Montgomery County ADU Rules & Costs (2026): Bethesda, Rockville & Gaithersburg
Regulations last verified: July 2026
The short version
- Yes, Montgomery County allows ADUs by-right. You get a Class 3 license from DHCA and building permits from DPS, no special-exception hearing for a standard unit.
- Detached ADUsare capped at the least of 10% of the lot, 50% of the home’s footprint, or 1,200 sq ft; attached ADUs up to 1,200 sq ft.
- Owner-occupancy is required, one ADU per lot, and no more than two adults; short-term rentals like Airbnb are not allowed.
- Parking:one extra space, waived if you’re within one mile of Metrorail, the Purple Line, or MARC.
- Rockville and Gaithersburg permit their own city addresses; Bethesda, Chevy Chase, and unincorporated areas go through county DPS.
The rules below are current as of 2026 and are a starting point, not legal advice. Always confirm the current requirements with Montgomery County DHCA and DPS (or your city) before you design or build.
Can you build an ADU in Montgomery County?
Yes. Montgomery County allows accessory dwelling units by-right, which means a standard ADU no longer needs the special-exception hearing the county once required. The process runs through two offices: you apply for a Class 3 Accessory Dwelling Unit license with the Department of Housing and Community Affairs (DHCA), and you pull building permits from the Department of Permitting Services (DPS). Both an interior or attached unit and a detached backyard unit are allowed, subject to the size and site rules below.
How big can a Montgomery County ADU be?
| ADU type | Maximum size |
|---|---|
| Detached | The smallest of 10% of the lot area, 50% of the main home’s footprint, or 1,200 sq ft |
| Attached | Up to 1,200 sq ft |
| Basement / cellar | May match the footprint of the primary dwelling |
For a detached unit, the “least of” test is what usually governs: on a typical suburban lot, 10 percent of the lot area or half the home’s footprint often lands well under the 1,200 square foot ceiling.
The rules that actually matter
Owner-occupancy and occupancy limits
An ADU must sit on the same lot as an owner-occupied single-family home, so you live in either the main house or the ADU. Only one ADU is permitted per lot, and it may be occupied by no more than two adults. It has to remain subordinate to the principal dwelling.
No short-term rentals
Short-term rentals like Airbnb, and other separate rental uses, are not permitted on a property that has an ADU. In practice that makes an ADU a long-term rental (leases of 30 days or more) or space for family, not a nightly-rental unit.
Parking
Montgomery County generally requires one additional off-street parking space for an ADU. The exception matters in this county: if your property is within one mile of a Metrorail, Purple Line, or MARC station, no additional space is required. The county also removed the former rule requiring a minimum distance between ADUs, which had limited how many could be approved in a given area.
Bethesda, Rockville, Gaithersburg: who issues the permit
The county-wide zoning rules are the same, but the permitting office depends on your address, and this trips people up. Bethesda and Chevy Chase are unincorporated, so they go through Montgomery County DPS. The City of Rockville and the City of Gaithersburg, however, run their own permitting for homes inside their city limits. A property just outside those city lines is back under county DPS. We confirm which office governs your lot before filing, so nothing stalls on a wrong-jurisdiction submission. See our service-area pages for Bethesda, Rockville, and Gaithersburg.
Maryland’s statewide ADU law (HB 1466)
Worth knowing for 2026: Maryland passed HB 1466 in 2025, which requires counties and municipalities with planning authority to adopt local laws allowing ADUs on single-family detached lots by October 1, 2026. Montgomery County already has established, by-right ADU rules, so its homeowners are ahead of the curve, but the law signals a broader statewide push toward easier ADU approval.
What a Montgomery County ADU costs
Cost depends far more on the type of unit than on any single per-foot figure. A basement or interior conversion generally sits at the lower end, an attached unit in the middle, and a fully detached backyard unit at the higher end, once you account for design, permits, and utility connections. Montgomery County’s established neighborhoods and higher-end finishes can push a project up. For planning ranges by ADU type, see our DC-metro ADU cost guide, and if you’d rather build now and pay over time, our RenoFi financingis based on your home’s after-renovation value.
Frequently asked questions
Can you build an ADU in Montgomery County, Maryland?
Yes, by-right. You apply for a Class 3 Accessory Dwelling Unit license from the Department of Housing and Community Affairs (DHCA) and pull building permits from the Department of Permitting Services (DPS). A standard ADU no longer requires the special-exception hearing the county used to mandate.
How big can an ADU be in Montgomery County?
A detached ADU is capped at the smallest of 10% of the lot area, 50% of the main home's footprint, or 1,200 square feet. An attached ADU can be up to 1,200 square feet, and a unit in a basement or cellar can match the footprint of the primary dwelling.
Does Montgomery County require owner-occupancy for an ADU?
Yes. The ADU must sit on the same lot as an owner-occupied single-family home, so you live in either the main house or the ADU. Only one ADU is allowed per lot, and it may be occupied by no more than two adults.
Can I use my Montgomery County ADU as an Airbnb?
No. Short-term rentals and other separate rental uses are not permitted on a property that includes an ADU. An ADU is intended for long-term occupancy, so it works as a long-term rental or as space for family, not as short-term lodging.
Do I need extra parking for an ADU in Montgomery County?
Usually one additional off-street space. But if your property is within one mile of a Metrorail, Purple Line, or MARC station, no additional parking space is required. The county also removed the old rule that required minimum spacing between ADUs.
Do Bethesda, Rockville, and Gaithersburg have the same ADU rules?
The county zoning rules apply across Montgomery County, but permitting differs by address. Homes inside the City of Rockville or City of Gaithersburg are permitted by that city; Bethesda, Chevy Chase, and unincorporated areas go through Montgomery County DPS. We confirm which office governs your property.
Related guides
- ADU rules in DC, Maryland & Virginia compared
- ADU cost guide for the DC metro
- Our turnkey ADU service: design, permits & build
- Financing an ADU through RenoFi
Where we build
Excell Homes designs, permits, and builds ADUs across the DC metro. Explore our work by area:
This guide is for general informational purposes only and does not constitute legal, zoning, or professional advice. Accessory dwelling unit rules vary by property, zone, and lot size, and they are actively changing. Several jurisdictions in the DC metro are currently reviewing or revising their ADU regulations, including Fairfax County. The information here reflects our understanding at the time of writing and may become outdated. Nothing here is a guarantee that a given project is permittable. Always confirm current requirements for your specific property directly with the relevant authority (the DC Office of Zoning and Department of Buildings, the Montgomery County Department of Housing and Community Affairs and Department of Permitting Services, or Fairfax County Planning and Development) before making decisions or beginning a project. Excell Homes is not responsible for actions taken based on this information.