Save Seattle's Exceptional Trees from New ADU's

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Petition by Treepac Board Member Richard Ellison, MS Botany

The City of Seattle wants to encourage single family homeowners to build additional ADU’s (accessary dwelling units, sometimes called mother-in-law units), both attached and unattached. Currently, you can build one or the other, but not both, on the same property. The City proposes allowing both, while also reducing minimum lot sizes from 4000 sq ft to 3200 sq ft and increasing the backyard structural footprint from 40% to 60%.

We believe the ADU-DEIS is highly flawed in claiming there will be no significant impacts to Exceptional Trees, Significant Trees and Tree Groves as a result of the proposed changes. Cumulative impacts, unavoidable adverse impacts, impacts to critical root zones, increased stormwater runoff, urban island heat effects, are all issues poorly addressed, if at all.

It is inconceivable to us that the ADU-DEIS cannot identify even one adverse impact, considering Single Family (SF) zones are 67% of the land and have 72% of Seattle’s tree canopy on 135,000 lots. The DEIS is silent as to how increases in lot coverage and reductions in minimum lot size, among many other issues, will be such that “No significant adverse impacts are anticipated to land use; therefore, no mitigation measures are proposed.” (The DEIS in 4.3.3 Mitigation Measures, Page 4-120 notes)

1) COMMENT PERIOD AND PUBLIC OUTREACH. We request an extension of the comment period and more public outreach on the ADU issue. HALA/MHA covers impacts to less than 33% of Seattle land cover, but has had over a year of public outreach and 6 major public outreach meetings and 6 public hearings. The ADU proposal, which would affect 67% of Seattle’s land cover, had limited public outreach and only 1 public hearing. 

The ADU proposal, effectively up-zoning 67% of Seattle, which includes reducing minimum lot sizes for ADU’s (allowing potentially increased lot subdivision in SF zones), and increasing lot coverage of structures from 40% to 60% in the back lots, with its resulting increased density and traffic, and cumulative impacts to tree canopy, should include more public outreach.

2) CUMULATIVE IMPACTS. Single Family (SF) zones have 72% of Seattle’s tree canopy on 135,000 lots. Any cumulative impacts of increased ADU development in SF zones, combined with up-zoning for HALA/MHA and Urban Villages could have significantly negative impacts to Seattle’s tree canopy, Heritage Trees, Exceptional Trees and Tree Groves throughout the city.

Cumulative impacts are not addressed. Cumulative long term impacts to canopy need to be calculated assuming the potential maximum buildout of ADU’s in SF homes. Underestimation of the number of ADU’s that might be built, and ignoring the AADU’s attached to primary dwellings, in addition to ignoring impacts to Critical Root Zones, will likely inhibit the City reaching its intended Tree Canopy Cover goals of 30% in the short term, and 40% in its long term goal. As this lost open space would be cumulative, this would be a potential long term impact that is not acknowledged in the DEIS.

3) UNAVOIDABLE ADVERSE IMPACTS. The ADU-DEIS is significantly flawed in its claim there are “No significant unavoidable impacts to tree canopy.” This is based upon faulty assumptions of extremely few ADU’s being built in low canopy cover neighborhoods. It also assumes no impacts to Critical Root Zones, no cumulative impacts from other citywide land use proposals such as the MHA/ HALA up-zones and long term buildouts of new ADU’s in SF zones.

4) CRITICAL ROOT ZONES. The DEIS fails to discuss impacts from new structures, walkways, utilities, driveways, etc. to Critical Root Zones of Exceptional and Significant trees and Tree Groves.

5) TREE GROVES. The DEIS must discuss potential impacts to existing Tree Groves and groves status. Trees should not be allowed to be removed if it causes a grove to lose status and protection as a Tree Grove. If one or more trees are removed from a grove that would ordinarily qualify for protection, then what would the impacts to groves be from increased ADU development? In the 2016 LIDAR study, less than 2300 Tree Groves remained in Seattle.

6) LACK OF ACURATE CANOPY SAMPLING. Impacts to canopy should be calculated from actual canopy samples from neighborhoods like Wallingford and Wedgwood, which currently have high canopy cover, and compared to neighborhoods like the University District and South Park with low canopy cover. The DEIS figure 4.3-15, and similar figures, assumes neighborhoods only have a few small trees currently, so impacts are assumed to be minimal with buildout, and these assumptions are incorrect for many neighborhoods.

7) STORMWATER RUNOFF. The EIS needs to evaluate how much increased runoff from increased imperious surfaces from both new ADU’s and loss of tree canopy. How much more runoff will be added, stressing an already overwhelmed combined sewage and street runoff system during heavy rains and peak storm events?

The new Ship Canal Water Quality Project, designed to reduce pollution from water overflows into the ship canal, will need to capture and store more stormwaters during heavy rains. How much more runoff will need to be stored from reductions in existing neighborhood canopy cover in the Ballard, Fremont, and Wallingford neighborhoods?  This could be estimated from existing canopy cover data, but the ADU-DEIS fails to evaluate impacts to this project.

8) URBAN ISLAND HEAT EFFECT. Seattle is currently #10 in the nation’s highest Urban Island Heat Effects. With many record high temperatures this last decade, combined with record droughts in summer, any significant loss of tree canopy will exacerbate impacts from Urban Island Heat Effects. The EPA says “Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality.” The assumption of no impacts to tree canopy misses opportunities to mitigate these heat impacts.

9) TREE MAPPING REQUIREMENTS. ADU site plans must include maps noting the location, DBH, canopy cover, and species of trees. Currently, ADU site plans are exempt from showing tree information.

10) PERMITS REQUIRED. Tree removal permits should be required for any Exceptional or Significant tree removed in development or otherwise in SF zones.

11) MAXIMUM ALLOWED LOT COVERAGE + REDUCED LOT SIZE IMPACTS. With minimum lot size being proposed to being reduced to 3200 square feet (from 4000 currently), how is it even possible to preserve Exceptional Trees and build additional ADU’s, especially if back lot coverage increases to 60% ?

12) LIMIT EXEMPTIONS FOR MAXIMUM BUILDOUT. Limit removal of Exceptional trees as much as possible, even if that would prevent the maximum allowed lot coverage. Encourage alternative designs and make penalties for removal strong so to discourage tree removals. This is especially true if rear lot coverage is increased from 40% to 60%. Residential small lots should include stronger tree protection and tree replacement requirements.



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