Can You Build an ADU in DC, Maryland, or Virginia? 2026 Rules by Jurisdiction

Regulations last verified: July 2026

The short answer

Rules change often and vary by property, so always confirm the current requirements with your jurisdiction before you design or build. This guide is a starting point, not legal advice.

Quick comparison: ADU rules by jurisdiction (2026)

Washington, DC

DC allows ADUs, officially called accessory apartments, by-right in most residential (R) zones following the 2016 zoning update. A detached ADU’s size is governed by footprint and height rather than a single square-footage cap: the accessory building’s footprint is limited to the greater of 450 square feet or 30 percent of the required rear yard, with a height of up to two stories (about 20 to 22 feet), so the actual size depends on your lot. An attached or interior ADU can be up to 35 percent of the main home’s gross floor area. Permits go through the DC Department of Buildings (DOB), and DC does not require parking for ADUs given its transit network.

Two things to watch in DC. First, owner-occupancy is required in R zones: either the principal dwelling or the accessory apartment must be owner-occupied for the duration of the accessory apartment use, and this requirement cannot be waived (RF zones and two-unit flats can differ, so confirm your specific zone). Second, roughly 30 percent of DC properties sit in a historic district (Georgetown, Capitol Hill, Dupont Circle, and others), where exterior work on an ADU needs historic review before a building permit. If your row house has alley access, a rear-loading detached ADU is often the most feasible path. If you plan to rent the unit, DC caps accessory apartment occupancy at three persons and requires a Certificate of Occupancy plus a Residential Rental Business License.

Montgomery County, Maryland (Bethesda, Rockville, Chevy Chase, Gaithersburg)

Montgomery County allows ADUs by-right, and the process runs through two departments: you apply for a Class 3 Accessory Dwelling Unit license with the Department of Housing and Community Affairs (DHCA), and you pull building permits from the Department of Permitting Services (DPS). A detached ADU’s maximum size is the least of 50 percent of the main home’s footprint, 10 percent of the lot area, or 1,200 square feet. Attached ADUs can run up to 1,200 square feet.

Key Montgomery County specifics: the property must be your primary residence (you can live in either the main house or the ADU), only one ADU is allowed per lot, and short-term rentals like Airbnb are not permitted in a property with an ADU. Parking is generally one added space unless the property is within one mile of a Metrorail, Purple Line, or MARC station. Historic districts require a Historic Area Work Permit, and Takoma Park handles its own rental licensing. This is home turf for our team, our office is in Gaithersburg.

One operational note for Maryland projects: we handle design in-house here, so drawings, permits, and construction stay under one roof and one contract. In Virginia and DC we work with a licensed architect partner for the design drawings, then manage permitting and construction the same way.

Fairfax County, Virginia (and Northern Virginia)

In Fairfax County, ADUs are officially called Accessory Living Units, or ALUs, and they are allowed only on single-family detached lots, one per property, capped at two bedrooms and two occupants, with owner-occupancy of either the principal dwelling or the ALU required.

The size limit depends on the type, and this is exactly where the confusion tends to start. An interior or attached ALU is capped at 800 square feet or 40 percent of the main home’s gross floor area, whichever is less, and can be approved through an administrative permit (a larger interior size can be requested by special permit). A detached ALU, a separate backyard structure, can be larger, up to 1,200 square feet, but it is only allowed on lots of at least 2 acres and requires a special permit with a public hearing before the Board of Zoning Appeals (BZA). On lots under 2 acres, a detached ALU is not currently permitted, so the interior or attached route is the path. In other words, your lot size is what determines which option is on the table, and the 800 square foot figure is the interior cap, not a blanket limit. Properties on well or septic also need Health Department approval before a permit is issued.

You can confirm the current rules on the official Fairfax County Accessory Living Unit page. For a deeper dive on our side, see our Fairfax County ADU zoning guide and our Fairfax County ADU permit process guide.

New in Maryland: the statewide ADU law (HB 1466)

Here is a 2026 development worth knowing. Maryland passed HB 1466 in 2025, which sets a state policy encouraging ADUs and requires counties and municipalities with planning authority to adopt local laws allowing ADUs on land with a single-family detached home by October 1, 2026. Montgomery County already has established, by-right ADU rules, so its homeowners are ahead of the curve, but the law signals a broader statewide push toward easier ADU approval. If you are in a Maryland jurisdiction that has been restrictive, your options may be expanding, confirm your local ordinance’s current status.

Coming in 2027: Virginia’s statewide ADU law (and why it does not change Fairfax)

In April 2026, Governor Spanberger signed Senate Bill 531 (SB 531), a statewide ADU law that takes effect July 1, 2027. For the localities it covers, it requires ADUs to be allowed by administrative permit, caps ADU permit fees at $500, and removes any requirement that the ADU occupant be related to the homeowner.

Here is the part that trips people up: SB 531 exempts any locality that adopted an accessory dwelling unit ordinance before January 1, 2026, so it does not apply to Fairfax County, which adopted its ALU standards in 2021. The state law does not automatically loosen Fairfax’s rules; it is aimed at Virginia localities that did not already regulate ADUs. Arlington and Alexandria have their own existing ordinances and may also be exempt, but confirm that with the locality. If you are planning an ADU in Fairfax, the county’s own rules are what govern, not SB 531.

Fairfax County is rewriting its ADU rules: what is being proposed (2026)

Here is the number that matters. By Fairfax County’s own analysis, only 4.7% of single-family detached homes in the county sit on lots of two acres or more, which is why detached ALUs have been so rare. Under the proposed 15,000 square foot minimum, 39% of single-family detached homes would qualify, roughly an eightfold increase in eligible properties.

What would stay the same: One ALU per lot; a maximum of two bedrooms; short-term lodging (under 30 days) still prohibited on a lot with an ALU; and new external entrances must be on the side or rear.

Timeline: The Planning Commission's Land Use Process Review Committee met on July 9, 2026, with community outreach through summer 2026 and possible authorization and public hearings by the end of 2026. Nothing changes until the Board of Supervisors adopts an amendment.You can follow the amendment on the county’s Accessory Living Units amendment page.

What about cost?

ADU cost depends heavily on type and jurisdiction, but as a general planning range across the DC metro in 2026: basement or interior conversions tend to run lower, attached ADUs in the middle, and fully detached backyard units the highest, once you account for design, permits, and new utility connections. For a full breakdown, see our ADU cost guide. Every project is different, and the only real number comes from a site visit.

Frequently asked questions

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