ADU rules in Montgomery County, MD
Yes. An ADU is a new dwelling unit and needs a building permit.
Since October 5, 2026 an ADU can be up to 75% of the size of the house (ZTA 26-06).
A detached ADU uses the accessory-structure setbacks, for example 5 ft side and rear in R-60.
Rules last checked by a person on October 20, 2026. Guidance only; confirm with Montgomery County before you build.
Limits
| ADUs per lot | 1 |
|---|---|
| Size | Up to 75% of the size of the house |
| Detached ADU setbacks | Same as an accessory structure in the zone (5 ft side and 5 ft rear in R-60 and R-90) |
| Detached ADU height | The accessory-structure height limit (20 ft in R-60) |
| Parking | None extra within 1 mile of Metrorail, Purple Line or MARC; otherwise generally 1 space (waiver possible) |
Good to know
- ZTA 26-06 took effect October 5, 2026. Plans reviewed under the old rules (12 ft rear setback, the 1,200 sq ft / 10% / 50% size cap) no longer apply. Confirm with DPS how the 75% is measured.
- Call 811 at least a few days before you dig footings or post holes. Utility locating is free.
- HOA covenants and deed restrictions can be stricter than city rules. Check yours before you build.
Check your lot
Where to apply
- DPS eServices
- Department of Permitting Services
- MCAtlas zoning map
- Phone: 311 (240-777-0311)
Sources
- Montgomery County DPS, Guidelines for Accessory Dwelling Units (revised October 2026; reflects ZTA 26-06, Ordinance 20-36, effective October 5, 2026). Checked 2026-10-20.
An ADU may not be greater than 75% of the size of the primary single-family detached dwelling... [An ADU] must comply with the setback, height, and building lot coverage standards applicable to an accessory structure in the underlying zone... [Within one mi…
- 44 CFR 59.1, definition of development (NFIP). Checked 2026-10-19.
Development means any man-made change to improved or unimproved real estate, including but not limited to buildings or other structures...
- 44 CFR 60.3, floodplain management criteria. Checked 2026-10-19.
Require permits for all proposed construction or other development in the community...
- Montgomery County DPS, R-60 Zone Development Standards (Sec. 4.4.9, guidance). Checked 2026-10-19.
Accessory Buildings: Rear yard only... 5 ft. Side setback... 5 ft. Rear setback, if not otherwise addressed... 20 ft. maximum height... footprint must not exceed 50% of the footprint of the main building or 600 sq/ft whichever is greater.