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

700, codified at Charles County Code § 297-212(23), permits a single-family residence with an accessory apartment as a use with conditions across most residential and mixed-use zones, including HVC, HVG, HVR, AC, RC, RR, RV, RL, RM, RH, RO, MX, TOD, WCD, CER, CMR and CRR. An accessory apartment can sit either inside the principal dwelling or inside an accessory building such as a converted garage. Sizing is strict: within a principal dwelling, the apartment must be at least 300 square feet but can never exceed 50% of the house's gross floor area; within an accessory building, the minimum is also 300 square feet, but the county adds a cap of no more than two bedrooms and a limit of 50% of the accessory structure's floor area.

Only one accessory apartment is allowed per existing single-family dwelling, so a homeowner cannot add a second unit later. The code also regulates appearance: where the apartment sits inside the main house, its entry and design must, so far as reasonably feasible, match the single-family look of the house, and no new exterior entrance facing a road or street may be added to either the principal dwelling or an accessory building. Before the county issues a zoning permit for a new accessory apartment or converts an existing accessory building into one, the property owner must get the Charles County Department of Health to approve the proposed water supply and sewage disposal method, and off-street parking must meet Article XX's parking standards.