Washington, DC
Accessory Dwelling Units

Accessory Dwelling Units (ADUs) are now permitted as a matter of right in most residential zones in the District of Columbia after the adoption of the new Zoning Regulations in 2016. Accessory Dwelling Units, by definition, are secondary to the principal dwelling unit in both size and intensity of use. They can either be part of the structure of the principal dwelling or can be a detached building. Basement apartments, in-law suites, converted garages, and detached “tiny homes” all fall under the umbrella of the term ADU. The portion of the DC Zoning Handbook pertaining to Accessory Dwelling Units along with the District of Columbia Municipal Regulations regarding accessory apartments can be found here: http://handbook.dcoz.dc.gov/use-categories/other-uses/accessory-dwelling-units/. Outlined below are some important considerations when contemplating the design and construction of an Accessory Dwelling Unit.


Zoning regulations are different than building codes in that codes tells you how to build your house, while zoning tells you where on your property and how large you can build. Rear yard area, side and rear setbacks, and lot occupancy all must be taken into consideration. A Preliminary Design Review Meeting (PDRM) with a Zoning Technician at DCRA is highly recommended to identify non-compliance issues prior to submitting for a permit. Applying for a variance is an option, although the District of Columbia is not as progressive as other states in this regard. A few of the main zoning guidelines include:

Rear yard ratio – the maximum building area for a detached ADU is either 30% of the required rear yard area, or 450 square feet; whichever is greater.

Maximum square footage – an attached accessory apartment cannot be more than 35% of the gross floor area of the principal dwelling.

Set backs – the ADU may be built within a rear yard but must be located a minimum of 12ft from the centerline of the alley, if applicable. For most properties in DC, Accessory Dwelling Units cannot be located within the required side yard setbacks simply because typical lots are relatively narrow.

Height restrictions – two stories and 20 feet for detached ADU’s is the limit.

Parking requirements – a parking space is not required for an Accessory Dwelling Unit, only for the principal dwelling on the property.

Renting an accessory apartment – the property owner must obtain a Residential Rental Business License from DCRA and accessory dwelling will need to be inspected for code compliance.


As with any construction project, Accessory Dwelling Units present their own unique set of design challenges. Due to the smaller footprint, space can become an important commodity, both on the interior of the building as well as the overall form of the building on the property. You don’t, however, want to simply build a box that will satisfy the zoning requirements. Some important considerations when designing an ADU, particularly a detached unit, include:

How the design will compliment that of the existing home, and this doesn’t necessarily mean “match” the existing structure.

The extent of aging-in-place design techniques and practices if the ADU is intended for an elderly family member.

How to maximize use of the small floor plan. Due to the limited square footage, maximizing space can become an exciting design challenge. Hidden storage, pocket doors, open floor plans, loft spaces, and built-ins are just a few of the design elements that can help with such a challenge.

How to bring in natural light to the small spaces. This can be accomplished with strategically placed skylights, clerestory windows, and/or by designing open floor plans with loft spaces.

Aesthetic and functional differences when designing for a rental, an aging family member, or a play space for children. The intended use can help inform the fixture and finish selections, thus affecting the overall cost.


Even though detached ADUs are small, you are in fact building a full home from the ground up. We have found that building detached Accessory Dwelling Units in Washington, DC can range from $450 – $600k. Although there are plenty of prefab units advertised at low rates, they do not include some of the larger cost drivers such as:

Foundations and plumbing groundwork.

Need to heavy-up the electrical service and upgraded the panel at the existing home.

Need to upgrade the incoming water service line. This is not always required but should be taken into consideration.

Bringing the utilities from the primary dwelling to the ADU, which entails digging a trench across the rear yard.

Fire sprinkler systems, which are required for new construction.

Window and door packages, which are often specialty items due size constraints.


Building a detached accessory dwelling is an exciting project, as you can watch a tiny home emerge from the ground before your eyes. Further, the construction offers less of a disruption to your daily life than a principal dwelling renovation would. There are a few additional components of the design and construction that need to be considered, some of which are not as apparent in the early stages of the project:

Landscaping – it is likely that a portion, if not most, of the rear yard will be dug up for utility work, and potentially the front yard if the water service line needs to be upgraded. Depending on the intended use of the new unit, you may want an access path between the two houses, an access path from the main street back to the ADU, or both.

Storm water management – a new building equals more impervious surface on your property, which means more potential storm water runoff. Depending on the size of your lot, more in-depth storm water management practices may need to be implemented, such as installing drywells, bioswales, or other techniques to mitigate runoff.

Construction fences and site control – The Department of Energy & Environment (DOEE) will ensure compliance with site control in the form of construction fences, designated construction entrances for large equipment, and staging areas for construction materials. You will also need to inform the District Department of Transportation (DDOT) ofthe location of the proposed construction entrances in addition to how pedestrian and vehicular traffic will be directed to the site. A plan for how any applicable public space (i.e. alleys) will be protected from potential damage due to construction will need to be submitted as well.

If you are in the market for an Accessory Dwelling Unit, or have questions pertaining to anything related, please contact us. Our team is experienced in the design and construction of these projects in Washington, DC.

“I was fortunate to find Four Brothers to help me design and build my ADU. Their architects thoroughly understand the nuances of the zoning code and permitting procedure; both of which were confounding to me. Furthermore, they are talented designers and kept an eye on the bigger picture items such as actual construction and overall budget, all while providing an exceptional level of personal attention. I will look nowhere else when it comes time to renovate my principal dwelling.”

Washington, DC

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