Build Your ADU in Montgomery County, MD

Yes. Montgomery County permits one ADU per lot on properties with a single-family detached home, either attached (a basement or addition) or detached. ADUs are approved through the Department of Permitting Services (DPS), and a DHCA rental license is required before the unit is rented or occupied.

Under ZTA 26-06, an ADU — attached or detached — may be up to 75% of the size of the primary home , replacing the county's earlier caps (the least of 1,200 sq ft, 50% of the home's footprint, or 10% of the lot). Accessory-structure height, lot-coverage, and stormwater limits still apply, so they can further constrain a detached unit.

ZTA 26-06 is Montgomery County's zoning text amendment updating its ADU rules to comply with Maryland HB 1466 / SB 891. It raises the ADU size limit to no more than 75% of the primary home (replacing the old 1,200 sq ft / 50% footprint / 10% lot caps), expands eligible zones , excludes ADUs from density calculations in all zones, lowers detached-ADU setbacks to the standard accessory-structure setbacks, and reduces parking requirements. It is expected to take effect on or around October 1, 2026 , the state-mandated deadline.

Yes. The county requires a rental license from the Department of Housing & Community Affairs (DHCA) before an ADU is rented or occupied, and the owner must occupy either the main home or the ADU as a primary residence. Do I need a Class 3 license before I build an ADU in Montgomery County? Yes — the process starts before construction. You file the Class 3 application with DHCA first; construction can't begin until DHCA completes its preliminary inspection report, and the license itself is issued after a final inspection. It's required whether or not you rent the ADU.

Yes. Montgomery County requires the Class 3 license whether the ADU is rented or not — aging-in-place and multigenerational living included. Short-term rentals and Airbnb are not permitted.

$615 at submission — a $260 filing fee, a $230 public-notice sign fee, and a $125 annual license fee. Properties in the City of Takoma Park pay $490, since the rental license there is handled by the city.

Under ZTA 26-06, effective on or around October 1, 2026, a detached ADU must meet the same side and rear setbacks as any other accessory structure . This replaces the county's earlier rule, which required a 12-foot rear setback plus an added setback for units wider than 24 feet.

A detached ADU uses the accessory-structure side and rear setbacks for its zone. In the most common ADU zones that is roughly: RE-2 / RE-1 — 15 ft side, 10 ft rear; R-200 — 12 ft side, 7 ft rear; and R-90, R-60, and R-40 — 5 ft side and 5 ft rear. Corner lots, older recorded lots, and larger structures can change what applies — check your exact standards on Montgomery Planning's zoning fact sheets or with DPS.

Parking requirements are being reduced under ZTA 26-06 to comply with state law. Under the prior rule, an additional off-street space was required, though it was waived within one mile of a Metro, Purple Line, or MARC station. Confirm the current requirement for your address with DPS.

Newly built detached ADUs generally require fire sprinklers. Properties within a designated historic district also need a Historic Area Work Permit (HAWP) and design review, which adds time to the permitting schedule.

A turnkey one-bedroom / one-bathroom detached ADU typically runs from about $250,000 to $350,000 depending on size, site, and finishes; basement and garage conversions cost less. A Makara ADU can be move-in ready in as little as 120 days from permit approval.