Current law: the essentials
Montgomery County has allowed ADUs as a limited use (no conditional-use hearing) since 2019, under County Code Chapter 59, Section 3.3.3. Attached ADUs are allowed in all residential zones; detached ADUs are allowed in all residential zones too — the old 1-acre minimum for detached was removed in 2019. ADUs are not permitted in townhouses, condominiums, duplexes, or mobile homes.
Size (current): attached ADUs max 1,200 square feet (a basement-only ADU may equal the full basement). Detached ADUs max the least of: 50% of the main home's footprint, 10% of the lot area, or 1,200 square feet.
Owner-occupancy: YES, currently. Either the main house or the ADU must be the owner's primary residence.
Occupancy: maximum 2 adults (18+) in the ADU.
Short-term rentals: NOT allowed. No Airbnb or short-term rental on a lot with an ADU — long-term rental or family use only.
License: a Class 3 Accessory Dwelling Unit license from DHCA is required, and the application requires an affidavit that your HOA does not prohibit the ADU.
Drainage: a drainage plan is required for any new detached ADU, regardless of lot size.
Sources: Montgomery County Code Ch. 59 §3.3.3; DHCA Class 3 ADU info sheet (montgomerycountymd.gov); County Executive newsletter (montgomerycountymd.gov).
Parking (current)
With an existing driveway: 1 additional onsite space beyond the main home's required parking. If a new driveway must be built for the ADU: 2 spaces total (minimum 480 square feet). No onsite space required within 1 mile of a Metrorail, Purple Line, or MARC station. If you cannot meet the requirement, you can apply for an OZAH parking waiver with adequate on-street parking.
Sources: DHCA info sheet; Zoning Ordinance §59.6.2.4B.
Pending changes: read before you act
ZTA 26-06 ("Accessory Residential Uses – Accessory Dwelling Unit") amends the ADU standards to align with state Chapter 197 — zones, minimum size, density, setbacks, and parking. A vote was expected September 15, 2026; final enactment UNVERIFIED. If enacted, current parking, setback, and minimum-size rules may have already changed.
Bill 41-26 would eliminate the owner-occupancy requirement entirely. Public hearing October 6, 2026 — it is not law. Until it passes, owner-occupancy stands.
Sources: Montgomery County Council meeting notice (govdelivery.com, Sept 2026).
Septic note
On septic in Montgomery County: a detached ADU requires a separate reserve area and a new system under its own permit, with soil and water-table testing. The county's Department of Permitting Services publishes this plainly — budget and plan for it early.
Sources: Montgomery County DPS (montgomerycountymd.gov).
For the state-law floor under all of this, see Maryland's ADU law in plain English. Comparing counties? See Charles County's rules and Prin ce George's County's rules.