Accessory Dwelling Units in Montgomery County
Montgomery Planning provides information and guidance around Accessory Dwelling Units (ADUs) in Montgomery County, Maryland. According to Montgomery County code, an ADU is defined as a second dwelling unit that is subordinate to the principal dwelling on a property. This can take the form of an attached ADU — such as an addition or basement conversion — or a detached ADU, which is a separate structure built on the same lot as the primary home.
Building an ADU in Montgomery County is largely a by-right process. It is facilitated through two county agencies: the Department of Housing and Community Affairs (DHCA) and the Department of Permitting Services. Property owners looking to move forward with an ADU project are directed to DHCA's resources to learn about applying for an Accessory Dwelling Unit Class 3 License.
Key Changes Introduced by ZTA 19-01
In January 2019, Zoning Text Amendment 19-01 (ZTA 19-01) was introduced and subsequently approved in July 2019. This amendment revised the limited use provisions for both attached and detached Accessory Dwelling Units in Montgomery County, making several notable changes to the rules governing ADU construction and use.
Among the most significant updates, ZTA 19-01 removed the previous prohibition on detached ADUs on lots smaller than one acre. This opened up the possibility of building a detached ADU — sometimes referred to as a backyard cottage, granny flat, or casita — on a wider range of residential properties.
The amendment also established size limits for detached ADUs. The allowable size is the smaller of the following:
- 10 percent of the total lot size
- 50 percent of the footprint of the principal dwelling
- 1,200 square feet
Existing rules that govern accessory structures continue to apply. These include height limits, maximum lot coverage requirements, and stormwater management requirements. The greater side and rear setbacks previously required for detached ADUs also remain in effect.
Additional Provisions Under ZTA 19-01
The amendment made several other practical changes to ADU regulations in Montgomery County:
- Parking requirements near transit: The requirement for an additional off-street parking space has been removed for properties located within one mile of Metro, Purple Line, or MARC stations. For ADUs proposed outside of that one-mile radius, three off-street parking spaces are still required — one for the ADU and two for the primary detached house.
- Conversion of existing structures: The amendment allows for the conversion of existing, legally built structures into ADUs, which may include garage conversions or similar projects.
- Short-term rental restrictions: The amendment clarifies that other rental uses, such as short-term rentals through platforms like Airbnb, are prohibited on properties that include an ADU.
- New construction: The previous prohibition on including ADUs in new construction has been removed.
- Distance restrictions lifted: The requirement that restricted ADUs from being built within 300 to 500 feet of an existing ADU has also been eliminated.
These regulatory changes reflect a broader effort to expand housing options in Montgomery County. For property owners considering an ADU — whether a detached unit, an in-law suite, or a garage conversion — understanding these updated rules is an important first step in the planning process.
