Accessory Buildings, Structures, and Dwellings

Rockville allows accessory buildings and will allow attached accessory dwelling units (ADUs) to be constructed on properties with single-unit detached homes, given that certain conditions are met. The city is considering amendments to the Zoning Ordinance that would revise the regulations to allow detached ADUs as a conditional use in residential zones. An overview of existing regulations and the proposed zoning amendment is provided below.

  • An accessory building is a detached structure in the rear yard, such as a shed or garage.
  • An attached accessory dwelling unit (or accessory apartment) is a secondary living unit enclosed within or attached to a single-unit detached house. Examples include a basement conversion or home addition.
  • A detached accessory dwelling unit is a secondary living unit on the same grounds as a single-unit detached house. Examples include an apartment over a detached garage or a smaller structure (on a foundation) in the backyard.

Learn more about how to submit a development application.

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Rockville, MD 20850
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Regulations for Accessory Buildings and Structures

Accessory Building Height

  • The maximum height at the minimum setback is 12 feet. Accessory buildings taller than 12 feet must be set back an additional 3 feet for each additional foot of height, up to 15 feet. Accessory buildings may exceed 15 feet in height, up to a maximum of 20 feet, with no additional setback required.
  • Height is measured to the peak of the roof of an accessory building. In addition, accessory building heights are measured from the lowest point of the finished grade rather than at the front of the building.
  • The height of any accessory building may not exceed the height of the main dwelling.

Accessory Building Area

  • The maximum footprint (i.e., area on the ground) for any one accessory building is 750 square feet and cannot exceed the footprint of the main building. The maximum footprint of all accessory buildings is 1,000 square feet.
  • The maximum rear yard building coverage is 15% in R-150, R-200, and R-400 zones and 25% in all other residential zones.

Accessory Building Setbacks

  • A minimum requirement of 3 feet of building setback is required from any lot line, with additional setbacks required for accessory buildings taller than 12 feet.

Review the full regulations for accessory buildings and structures in the Zoning Ordinance.

Regulations for Attached Accessory Dwelling Units (Effective April 9, 2024)

The Mayor and Council passed a zoning amendment (TXT2023-00263) on Monday, Oct. 9, 2023, to allow attached ADUs as a conditional use in residential zones.

This legislation will go into effect on April 9, 2024. At that time, homeowners who wish to either build an attached ADU under the new regulations or bring an existing unpermitted attached ADU into compliance may begin the application process. Before that date, homeowners may still apply to construct an accessory apartment as a special exception.

Attached ADU Regulations

  • Design requirements: The attached ADU must have a separate entrance. Any external modifications to the single-unit detached home must be compatible with the existing dwelling and any neighborhood design guidelines.
  • Owner occupancy: The property owner must occupy either the main building or the attached ADU for at least six months of the year.
  • Size: The gross floor area of the attached ADU must be equal to or less than 50% of the total floor area of the primary dwelling, unless the attached ADU is located in the basement or cellar, in which case it may equal the square footage of the basement or cellar.
  • Parking: Lots with attached ADUs that require modifications to the exterior of the existing building (for example, a home addition) must have two off-street parking spaces unless located within 7/10th of a mile walking distance to a Metro station, in which case the lot must have at least one off-street parking space. Applicants may seek a waiver from this requirement from the Chief of Zoning. Lots with attached ADUs that do not require modifications to the exterior of the existing building (for example, a basement conversion) are not required to provide off-street parking spaces.

Review the full future regulations of attached ADUs in the approved Zoning Text Amendment.

Grace Period for Existing Unpermitted Attached ADUs

If you have an attached ADU in your home that has not been approved by the city, you will be able to apply for approval beginning April 9, 2024. Conditional use approval must be received by April 9, 2025 – after that date, this clause will no longer be in effect and any unpermitted attached ADU will constitute a violation. All applications from existing attached ADUs received during this grace period will need to conform with the regulations listed above, except for the gross floor area requirements.

Accessory Apartment Regulations

Accessory apartments are the former name used by the city for attached ADUs. Prior to April 9, 2024, homeowners may still submit a special exception application to construct an accessory apartment.

  • Design Requirements: The accessory apartment must have a separate entrance. Any external modifications to the single-unit detached home must be compatible with the existing dwelling and surrounding properties.
  • Owner Occupancy: The property owner must occupy either the main building or the accessory apartment for at least six months of the year.
  • Parking: Both the main dwelling and the accessory apartment must comply with all current development standards, including off-street parking requirements.
  • Additional Restrictions: The Board of Appeals must find that the accessory apartment will not constitute a nuisance and will not adversely impact the parking or traffic situation in the neighborhood.

The special exception is granted solely to the owner/applicant and does not run with the land. No variance may be granted to accommodate an accessory apartment. Review the full regulations for accessory apartments in the Zoning Ordinance.

Public Input Sought on Proposed Revisions to Regulations for Detached Accessory Dwelling Units

The Mayor and Council is seeking feedback on the proposed text amendment (TXT2019-00255) to allow a detached ADU on the same lot as a single-unit detached home. In addition to the requirements for accessory buildings, as described above, detached ADUs would be subject to other requirements, including a maximum gross floor area restriction of 50% of the gross floor area of the main home or 750 square feet, whichever is less. Detached ADUs would not be permitted on a property that already has an attached ADU.

The Planning Commission held a public hearing for this proposal on May 10, 2023 and made recommendations to the Mayor and Council at its July 12, 2023 meeting. The Mayor and Council will hold a public hearing and consider Planning Commission, staff recommendations, and community feedback at future meetings. Community Planning and Development Services staff are available to answer questions through virtual meetings or over the phone, as requested.