Accessory dwelling unit regulations vary but results are consistently underwhelming in the D.C. region.

"Even though accessory apartments have become legal in parts of DC, Maryland, and Virginia over the past few decades, we have yet to see a renaissance of this housing type in our region," writes Tracey Hadden Loh.
The answer to the question of why accessory dwelling units (ADUs) are so hard to deliver at scale with the stated goals of zoning codes varies by jurisdiction, according to Loh. The article includes a list of current ADU regulations in the city of Alexandria, the District of Columbia, Fairfax County, Prince George's County, Montgomery County, and Arlington County, finding substantial differences in the "state of play" for each.
However, one councilmember in Montgomery County, Maryland has proposed legislation that would make eight changes to the county's zoning code to streamline ADU permitting to help produce results more in line with intensions.
Montgomery County Councilmember Hans Riemer's bill would: "Remove the prohibition on detached accessory apartments in small lot single family zones (R-200, R-90, and R-60)," and " Remove the prohibition on detached accessory apartments on lots smaller than one acre," among the six other changes.
Riemer's zoning text amendment is getting a public hearing today in Rockville, Maryland.
FULL STORY: Accessory apartments in Montgomery County: Still looking for the secret sauce

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