Rockville, MD Accessory Structures: ADU Rules (2026)
ADU rules in Rockville, MD, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Foundation
- Must sit on a permanent foundation
- Size, principal ≥800 sf
- Up to 75% of its area
- Size, principal <800 sf
- Capped at 600 square feet
- Also governed by
- Development standards in § 25.8.1.3
Summary
A detached Accessory Dwelling Unit in Rockville must sit on a permanent foundation and stay at or under 75% of the main house's floor area, or up to 600 square feet if that house is under 800 square feet.
B. Detached Accessory Dwelling Units must be permanently attached to the ground
by a foundation.
C. Size limitation. Detached Accessory Dwelling Units may have a gross floor area:
1. If the principal dwelling unit has a gross floor area of 800 square feet or more,
equal to or less than 75 percent of the total floor area of the principal
dwelling unit; or
2. If the principal dwelling unit has a gross floor area of less than 800 square
feet, the Detached Accessory Dwelling Unit may be up to 600 square feet.
(c) Conditional use standards.
(1) In all MXTD zones, Accessory Dwelling Units are permitted only when located on the
same lot as a Single-Unit Detached Dwelling that was in existence on August 1, 2026.
Full Breakdown
§ 25.6.3.6(b)(3) of the 2026 Zoning Ordinance sets the specific standards a Detached Accessory Dwelling Unit must meet to get a permit: it must be permanently attached to the ground by a foundation, no skid-mounted or portable units, and its gross floor area is capped relative to the principal house. Where the principal dwelling has 800 square feet or more, the detached ADU can run up to 75% of that floor area; where the principal dwelling is smaller than 800 square feet, the detached ADU is capped at a flat 600 square feet. Detached ADUs are also subject to the accessory-building setback, height, and rear-yard placement standards of § 25.8.1.3.
Violations & Fines
A detached ADU built without a permanent foundation, or over the 75%/600-square-foot size cap, violates § 25.6.3.6(b)(3): a municipal infraction under § 25.13.2.1(a) and § 1-9, up to $400 per day and a possible stop-work order.
Frequently Asked Questions
How big can a detached ADU be in Rockville?
Does a detached ADU need a foundation in Rockville?
Sources & Official References
Other rules in Rockville
How Rockville compares: Easiest Cities to Build an ADU·Compare Rockville to another location·View the Maryland accessory structures overview
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ADU Rules in Nearby Cities
How other cities in Montgomery County handle adu rules.