Updated October 13, 2026. These rules are changing. A 2025 Maryland law requires counties to allow ADUs on single-family lots, and the county council is considering an update to its zoning code (Bill 61-26) this month. Confirm the current rules with the Office of Planning and Zoning (410-222-7460) before you design.
1. What the county allows today
Since 2023 (Bill 6-23), an ADU is a conditional use in the RA, RLD, R1, R2, R5, R10 and R15 residential zones: it is allowed if it meets the conditions in the code. ADUs are not allowed in R22 or in the Critical Area's Resource Conservation Area. There is no minimum lot size. One ADU is allowed per lot, and it can be inside the house, connected by a breezeway, in an attached or detached garage, or in a separate building; a mobile or manufactured home doesn't qualify.
2. Size and design
A detached ADU can be no larger than 800 square feet or 50% of the main house, whichever is less. An ADU inside or attached to the house must be smaller than the house. It needs its own entrance, shares the house's street address as "Unit B", and a detached unit follows accessory-structure setbacks. Setback variances aren't available, except to convert a building that existed before May 27, 2023.
3. Parking, fees and renting
The county's current rules require no extra parking space, and ADUs are exempt from impact fees unless built together with a new house. Owner-occupancy is tied to short-term rentals: either unit may be rented short-term if the house or the ADU is owner-occupied. HOA covenants may still restrict ADUs, so check yours.
4. Septic and the Critical Area
On a septic lot, the Health Department reviews the project: the system must be sized for the added living space and bedrooms, and the lot must have room for two future replacement systems. That is often the deciding factor; see our septic guide. Near the water, an ADU adds lot coverage and must fit the Critical Area limits, and it is not allowed at all in the Resource Conservation Area.
5. What may change
Maryland's 2025 ADU law (Chapter 196) requires local rules allowing ADUs on single-family land by October 1, 2026, with approval decisions within 90 days, objective standards, no new parking near transit and no impact fees for ADUs under 750 square feet. The county's pending Bill 61-26 would, among other changes, cap attached ADUs at 75% of the house and allow manufactured homes as ADUs. We will update this guide when the county acts.
6. Inside Annapolis
The City of Annapolis has its own zoning rules for ADUs, including size limits, an owner-occupancy rule and a city rental license, and historic-district approval for exterior work. Check with the city's Planning and Zoning department.
