Herndon, VA Accessory Structures: ADU Rules (2026)

Accessory dwelling units (ADUs), also called in-law suites, accessory apartments, or granny flats, are regulated by the Herndon Zoning Ordinance consistent with the enabling authority of VA Code §15.2-2292.1, which encourages localities to allow ADUs but does not mandate them. Herndon's zoning permits ADUs in single-family residential districts under specific conditions, which may include either administrative approval or a special exception (special use permit) depending on the district and design.

Typical ADU requirements in Herndon include: the ADU must be clearly subordinate in size to the primary dwelling (commonly capped at 35 percent of the main unit's floor area or a fixed square footage limit such as 800 to 1,000 sq ft); the property owner must occupy either the primary dwelling or the ADU as their principal residence; only one ADU is permitted per lot; and the ADU must have its own kitchen, bathroom, and separate entrance. Minimum lot size and setback requirements apply, and one additional off-street parking space is typically required.

ADUs can be configured as attached (internal conversion, basement unit, or addition) or detached (converted garage, carriage house, or freestanding structure). Detached ADUs face the same setback and height rules as other accessory structures. Construction must comply with the Virginia USBC, including separate egress, smoke and CO alarms, and energy code compliance. ADUs cannot be subdivided from the primary lot or sold separately. Short-term rental use of an ADU is subject to separate STR regulations and may be prohibited unless the owner occupies the property. HOAs in many Herndon neighborhoods impose their own ADU restrictions or outright prohibitions, which are independently enforceable on top of town zoning.