Montgomery County, MD Accessory Structures: ADU Rules (2026)

Montgomery County adopted Zoning Text Amendment 19-01 (ZTA 19-01), effective October 30, 2019, making it one of the most ADU-friendly jurisdictions in the Washington DC region. ADUs are permitted by right as a limited use in all residential zones without a conditional use or special exception. An ADU may be an internal conversion, an addition to the principal dwelling, or a detached structure. Maximum size is 50 percent of the gross floor area of the principal dwelling or 1,200 square feet, whichever is less. The ADU must have a separate entrance, a kitchen, a bathroom, and a sleeping area. The property owner must occupy either the principal dwelling or the ADU as their primary residence and must file a license with DPS. No additional parking spaces are required for the ADU beyond what the zoning ordinance requires for the principal dwelling. The ADU must meet all building code, fire safety, and health requirements. Detached ADUs must comply with accessory structure setbacks (typically the same as the principal structure side and rear setbacks). ADUs cannot be used as short-term rentals for fewer than 30 days. A building permit from DPS is required for all new ADU construction or conversion. DPS created a streamlined ADU permit review process. Height of detached ADUs is limited to 25 feet in most zones. The ADU is not counted as a separate dwelling unit for density purposes.