common accessory apartment format is the basement apartment. However, based on the property size and location, freestanding structures are also a popular option in less dense neighborhoods.

For example, a townhouse with an existing unoccupied basement can be outfitted to accommodate an accessory apartment with minimal alterations. A house with a large backyard could accommodate a free-standing model. Some homeowners have converted garages, attics, carriage houses, sheds, and more.

I'm thinking of building an accessory apartment. What should I know?

When considering adding an accessory apartment, there are a few things a homeowner should keep in mind. First, they should balance zoning and building code requirements with the economic impact of the investment.

If you don't know the difference between a zoning code and a building code, fear not. Zoning regulations are the rules that organize elements between the outside of the building (aka building envelope) and the city that surrounds it, for example: setbacks, height, area, lot coverage, and other elements required by the zone where the property is located.

The building code regulates everything from the building envelope to the inside of the structure, for example: structure, infrastructure, fenestration, sustainability, and safety requirements (such as fire separation). Examples of building code rules include room sizes, ceiling clearance, size of windows, and plumbing requirements, among many other details.

Zoning rules vary by location: DC has its own rules, which are different from Fairfax County, which is different from Montgomery County. In fact, Montgomery County Councilmember Hans Riemer recently introduced a zoning amendment that would remove more barriers to building these apartments. However, the building rules for all three jurisdictions are the same, because the International building Code (IBC) and the International Residential Code (IRC) regulate what’s allowed and what’s not.

There are many benefits to accessory apartments, but building one can be a challenge and they're not right for everyone. That's why there are various professionals to help with everything from design to permitting and construction. It's advisable to start by reaching out to a local architect with experience in building and zoning codes—their input early in the process is will help you determine the overall feasibility of the project.

Architects evaluate the existing conditions and potential ways to implement an accessory apartment on your property. They will also be able to design and document the project in order to present it to the local regulating agency that will provide building permits. Local general contractors with experience in residential buildings will be able to provide a estimate and complete construction based on the building permit.

How can I get started?

There are several resources a homeowner can use to evaluate if and how to move forward with an accessory apartment. First, you'll want to verify that accessory apartments are allowed in your area, which you can find in the zoning website for your jurisdiction. (Plug your address into the DC Department of Zoning or the Montgomery County Office of Zoning websites to check.)

Another question to answer is: Who is this for? Will this be rented for income? Will I move in the accessory apartment to reduce my living space and expenses as I age? Will the accessory apartment be used as a nanny flat or for short-time guests? There are a lot of options, and the future occupant will define a lot of design decisions.

Next, you want to evaluate where and how you visualize an accessory apartment on your property. Is the basement a good place? Is a freestanding structure better? What are the pros and cons of each option? An architect can help you evaluate each option by considering the existing conditions of the building, potential growth, access, and other factors that will impact your decision of where to add another apartment.

Once you have decided of the where, who, and why, it's time to reach out to a professional who will assist you in the what and how.

Creating accessory apartments in our region helps increase property values, boost affordable housing, and allow residents to stay in their neighborhoods as their family situation changes. As we welcome more and different types of families, accessory apartments can help with our region's current shortage of affordable housing.