Everything homeowners need to know about building an accessory apartment in the District of Columbia - size limits, the alley-access rules that shape backyard cottages, and the owner-occupancy requirement that still applies here even though many other cities have dropped it.
Last verified: August 2026
6/10 · Moderately ADU-friendly
450 sq ft*
Max ADU size
22 ft
Max height
Required*
Owner-occupancy
By-right
Permit type
This page summarizes Title 11 DCMR (District of Columbia Municipal Regulations), Subtitle U § 253 and Subtitle D, Chapter 50 (accessory apartment and accessory building standards) for general reference - it is not legal advice. Washington, D.C. is not part of any state and has no county government; the DC Zoning Commission and DC Council are the top-level authorities, with no separate state law layered underneath. Regulations change; always confirm current requirements with the DC Department of Buildings (DOB) before designing or applying for permits. See sources below.
Zoning & size requirements
What you're allowed to build
ADU type
Max size
Max height
Notes
Accessory apartment within the principal dwelling (basement, above-garage, or attached addition)
Up to 35% of the principal dwelling's gross floor area (GFA)
Same as the principal dwelling
The house itself must have at least 2,000 sq ft of GFA in the R-1-A, R-1-B and R-19 zones, or 1,200 sq ft in most other R zones, to qualify
Accessory apartment in a detached accessory building (backyard cottage, garage apartment, carriage house)
Greater of 450 sq ft or 30% of the required rear yard area (450 sq ft is the larger, controlling figure on most lots)
2 stories and 22 ft
Must be reached by an 8 ft-wide path to the street, a 24 ft-wide alley, or a 15+ ft-wide alley within 300 ft of a public street
Allowed zoning districts
DC's zoning code calls this use an "Accessory Apartment" (Title 11 DCMR, Subtitle U § 253) - "ADU" and "accessory dwelling unit" are used interchangeably in practice and on this page. Accessory apartments are allowed as a matter-of-right (no discretionary approval needed) in nearly every Residential (R) zone citywide, including R-1-A, R-1-B, R-2, R-3, and R-6 through R-21. The two Georgetown zones, R-19 and R-20, are the exception: an accessory apartment there requires a special exception from the Board of Zoning Adjustment. Confirm your parcel's zone on DC's official zoning map before designing.
Additional standards
Setbacks, parking & other rules
Setbacks
Detached accessory buildings must sit at least 7.5 ft from the centerline of a rear alley; no side yard is required for the accessory building itself
Parking
No additional off-street parking is required for the accessory apartment itself. The one parking space required for the principal dwelling can't be eliminated to make room for the ADU without being replaced
Units per lot
1 accessory apartment permitted per lot, in addition to the principal dwelling, in R zones
Owner-occupancy
Required, but flexible: either the principal dwelling or the accessory apartment must be owner-occupied for the duration of the accessory apartment use. You don't have to live in the main house specifically, but the property can't be entirely non-owner-occupied while the ADU is in use
Occupancy limit
No more than 3 people may live in the accessory apartment (up to 6 combined with the principal dwelling in the R-19 and R-20 zones)
Building code
Must meet DC's amended versions of the International Residential Code or International Building Code, including DC-specific ceiling-height and egress requirements
Certificate of Occupancy
Not required for an accessory apartment in an R zone, since it's treated as a secondary use of a one-family property. A second dwelling added in an RF or RA zone is a different category and does require one
Rental license
A One-Family Residential Rental Business License (Basic Business License) from DLCP is required to rent out the ADU or the primary home, plus registration with DHCD's Rental Accommodations Division
Roof decks
Not permitted on accessory buildings
Ordinance history
How D.C.'s ADU rules got here
2016Zoning Regulations of 2016 take effect (September 6). DC's first comprehensive zoning rewrite since 1958 made accessory apartments a matter-of-right use in nearly all R (Residential House) zones for the first time - previously they were highly restricted and rarely permitted in accessory buildings like garages.
2019ADU DC initiative launches. The United Planning Organization and Coalition for Smarter Growth, with founding support from Citi Community Investing, launched a pilot effort to help low- and moderate-income DC homeowners navigate zoning, permitting, and financing for accessory apartments.
2021Zoning Commission eases accessory-building standards (Order 20-19). The height limit for detached accessory buildings rose from 20 ft to 22 ft, and the required alley setback was reduced from 12 ft to 7.5 ft from the alley centerline, making backyard cottages easier to fit on typical DC lots.
2022DC launches the Residential Accessory Apartment Program (RAAP). A city-funded pilot offering forgivable assistance of up to $75,000 to income-qualified homeowners who build an accessory apartment and rent it at an affordable rate, tied to Mayor Bowser's citywide housing-production goals.
2022DCRA splits into two agencies (October 1). The Department of Consumer and Regulatory Affairs was dissolved; building permitting, inspections, and zoning administration moved to the new Department of Buildings (DOB), while business licensing moved to the new Department of Licensing and Consumer Protection (DLCP).
Worth knowing: even with matter-of-right zoning in place since 2016, ADU production in DC has stayed modest - the DC Office of Planning has reported fewer than 50 accessory apartments approved per year citywide, and reporting on the city's own RAAP subsidy pilot found only a handful of its target units actually completed as of mid-2025. Financing, DC's high construction costs, and a permitting process built for larger projects are the barriers most often cited by advocates and city planners.
Permit process
How D.C. ADU permitting works
1
Confirm your zoning and eligibility
Look up your property's zone on DC's official zoning map and confirm your lot qualifies for an accessory apartment. Most R zones do by-right; R-19 and R-20 require a special exception. A pre-application check with DOB's Zoning Administrator can catch problems early.
2
Hire an architect and prepare a full drawing set
DC requires a complete architectural, structural, plumbing, mechanical, electrical, and fire-protection drawing set for any new dwelling unit - a registered architect or engineer is required regardless of who manages the rest of the project.
3
Assemble supporting documents
Gather your plat from the DC Office of the Surveyor, a DC Water service worksheet, an Energy Verification Sheet for the Department of Energy and Environment, and a structural certification if significant structural work is involved.
4
Submit your application to DOB
File online through DC's permitting portal, pay the initial invoice, and upload your drawing set for review. Properties in a historic district also need a separate Historic Preservation Office review, which can run alongside or before the DOB application.
5
Respond to plan review comments
Reviewers from each discipline - zoning, structural, plumbing, and more - return comments; expect at least one review cycle before your permit is approved. More complex projects or those needing zoning relief take longer.
6
Build, inspect, and license the rental
Once your permit issues, pull trade permits and schedule inspections as work proceeds. An accessory apartment in an R zone doesn't need a separate Certificate of Occupancy, but you'll need a One-Family Rental Business License from DLCP before renting it out.
Plan review has historically taken anywhere from several weeks to a few months depending on application completeness, and considerably longer if historic review or a special exception is involved. Fees and timelines change as DOB updates its systems and fee schedule - confirm current figures directly with DOB before budgeting.
Budgeting
What it costs to build an ADU in D.C.
Accessory apartment in an existing garage or accessory building
Roughly $130,000-$230,000; converting an existing structure keeps costs down, though DC's compact 450 sq ft cap keeps per-square-foot costs high
Basement/cellar apartment
Roughly $150,000-$260,000, more if underpinning is needed to meet DC's ceiling-height and egress requirements in a rowhouse basement
New detached accessory building
Roughly $150,000-$300,000+ for a fully code-compliant unit at DC's small permitted footprint
Permit and plan-review fees
Commonly estimated in the $1,000-$3,000 range depending on construction valuation - confirm DOB's current fee schedule when you apply
Architect fees
Typically 10-20% of construction cost for the full professional drawing set DC requires
These are general ranges drawn from local contractor estimates and reporting on completed DC projects, not quotes - actual cost depends heavily on lot access, whether underpinning or utility relocation is needed, and finish level. DC's small accessory-building footprint (450 sq ft in most cases) means detached units often cost more per square foot than larger ADUs allowed elsewhere, since a kitchen and bathroom cost roughly the same no matter how small the unit around them is.
Before you apply
Tips for a smooth approval
Decide which unit will be owner-occupied before you design. DC still requires either the principal dwelling or the accessory apartment to be owner-occupied - settle this early, since it shapes your financing and rental plans.
Check your access route before designing a detached unit. An accessory apartment in a backyard structure needs a qualifying path or alley - if your lot doesn't have one, a basement or attached conversion may be the only realistic option.
Budget separately for DOB fees, plan review, and your architect. DC requires a full professional drawing set regardless of project size, and these soft costs add up before construction even starts.
Ask about the Residential Accessory Apartment Program (RAAP) if you qualify by income. The city has offered forgivable assistance to income-qualified homeowners in exchange for affordable rents, though funding and availability change - confirm current status with DHCD.
Check for historic district review early. A property in one of DC's many historic districts needs Historic Preservation Office sign-off before or alongside your DOB application, which can add real time to the schedule.
FAQ
Common questions
Do I need to live on the property to build an accessory apartment in D.C.?
Not exactly, but one of the two units has to be owner-occupied. DC's rule lets the owner live in either the principal dwelling or the accessory apartment - you don't specifically have to live in the main house, but the property can't be fully non-owner-occupied while the accessory apartment use is active.
What does D.C.'s zoning code call an ADU?
"Accessory Apartment" is the defined term in Title 11 DCMR, Subtitle U § 253. "Accessory dwelling unit" and "ADU" are used interchangeably in practice and in DC's own planning materials, but "accessory apartment" is the term that actually appears in the zoning code.
How big can a detached ADU be in D.C.?
In most R zones, a detached accessory building is capped at the greater of 450 sq ft or 30% of the required rear yard area - in practice, 450 sq ft is usually the larger, controlling figure. It can be up to 2 stories and 22 ft tall.
How big can an attached or basement ADU be?
Up to 35% of the principal dwelling's gross floor area, and only if the house itself has at least 2,000 sq ft of GFA (R-1-A, R-1-B, R-19 zones) or 1,200 sq ft (most other R zones).
Do I need extra parking for an ADU?
No additional off-street parking is required specifically for the accessory apartment. The one parking space required for the principal dwelling can't be removed to make room for it without being replaced.
Is D.C. part of a state ADU law, the way California or Washington-state cities are?
No. DC is not part of any state and has no county government either - Title 11 DCMR, adopted by the DC Zoning Commission, is the top-level zoning authority with no separate state law layered underneath it. Congress retains limited oversight of DC legislation under the Home Rule Act, but this essentially never affects day-to-day zoning or building-permit administration.
Which agency handles ADU permits in D.C.?
The Department of Buildings (DOB), created in October 2022 when the former Department of Consumer and Regulatory Affairs (DCRA) was split into two agencies: DOB, which handles permitting, inspections, and zoning administration, and the Department of Licensing and Consumer Protection (DLCP), which handles business and rental licenses.
How much does an ADU cost to build in D.C.?
Converting an existing garage or accessory building tends to run roughly $130,000-$230,000, a basement conversion roughly $150,000-$260,000, and a new detached unit roughly $150,000-$300,000 or more, based on local contractor estimates and reporting on completed DC projects. DC's small permitted footprint and high construction costs keep per-square-foot pricing high even for compact units.
Is D.C. actually building many ADUs?
Not many yet. Even with matter-of-right zoning since 2016, DC's Office of Planning has reported fewer than 50 accessory apartments approved per year citywide, and the city's own RAAP subsidy pilot fell well short of its unit targets through 2025. Financing, construction cost, and a permitting process built for larger projects are the barriers most commonly cited.
