ADUs in Montgomery County
An Accessory Dwelling Unit as defined in Montgomery County code is a second dwelling unit that is subordinate to the principal dwelling. An ADU can be an addition or basement (attached) or separate structure on the same lot (detached).
Building an ADU in Montgomery County, MD is mostly a by-right process facilitated through Montgomery County’s Department of Housing and Community Affairs and Department of Permitting Services.
Visit DHCA’s website to learn about how to apply to for a Accessory Dwelling Unit Class 3 License.
Changes made in 2019 through ZTA 19-01
ZTA 19-01, introduced in January 2019 and approved in July 2019 revised the limited use provisions for attached and detached Accessory Dwelling Units.
Changes that were made in ZTA 19-01 include:
- Removes the prohibition on detached ADUs in lots smaller than one acre.
- The size of the detached ADUs must be the smaller of 10 percent of the lot size, 50 percent of the footprint of the principal dwelling; or 1200 sq. ft.
- Existing rules limiting the construction of accessory structures apply, including height limits, maximum lot coverage requirements, and stormwater requirements. In addition, the greater side and rear setbacks currently required for detached ADUs remain.
- Removes the requirement for an additional parking space within one mile of Metro, Purple Line or MARC stations. Proposed ADUs require one off-street parking space in addition to the parking required for each detached house (typically two spaces). For areas outside of one mile, three off-street spaces are still required.
- Allows for the conversion of existing, legally built structures into ADUs.
- Clarifies that other rental uses (such as Airbnb) on a property that includes an ADU are prohibited.
- Removes the prohibition on ADUs in new construction.
- Removes the distance requirement restricting ADUs from being built within 300 to 500 feet of an existing ADU.
Frequently asked questions
ADUs are a helpful tool in increasing supply and affordability without adding sprawl or creating wholly new developments. Especially in built-out single-family neighborhoods where space is an issue, they offer short-term feasibility and long-term flexibility that many homeowners find beneficial. They are great tools for aging in place and are considered a gentle density to help increase our housing supply.
Easing zoning and permitting requirements (as we did in 2019) has proven to be an important part of helping increase the production of ADUs, but other hurdles—like more financing options for the upfront costs and navigating homeowners’ association (HOA) restrictions may remain.
In housing and urban development, gentle density refers to an approach whereby the jurisdiction focuses on slight increases to the number and variety of homes in existing single detached-home neighborhoods. These small-scale changes add up to meaningful impact on our county’s overall housing supply, which is currently short of where Montgomery County needs to be.