Size caps
State ceiling (verified): every ADU in Maryland is capped at 75% of the primary home's size, and must be smaller than the primary. No state minimum.
Charles County (published draft): lots under 3 acres — 1,000 sq ft or 75% of the main home, whichever is smaller; lots of 3+ acres — 75% with no absolute cap. Max 2 bedrooms, never more than the main house; one ADU per lot; no fixed minimum (the old 300-sq-ft floor is deleted).
Montgomery County (current law): attached up to 1,200 sq ft; detached up to the least of 50% of the main home's footprint, 10% of lot area, or 1,200 sq ft.
Sources: Chapter 197, §§4-501, 4-504(C)(1); Charles County Bill 2026-06 / ZTA 25-189 (published draft, §§3.01.700.F, G); Montgomery County Code Ch. 59 §3.3.3.
Setbacks and height
Charles County (published draft): attached ADUs meet the main house's setbacks. Detached on lots under 3 acres: zone front setback, at least 6 feet behind the main home's front building line, 6 feet to side and rear lot lines, 6 feet from other buildings, never inside a recorded easement. Detached on 3+ acres: anywhere, but 50 feet from any public road right-of-way. Height: 2 stories or 24 feet max, never above the district maximum; no mobile homes. Attached entries and stairs must not face the street.
Montgomery County (current): detached ADUs meet the accessory-structure setbacks of the underlying zone, with greater side and rear setbacks — exact feet vary by zone, confirm per address.
Sources: Charles County Bill 2026-06 / ZTA 25-189 (published draft, §§3.01.700.E, H, I; §297-29.B); Montgomery County 2019 ADU release.
Parking
State ceiling (verified): within 0.75 mile of mass transit — no new off-street parking may be required. Beyond that — at most one space; more needs a parking study plus waiver.
Charles County (published draft): the parking table is amended to "NO OFF-STREET PARKING SPACES REQUIRED" for ADUs — beyond the state minimum ask. (Townhouse ADUs excepted: 2 spaces for a 1-bedroom principal unit, 2.25 for 2-bedroom, plus 1 per ADU.)
Montgomery County (current): 1 additional space with an existing driveway; 2 total if a new driveway is built; none required within 1 mile of Metrorail, Purple Line, or MARC. Waivers available with adequate on-street parking.
Sources: Chapter 197, §4-504(D)(4); Charles County Bill 2026-06 / ZTA 25-189 (published draft, amending Art. XX §297-335); Montgomery County DHCA info sheet; Zoning Ordinance §59.6.2.4B.
What this means for your lot
Three measurements decide feasibility before design starts: is there 6+ feet of side yard to work with (smaller lots), does the size cap fit the bedrooms you need, and does your HOA require parking the county does not. Our free feasibility study measures all three on your actual property — or see Charles County's full rules for the complete picture.