Myth vs. Fact: Accessory Dwelling Units 15

Myth: The Zoning Update will allow residents to build McMansions or apartment buildings in their back yards.

Fact: We’re proposing some modest changes to the rules for accessory dwelling units (or ADUs). ADUs are actually already allowed under the current zoning regulations (we now call them “accessory apartments,” but “accessory dwelling unit” is the more common term throughout the U.S.).

Under the zoning regulations in place today, residents in R-1, R-2, and R-3 zones can have an ADU within their home, but they are required to obtain a special exception from the BZA. There are a host of other rules that apply: the ADU can be no larger than 25% of the house, the owner must occupy either the main dwelling unit or the ADU, and so on.

Interestingly, homeowners in R-1 can also have an ADU above their garage by-right, under the current rules. There are very few restrictions governing these ADUs—the building can measure 20 feet in height and, as long as the main building and the garage stay within the lot occupancy restrictions of the zone, there are no limits on the size of the ADU. There is one restriction; the ADU must be for a “domestic employee.” While this rule might have made sense in an era where live-in housekeepers were common (at least for a certain part of the population), we’re not sure it applies to many families today.

So, as we engaged in our comprehensive review of the zoning regulations, a couple of things struck us as odd.

 First, the current regulations treat an ADU within your home much more restrictively than one in a separate building. Remember, as long as you can show some evidence that your tenant is a “domestic employee,” you’re allowed to add an apartment on to that garage without any public review of how big it is, or what impacts it might have. In contrast, ADUs inside the home, which are comparatively small and hidden from view, must go through a public hearing.

Second, for our R-1 zones, there’s nothing that says you can’t have both the ADU within your house and the one over your garage. In theory, you could have three independent dwelling units on the same property. (Granted, we’re not aware of anyone taking advantage of this.)

 At the same time, we expect much more interest in this type of living arrangement in the future. A recent Washington Post article showed how families in Virginia are using ADUs as a way to provide independent living for aging parents. It’s also a way for young homeowners to get some additional income to help with the mortgage, or to offer more choices to the greater number of small households arriving in the District every day.

 So, here’s what we’ve proposed:

  • If you want an ADU inside your house, you would be able to do it by-right (provided you follow all the rules limiting its size, maintain owner-occupancy, etc.).
  • If you want an ADU in an existing garage or carriage house (the catch-all term we use is “accessory building”), you could also do it by right. Again, there are some restrictions; see below.
  • If you want to build a new ADU in your backyard, you’d have to get a special exception; we’d also require a special exception if you were going to put an ADU in a garage or carriage house that was expanded after the new regulations went into effect.

 So, could you build a monster home in your backyard? Not really. Remember, the only way to add an ADU in an existing accessory building is to not enlarge the structure – the accessory building would have to remain its current size. We think this makes perfect sense in older neighborhoods, where many homeowners have carriage houses that can’t be readily adapted for other uses.  We see ADUs as great spaces for young residents looking for affordable housing options or elderly residents that want to be close to their families so they can help raise their grandchildren.

 If you’re building something new (or adding on to an existing accessory building), here’s what you’d have to do, in addition to seeking a special exception, which includes a public hearing so neighbors can voice any concerns:

  • Keep the footprint of the building to 450 square feet or less.
  • Keep all balconies or decks within that footprint, and make sure they don’t face the immediately adjoining houses (currently, those balconies and decks are unregulated).
  • Keep the total living area to 900 square feet or less (in other words, you could have two full stories).
  • Keep the height of the building to no more than 22 feet (remember, you can do 20 feet now for your “domestic employee” apartment; we’re proposing an extra two feet to account for the slope of a roof).
  • Stay at least 30 feet away from the main building (we want an adequate distance for light, air, open space, and fire separation).
  • Keep any other accessory uses out of an accessory building with an ADU in it, except for a garage (we’re assuming this is independent living space, not an opportunity for the homeowner to expand a home business, but we thought it best to be clear).

 If it sounds like we’re being a little detail-oriented here, that’s because we want to be sure that any new construction fits into the fabric of single-family neighborhoods. And it’s possible that, without some oversight, the greater interest we anticipate could lead to some unintended consequences. That’s why we’re closing loopholes, setting some reasonable limits, and giving neighbors an opportunity to weigh in on the most significant changes or additions.

 Would you want an ADU on your property? How about your neighbor’s? And what rules should we expect people to follow if they want to build one?


  1. Very informative post. There are still a couple of myths out there perhaps you can clarify.

    First, can you clarify if there can be multiple ADUs sited on a single property, Some opponents contend yes.

    Second, what is the logic behind not having a limit on the number of people who may live in an ADU? Claims have been made this means 5 people will be crammed into a single ADU. Please clarify how the limit is to apply.

  2. First: Under our proposal, you could never have more than one ADU on a property. Our draft regulations have always been very clear about this.

    Second: The proposed regulations say “the total number of persons that may occupy the building, including the principal and accessory dwelling units combined, shall not exceed six.” This is taken directly from the current regulations, which have the same provision. So in theory, it would be possible to have 5 people in the ADU, and one in the principal dwelling unit (or vice versa). However, if the household in the main dwelling unit is larger, the number of people living in the ADU would have to decrease. It’s meant to be a flexible rule that can accommodate changes in household size over time.

    And, as a point of clarification, our original draft appeared to say that the 6-person limit would only apply to ADUs within the principal building on the property. This was an error – the revised draft will clarify that this rule applies whether the ADU is in the main building or in an accessory building.

  3. This is really good information. Under your proposed changes, could newly built ADUs be placed in back yards in areas zoned R-1, R-2, and R-3? Thanks for clarifying!

  4. Yes, newly built ADUs could be placed in rear yards in the R-1, R-2, and R-3 zones. As indicated above, they’d have to go through a special exception approval process.

    • Can you describe the special exception process? There has been concern expressed that neighbors are not part of that process and cannot weigh in on proposed ADUs.

  5. Could I build a new ADU facing the 24′ wide alley behind my row house. My lot is 16.8′ wide and 174′ deep. I am in zone R-3.

  6. We’d need more information about your property to say for sure. First, not to sound like a broken record, but you’d need approval through the special exception process, which means having a hearing before the Board of Zoning Adjustment.
    There are a lot of criteria that you’d have to meet in order to gain approval. Will the main building and your ADU stay within the lot occupancy limit for your zone (in the case of R-3, it’s a 60% limit)? Can you separate your ADU at least 30 feet from the main building? Can you provide pedestrian access to the ADU through an alley, yard, or easement? All these, and many other questions, would need to be answered.

  7. Good day. Might you update us on the status of this proposal? We’re in middle of a full home renovation of an R-3 zoned home and it would be most cost-effective to build the basement as an ADU now…but unless code has been changed, we don’t think we’d pass inspection. If the change has not yet been approved, do you have a sense of how long it will take? We’re willing to consider delaying renovation if it means we will soon be able to renovate the home as we want it. The alternative is to renovate partially now and renovate the basement separately later once the ADU for R-3 is approved (and yes, we understand that we could go through the special exception process now).

  8. We live in R-1-A zoned neighborhood of many children. The street has very limited parking. Our neighbors are currently building an accessory apartment (without zoning approval), planning for short-term renters from Craig’s list, as well as renting their house during the summers. This is most disturbing; will this be permitted under the new proposal?

    • Thank you for your comment. The current zoning regulations permit a resident to take on a boarder or lodger within the home, but this does not permit the creation and rental of an accessory dwelling unit (an apartment with a full kitchen and bath) on the property. A Bed and Breakfast, for short term accommodation, is also a permitted use subject to conditions, but this also would not entail a separate dwelling unit. However, the R-1-A zone currently permits an accessory unit, but only for someone who works on the property (the regulations refer to a “domestic”) above an accessory building, such as a detached garage, on the property.

      Under the new ZRR proposals, an accessory unit within the house or, subject to conditions, above an accessory building would be permitted. The unit could not be used as a transient accommodation and would require minimum stay of 30 days. There is nothing in the existing or the proposed regulations that would prohibit the renting of the house in the summer, although the new regulations would not permit the house to be rented if there is an accessory apartment on the property that is also being rented. The proposed accessory dwelling unit provisions require that the owner reside on the property.

  9. What about parking? Does the property owner have to provide a minimum amount of parking for the accessory unit?

    • Under the current zoning regulations and under the ZRR proposal, which the Zoning Commission has taken proposed action to approve, an additional parking spot for a bed and breakfast, a boarder or lodger, or an accessory dwelling unit would not be required.

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