A typical ADU, built as a mother-in-law suite, in Arlington. Photo by Eden Brown.
Local tree protection advocates lost their battle to delay or reject the Arlington County Board vote on Accessory Dwelling Unit (ADU) setback when the board went ahead with a vote on Saturday, May 18, approving the five-foot setback for ADUs.
The vote was on amendments to Sections 12.9.2 and 16.2.4 of the Arlington County Zoning Ordinance (ACZO) to create new standards for establishing new detached accessory dwellings, to clarify standards for the creation of a detached accessory dwelling within existing accessory buildings, and to make other editorial amendments for clarity.
Tree canopy advocates argued the 5-foot setback for ADUs would reduce Arlington's shrinking tree canopy and increase storm water run-off. On smaller lots, where trees cluster around the perimeter and at the rear of lots, exactly where these ADUs would go, they noted, the trees would come down in favor of the building.
Arlington County planners believe the amendment will make it easier for homeowners to build ADUs which could function as additional lower cost housing. ADUs are usually designed for an older family member or children, but could also be for short-term rental.
Arlington Tree Action Group (ATAG) did not believe the ADU amendment would solve the problem of high housing costs in Arlington. Its view was that ADUs trump existing single family zoning by placing two dwellings for two families on a lot zoned for single-family dwellings. “A new ADU may be closer in proximity to a neighbor’s home than to the home of the ADU’s owner,” according to ATAG. ATAG’s second concern was for impervious surfaces, which they said was now at 45 percent of the county and climbing, exacerbating stormwater runoff.
In March 2019, the Urban Forestry Commission weighed in, proposing the county explore ways to incentivize applicants to preserve mature trees or deter applicants from placing detached accessory dwellings near mature trees. The commissioners asked if there was a way to monitor the impacts to trees or report the loss of trees.The commission also sent the county a letter requesting further changes to the proposed amendment, including adding other requirements regarding tree preservation, monitoring of impacts to trees, and providing notice to adjacent neighbors regarding impacts to trees.
However, because accessory dwellings are a by-right use in the ACZO (Arlington County Zoning Ordinance), and the potential impact of a detached accessory dwelling is viewed as similar to the development of an accessory building, and because the recommended five-foot setback for a new detached accessory dwelling could potentially mitigate impacts to existing trees on neighboring properties, while also providing placement flexibility to the owner to minimize potential impacts to existing trees on their own lot, staff did not change the amendment.
The Planning Commission voted unanimously for the ADU change but made recommendations that would require all trees above eight inches in diameter to be shown on the application drawing for any proposed accessory dwelling; that the applicant provide one replacement canopy tree for every two canopy trees above eight inches in diameter or more that would be removed due to the construction of the proposed new structure with the replacement tree(s) to be planted on a location within the property boundaries; that the applicant provide one replacement understory tree for every one understory tree eight inches in diameter or more removed due to the construction of the proposed new structure with the replacement tree(s) to be planted on a location within the property boundaries. It was not clear how staff would address those concerns.
For the full documentation on this zoning change, see: https://housing.arlingtonva.us/plans-reports/accessory-dwelling-ordinance-update/