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

Regulations last verified: September 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, as a matter of right in the residential (R) zones, except the R-19 and R-20 zones, following the 2016 zoning update. An attached or interior accessory apartment can occupy up to 35% of the house's gross floor area. A detached unit has no minimum lot size and no single ADU-specific square-foot cap, it is governed instead by the accessory-building standards for your zone, plus an entrance and alley-access rule (Subtitle U § 253.8) that decides whether a backyard unit is even feasible, see our DC ADU builder guide for that rule in detail. 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 from DOB and a rental housing license from the Department of Licensing and Consumer Protection (DLCP).

Montgomery County, Maryland (Bethesda, Chevy Chase and unincorporated areas)

Montgomery County allows ADUs by-right, and the process runs through two departments: you need a Class 3 Accessory Dwelling Unit license through DHCA, plus building permits through the Department of Permitting Services (DPS). Since October 5, 2026, under ZTA 26-06 (Ordinance 20-36), an attached or detached ADU must be “not greater than 75% of the size of” the primary single-family detached dwelling unit, and it must meet the setback, height, and building lot coverage standards of an accessory structure in the underlying zone. These are the county’s rules: the City of Rockville and the City of Gaithersburg have their own ADU zoning and licensing (see Rockville and Gaithersburg ADU rules).

Key Montgomery County specifics: under DHCA’s Class 3 license rules, the ADU must be in the owner's primary residence, and every owner must show proof of residency, and under the zoning code an ADU cannot be on a lot where a short-term residential rental exists or is licensed. Parking: one on-site space for the ADU in addition to any space required for the main house (only that one space if the main house has no on-site parking), none within one mile of a Metrorail, Purple Line or MARC station, or a Hearing Examiner waiver where on-street parking is adequate or the lot's size or shape rules out more on-site parking. 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% of the principal dwelling'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, is 1,200 sq ft, permitted only on lots of at least two acres, and requires a special permit with a Board of Zoning Appeals public hearing. On lots under that threshold, 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.

What does Maryland’s statewide ADU law (HB 1466) require?

Maryland’s HB 1466 requires counties and municipalities with planning and zoning authority to adopt a local law allowing accessory dwelling units on land with a single-family detached home on or before October 1, 2026. The law was enacted as Chapter 197, signed April 22, 2025, and took effect October 1, 2025.

Montgomery County already allowed ADUs by-right, and it then amended its rules with ZTA 26-06 (Ordinance 20-36), effective October 5, 2026, to amend the use standards for attached and detached accessory dwelling units consistent with State law (details in our Montgomery County ADU guide). The law matters most in Maryland jurisdictions that were previously restrictive: if yours was one, your options may have expanded, and the practical step is to confirm what your local ordinance now says.

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 is effective July 1, 2027, and it 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: public hearings October 29 and November 17, 2026

Here is the number that matters. By the county’s staff report, only about 5% of single-family detached homes in the county sit on lots of at least two acres, which is why detached ALUs have been so rare. Staff recommend allowing a detached ALU by right on lots of 36,000 square feet or more (another 9% of single-family detached homes are on lots from 36,000 square feet up to that size) and with a special permit on any smaller lot.

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: County staff published the staff report on September 29, 2026. Public hearings: the Planning Commission on October 29, 2026 at 7:30 p.m. and the Board of Supervisors on November 17, 2026 at 4:30 p.m., both in the Fairfax County Government Center auditorium. The Board can adopt staff's recommendations, one of the alternative options, or keep the current rules. Nothing changes until it adopts an amendment. Checked against the county’s staff report and FAQ on October 14, 2026. 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.

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