The ABCs of ADUs

Accessory Dwelling Units (ADUs) can help existing homeowners stay in their houses longer, while allowing new residents to live in an otherwise unaffordable area

The ABCs of ADUs

by Ileana Schinder

Additional Dwelling Units (ADUs) are one of the various types of housing solutions for current urban dwellers. Statistics show that family types have changed significantly. People are staying single longer, having fewer children, and frequently prioritize walkable urban neighborhoods over suburban ones. People are giving up large homes for smaller units, shared spaces, and urban amenities. Those cultural shifts, along with the high cost of living in the cities, have forced politicians and residents to rethink where and how new housing should be accommodated. ADUs help meet this need while minimally impacting infrastructure in cities.

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 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, parks, and schools.

Basement Apartment in Washington DC 1

What is an Additional Dwelling Unit?
Also called “accessory apartments” or “Granny flats”, an ADU 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. An ADU cannot receive a lien, taxed or mortgaged separately from the main house. A homeowner cannot sell an accessory apartment separately from the main property, but it can be leased in any terms allowed by local regulations. All utilities and uses between the dwellings are independent from each other. Separate metering depends on the regulations of local utility companies.

The typical examples of accessory apartments include refurbished garages and basement units. In the Washington, D.C. area, the most common accessory apartment format is the basement format as part of early 20th century townhouses.

Is an Additional Dwelling Unit feasible?
When considering adding an accessory apartment, there are a few things a homeowner should keep in mind. First, they should study zoning and building code requirements and educate themselves on the economic impact of the investment.

ADU at refurbished garage in Washington  1

Zoning and building codes are a set of rules that identify what is allowed in the ADUs. Zoning regulations are the rules that organize elements between the building envelope and the city that surrounds it. Elements ruled by zoning codes are setbacks, height, area, lot coverage, parking and other elements required by the zone in which the property is located.

The building code regulates everything from the building envelope to the inside of the structure, for example: 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.

Interior @ 1-bedroom at freestanding ADU 1

Benefits of Additional Dwelling Units
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 construction and financing. 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 will help determine the overall feasibility of the project.

Architects evaluate the existing conditions and potential ways to implement an accessory apartment on the 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 an estimate and complete construction based on the building permit. Appraisers and financial institutions will provide a loan product that matches the value of the investment.

How to afford an Additional Dwelling Unit?
The construction costs of ADUs vary greatly per region and type of construction. The affordability of such a project can be evaluated by calculating the current value of the house, the cost of adding an ADU, and the potential rental fee that will collected once the unit is complete. Also, by adding an ADU, the existing property value will increase so aside from the rent collection, the homeowner will obtain the return in investment once and if they decide to sell the property.

Talking with a financial institution can educate the homeowner on the options for loans and financial products that can be implemented to build a project like an ADU. The financial industry and appraisers are still working on how to evaluate the monetary contribution of an additional dwelling unit to the property.

Creating accessory apartments help 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 provide housing options that are currently unavailable.

Floor plan of renovated ADU @ Garage 1


Argentina-born Ileana Schinder is an architect in Washington, D.C. She has a passion for design challenges that include modern and creative solutions that meet her clients' needs. Having designed a number of ADUs in her area, Ileana has become the go-to ADU expert by media and local institutions. She graduated with a B.A. in Architecture from Universidad Nacional de Cordoba and an M.A. in Communications from the American University. Her company was awarded "Best Architecture Firm" by Washington City Paper's Best of D.C. competition. Ileana is licenced to practice architecture in Washington D.C., Virginia, and Maryland, and is a LEED-certified accredited professional as well as a Passive House Design Consultant.