• Jake Kirkwood

Accessory Dwelling Units. The What, How, and Why.

What is an Accessory Dwelling Unit?


An accessory dwelling unit, commonly referred to as an ADU, is a secondary housing unit on a single-family residential lot, built within the limits of an existing property. Also known as Accessory Apartments or Additional Dwelling Units (ADUs). ADUs come in many types and sizes and are frequently found throughout Washington, D.C.


The most common type of ADU is the basement apartment or "English Basement." English Basements inhabit hundreds (if not thousands) of rowhomes across the District and are often a focal point during redevelopment. Other types of ADUs include in-law suites, converted garages, and small, free-standing structures.

How do I find out if my property allows an ADU?


First and foremost, zoning and building code must be consulted during the planning stage to ensure that (a) an ADU is allowed by-right on your property and (b) there is at least the minimum available space on your property for an ADU. (We can evaluate this for you). Knowing the difference between building code and zoning code is important as well. Building code is the set of rules that governs everything inside of your ADU, such as ceiling height, window and room sizes, electrical, plumbing.....you get the idea. Zoning code governs everything outside of your ADU, such as building height, yard setbacks, etc.


The DC Zoning Handbook was updated in 2016 with new rules on ADUs and what zones they are allowed in by-right. Follow the link here to read more.


To summarize the updated zoning regulations, properties located in Zones designated "R" may have ADUs. When renovating an existing home to include an ADU, there are minimum gross floor areas (GFAs) that must be met to allow for the inclusion of an accessory apartment. In Zones R-1-A, R-1-B, and R-19, the minimum GFA must be 2,000 sq. ft. However, in Zones R-2, R-3, R-10, R-13, R-17, and R-20, the minimum GFA must be 1,200 sq. ft. If constructing a free-standing ADU, the following requirements must be met:

  • There must be permanent access to the accessory dwelling

  • The accessory building cannot be used simultaneously for any other use than as a private vehicle garage, an artist studio, or storage for a dwelling unit on the lot

  • An accessory building that houses an apartment may not have a roof deck

There are many more zoning regulations (i.e. rear yard ratio, maximum square footage, setbacks, parking requirements) pertaining to ADU construction; we advice you consult the DC Zoning Handbook and DCRA for more information, or give us a call for guidance.

Accessory Dwelling Unit (Free-Standing)

Why are ADUs important?


According to Ileana Schinder, a licensed architect in Washington, D.C.; "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."



ADUs certainly pose a lot of challenges during design and construction, but as a whole, they can be beneficial to the homeowner and the community. If you are interested in creating an ADU on your lot, give us a call (202) 386-7375 or send us an email (info@demarr-engr.com) and we would be glad to help!


This article is brought to you by DEMARR Engineering, PLLC, a Washington, DC Civil Engineering Firm. For a list of Civil Engineering and Land Surveying services we provide in Washington DC, visit https://www.demarrengineering-dc.com/services-dc or email us at info@demarr-engr.com for help with your property.

© 2020 by DEMARR Engineering, PLLC. 

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