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

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 is generally limited to around 1,000 square feet with dimensional and rear-yard limits, and 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.

Fairfax County, Virginia (and Northern Virginia)

In Fairfax County, ADUs are officially called Accessory Living Units, or ALUs, and the standards are specific. An ALU is allowed only on a single-family detached lot, one per property, and is limited to two bedrooms and two occupants, with owner-occupancy of either the principal dwelling or the ALU required.

Size and process depend on the type. An interior ALU, inside the existing home, can be up to 800 square feet or 40 percent of the principal dwelling’s gross floor area, whichever is less, and can be approved by an administrative permit, the streamlined path. A detached ALU can be up to 1,200 square feet, but it requires a special permit and is only allowed on lots of 2 acres or more, a significant restriction that rules out most typical suburban lots. Applications run through the county’s PLUS portal, and properties on septic systems need Health Department confirmation that the system can support the added unit. For a deeper dive, 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

Virginia is moving in the same direction as Maryland. The state passed ADU legislation, effective January 1, 2027, that requires localities to allow ADUs by-right on qualifying residential lots, subject to size, setback, and lot-size standards (generally a maximum of 1,000 square feet or 50 percent of the primary dwelling), and it preempts more restrictive local rules. In practice, that means Northern Virginia’s ADU rules, including Fairfax County’s, are set to loosen significantly in 2027. Because the law takes effect in the future, treat this as worth knowing and verify the current status and your locality’s implementation before you plan.

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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