ADUs in Maryland: What Buyers Need to Know in Prince George’s and Montgomery Counties

Maryland has told every county and city with zoning authority to allow accessory dwelling units on lots with a single-family detached home. In Montgomery County, that mostly meant updating a program that was streamlined in 2019. In Prince George’s County, it meant starting from zero.

Both counties now have a legal path for a basement apartment, an in-law suite or a backyard cottage. The paths are not the same, and neither one turns an unpermitted unit into a legal one. If you are looking at a home with a second unit, or a lot with room to add one, the details below decide whether it is an asset or a liability.

What Maryland’s ADU law requires

Maryland’s 2025 ADU law (HB 1466 / SB 891) took effect on October 1, 2025. Local governments had until October 1, 2026 to adopt a conforming law. Counties can be more generous than the state, but they cannot fall below these minimums. The main requirements:

• Where. ADUs must be allowed on land with a single-family detached home. The state sets no minimum lot size.
• Size. An ADU can be up to 75% of the size of the main home. Local rules cannot set the cap lower.
• Setbacks. Side and rear setbacks cannot be stricter than those for other accessory structures on the lot.
• Density. ADUs do not count toward density limits.
• Parking. A county that wants added off-street parking for an ADU must first complete a parking study and offer a waiver process.
• Attached and detached. Both types must be allowed in some form, such as a basement unit, an addition or a separate cottage.
• HOAs and deeds. Covenants and bylaws cannot unreasonably block building or renting an ADU. Limits on short-term rentals are still allowed, and properties listed or eligible for the National Register of Historic Places are exempt.

The state does not require by-right approval, does not waive building codes, and does not stop counties from regulating short-term rentals. New ADUs are also subject to the state’s residential sprinkler requirements.

Montgomery County: established, and just updated

Montgomery County has allowed accessory apartments for decades, and a 2019 rewrite made the process mostly by-right, as Planning staff describe it today. Its older rules conflicted with the state law, so the Council adopted ZTA 26-06 (Ordinance No. 20-36). The County’s permitting department reports it took effect October 5, 2026.

• Size. An ADU can now be up to 75% of the size of the main single-family detached home. That replaces the older caps, which included a 1,200-square-foot limit.
• Setbacks, height and lot coverage. They follow the accessory-structure standards of the underlying zone.
• Parking. One added space is required, on top of the house’s own parking, unless the lot is within a mile of a Metrorail, Purple Line or MARC station. A Hearing Examiner can grant a waiver.
• One ADU per lot, with no more than two adults age 18 or older living in it under the standard license.
• License and permits. You need a Class 3 ADU license from the Department of Housing and Community Affairs (DHCA) to live in or rent the unit, plus a building permit from the Department of Permitting Services (DPS) and WSSC approvals for water and sewer. DHCA lists a $615 application total ($260 filing, $230 public-notice sign, $125 annual license). Takoma Park properties are handled differently.
• Owner occupancy. Today, the owner must live in either the main house or the ADU to get the rental license. Bill 41-26 would remove that for rental licensing, but it is a proposal, not law.
• Short-term rentals. They are not allowed on a lot with an ADU.
• Cities with their own rules. Rockville, Gaithersburg and Takoma Park have their own zoning or add review layers, and homes in historic districts may need a Historic Area Work Permit first.

What this means if you’re buying

The new rules open the door. They do not make every existing basement apartment legal, and they do not guarantee you can build one on a given lot. Work through this list before you write an offer.

• Permit and license history. Ask whether the unit was permitted. In Montgomery County, ask whether it holds a DHCA Class 3 license; the County keeps a public list and map of licensed ADUs. Do not assume the new laws legalize an unpermitted unit.
• Rental income. Montgomery County requires an owner-occupant for the rental license today. Prince George’s rental rules were still being finalized. If your plan counts on renting both units, confirm the rule in writing first.
• Lot and standards. Size, height, setbacks and parking all have to fit the lot. Get current numbers from County staff, not from a listing.
• HOA and covenants. State law limits what an HOA can ban, but read the documents and ask early.
• Town rules. Municipalities can add their own permits, licensing and design review. The rules can differ between the County and your town.
• Historic districts. Homes in a historic district may need added design review before a building permit.
• Cost. Construction cost is still the main barrier, and new units must meet building codes and the state’s residential sprinkler rules. Price a conversion before you pay for the potential.