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

Section 10-41(2) treats detached and attached accessory dwellings differently by district. In the A-1 Agricultural and C-1 Conservation Districts, a detached accessory dwelling needs a parcel of at least five acres, is capped at two thousand square feet of floor area, cannot include duplex-style units, and where a parcel has more than one accessory dwelling, only one additional unit is allowed per twenty acres beyond the first. An attached accessory dwelling in those same two districts needs only two acres but is still capped at two thousand square feet, cannot get its own electrical meter, and requires written approval from the Virginia Department of Health on the drain field before it can be established, plus a building permit.

In the R-Rural, R-1, R-2 and R-3 Residential Districts, a detached accessory dwelling needs a parcel of at least one acre, with no more than one accessory dwelling permitted per parcel beyond the principal dwelling; an attached unit needs only a half acre. Residential-district accessory dwellings are capped at twelve hundred square feet, require at least one additional off-street parking space outside the required front yard, and a parcel can never carry more than two dwelling units total. Every accessory dwelling requires its own zoning permit and compliance with the applicable base district's other regulations.