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Montgomery County ADU Rules & Costs (2026 Update): Bethesda, Rockville & Gaithersburg

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Modern accessory dwelling unit built by Excell Homes in Montgomery County, Maryland

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The short version

  • Yes, Montgomery County allows ADUs by-right. You get a Class 3 license from DHCA and building permits from DPS, no special-exception hearing for a standard unit.
  • Size: since October 5, 2026, an attached or detached ADU must be “not greater than 75% of the size of” the primary single-family detached dwelling unit.
  • Owner-occupancy: the ADU must be in the owner's primary residence, and every owner must show proof of residency. The zoning code allows one ADU per lot, with no more than two occupants aged 18 or older, and 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.
  • Rockville and Gaithersburg have their own ADU rules; the county rules here cover Bethesda and other unincorporated areas. See the city section below.

What changed on October 5, 2026

Montgomery County’s ZTA 26-06 (Ordinance 20-36) was adopted September 15, 2026 and took effect October 5, 2026. It rewrote the county’s ADU rules (read the ordinance):

  • Size cap: the fixed square-foot caps are gone. An attached or detached ADU must now be “not greater than 75% of the size of” the primary single-family detached dwelling unit.
  • Basement units: the old exception that let a basement or cellar ADU use the whole basement was deleted, so a basement unit falls under the same size cap.
  • Parking: the extra space once required when a new driveway is built was deleted. The rule now is 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.

In full, the ordinance deleted the 1,200 sq ft cap on attached ADUs (which let a basement or cellar unit use the whole basement), the detached cap of the least of 50% of the house's footprint, 10% of the lot area or 1,200 sq ft, the 12-foot rear setback and the extra setback for detached ADUs longer than 24 feet along a lot line, and the rule requiring two on-site parking spaces when a new driveway is built. These are the rules for unincorporated Montgomery County; homes inside Rockville or Gaithersburg follow those cities’ own rules (see below).

The county rules below cover unincorporated Montgomery County, including Bethesda. Homes inside the City of Rockville or the City of Gaithersburg follow the city’s own rules instead (see Rockville and Gaithersburg). These rules are a starting point, not legal advice: confirm the current requirements with DHCA and DPS, or with your city, before you design or build.

Can you build an ADU in Montgomery County?

Yes. Montgomery County allows accessory dwelling units by-right, which means a standard ADU no longer needs the special-exception hearing the county once required. The process runs through two offices: you need a Class 3 Accessory Dwelling Unit license through DHCA, plus building permits through the Department of Permitting Services (DPS). Both an interior or attached unit and a detached backyard unit are allowed, subject to the size and site rules below.

How big can a Montgomery County ADU be?

Since October 5, 2026, the county zoning code defines an ADU as one that is “not greater than 75% of the size of” the primary single-family detached dwelling unit. The same limit applies to an attached unit (an addition or a basement) and a detached backyard unit. We confirm with DPS how the size of each design is measured against that limit.

Size is not the only limit. Every ADU must also meet the setback, height, and building lot coverage standards of an accessory structure in the underlying zone, and its footprint, together with the other structures on the lot, counts toward the zone’s total lot coverage limit.

Find out if your lot qualifies →

The rules that actually matter

Owner-occupancy and occupancy limits

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. The zoning code allows one ADU per lot, and the unit may have no more than two occupants aged 18 or older.

No short-term rentals

Under the county zoning code, an ADU cannot be on a lot where a short-term residential rental exists or is licensed. In practice that makes an ADU a long-term rental or space for family, not a nightly-rental unit.

Parking

Montgomery County requires 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. Separately, the rule that kept a new ADU 300 to 500 feet from an existing one was removed in 2019 (ZTA 19-01).

Rockville and Gaithersburg: their own ADU rules, not the county’s

Bethesda is unincorporated, so the county rules on this page apply there: the DHCA Class 3 license and DPS permits. Several Chevy Chase communities are incorporated municipalities with their own approval step: Chevy Chase Village, the Town of Chevy Chase, Chevy Chase Sections 3 and 5, the Village of North Chevy Chase, and Chevy Chase View. The City of Rockville and the City of Gaithersburg are different. They are municipalities with their own zoning, so a home inside either city’s limits follows the city’s ADU rules, not the county’s, and ADUs inside the City of Rockville and the City of Gaithersburg are not licensed by Montgomery County DHCA. A property just outside those city lines is back under the county rules. We confirm which rules govern your lot before design starts.

City of Rockville

  • Size: up to 75% of the main home's total floor area; an attached ADU in a basement may use the entire basement, and a detached ADU behind a home of less than 800 sq ft may be up to 600 sq ft.
  • Parking: no parking space is required for an ADU.
  • Owner occupancy: the owner must live in the main house or the ADU for at least six months of the year, and both must stay under common ownership.
  • Permits: a City of Rockville building permit for every ADU, and a City rental license if it is rented to anyone other than the owners' relatives; detached ADUs also need an automatic fire sprinkler system. One ADU per lot, on a lot with a single-unit detached home.

Source: the City of Rockville’s ADU regulations and ADU guidance (Zoning Ordinance Sec. 25.6.3.6), checked October 15, 2026.

City of Gaithersburg

The City of Gaithersburg has its own zoning code (City Code Chapter 24), so the county limits above do not govern homes inside the city. We confirm the city’s current ADU rules with the City of Gaithersburg for your address before design. See our service-area pages for Bethesda, Rockville, and Gaithersburg.

Maryland’s statewide ADU law (HB 1466)

Maryland’s HB 1466, enacted as Chapter 197 and effective October 1, 2025, requires counties and municipalities with planning and zoning authority to adopt a local law allowing ADUs on single-family detached lots on or before October 1, 2026. Montgomery County already allowed ADUs by-right, and it then amended its rules with ZTA 26-06 (Ordinance 20-36) to amend the use standards for attached and detached accessory dwelling units consistent with State law (see what changed).

What a Montgomery County ADU costs

Cost depends far more on the type of unit than on any single per-foot figure. A basement or interior conversion generally sits at the lower end, an attached unit in the middle, and a fully detached backyard unit at the higher end, once you account for design, permits, and utility connections. Montgomery County’s established neighborhoods and higher-end finishes can push a project up. For planning ranges by ADU type, see our DC-metro ADU cost guide, and if you’d rather build now and pay over time, our RenoFi financing is based on your home’s after-renovation value.

Frequently asked questions

Can you build an ADU in Montgomery County, Maryland?

Yes, by-right. You need a Class 3 Accessory Dwelling Unit license through DHCA, plus building permits through the Department of Permitting Services (DPS). A standard ADU no longer requires the special-exception hearing the county used to mandate. These are the county rules for unincorporated Montgomery County; the cities of Rockville and Gaithersburg set their own.

How big can an ADU be in Montgomery County?

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. Every ADU must also meet the setback, height, and building lot coverage standards of an accessory structure in the underlying zone, and its footprint counts toward the zone's total lot coverage limit.

Does Montgomery County require owner-occupancy for an ADU?

Yes. 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. The zoning code allows one ADU per lot, with no more than two occupants aged 18 or older.

Can I use my Montgomery County ADU as an Airbnb?

No. Under the county zoning code, an ADU cannot be on a lot where a short-term residential rental exists or is licensed. An ADU works as a long-term rental or as space for family, not as short-term lodging.

Do I need extra parking for an ADU in Montgomery County?

The county requires 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. Separately, the rule that kept a new ADU 300 to 500 feet from an existing one was removed in 2019 (ZTA 19-01).

Do Bethesda, Rockville, and Gaithersburg have the same ADU rules?

No. The county rules on this page, including the DHCA Class 3 license, cover Bethesda and the rest of unincorporated Montgomery County; several Chevy Chase communities are incorporated municipalities (Chevy Chase Village, the Town of Chevy Chase, Chevy Chase Sections 3 and 5, the Village of North Chevy Chase, and Chevy Chase View). The City of Rockville has its own ADU zoning. An ADU there may be up to 75% of the main home's total floor area; an attached ADU in a basement may use the entire basement, and a detached ADU behind a home of less than 800 sq ft may be up to 600 sq ft; no parking space is required for an ADU; and the owner must live in the main house or the ADU for at least six months of the year, and both must stay under common ownership. Rockville also requires a City of Rockville building permit for every ADU, and a City rental license if it is rented to anyone other than the owners' relatives; detached ADUs also need an automatic fire sprinkler system. The City of Gaithersburg has its own zoning code (City Code Chapter 24), and ADUs inside the City of Rockville and the City of Gaithersburg are not licensed by Montgomery County DHCA. We confirm the current Gaithersburg ADU rules with the city for each address.

Recent Montgomery County work: Exterior Renovation in Chevy Chase, MD, and Home Addition in Bethesda, MD.

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Where we build

Excell Homes designs, permits, and builds ADUs across the DC metro. Explore our work by area:

This guide is for general informational purposes only and does not constitute legal, zoning, or professional advice. Accessory dwelling unit rules vary by property, zone, and lot size, and they are actively changing. Several jurisdictions in the DC metro are currently reviewing or revising their ADU regulations, including Fairfax County. The information here reflects our understanding at the time of writing and may become outdated. Nothing here is a guarantee that a given project is permittable. Always confirm current requirements for your specific property directly with the relevant authority (the DC Office of Zoning and Department of Buildings, the Montgomery County Department of Housing and Community Affairs and Department of Permitting Services, or Fairfax County Planning and Development) before making decisions or beginning a project. Excell Homes is not responsible for actions taken based on this information.

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