Gaithersburg, MD Accessory Structures: ADU Rules (2026)

Gaithersburg has adopted ADU-friendly zoning regulations that allow accessory apartments in most single-family residential zones. ADUs may take the form of an attached addition, a conversion of existing space such as a basement or garage, or a detached accessory structure on the same lot. The ADU may not exceed 50 percent of the primary dwelling gross floor area or 1,200 square feet, whichever is less. The unit must contain a separate kitchen, bathroom, and sleeping area. The property owner must occupy either the primary dwelling or the ADU as their principal residence. One additional off-street parking space is required for the ADU unless the property is within one-quarter mile of a transit station, in which case the parking requirement may be waived. ADUs must meet all building code requirements including separate egress, fire separation, and adequate natural light and ventilation. Detached ADUs must meet accessory structure setback requirements for the zoning district. Design standards require the ADU to be compatible with the architectural character of the primary dwelling. A building permit and zoning compliance review are required before construction. Montgomery County also allows accessory apartments under its zoning code, but within Gaithersburg city limits the city's own regulations govern. Contact the Department of Planning and Code Administration at (301) 258-6330.