Green Factor measurement
Seattle Mun. Code § 23.86.019
in effectA. Development standards for certain areas require landscaping that meets a minimum Green Factor score. All required landscaping shall meet standards promulgated by the Director to provide for the long-term health, viability, and coverage of plantings. These standards may include, but are not limited to, the type and size of plants, spacing of plants, depth, and quality of soil, use of drought-tolerant plants, and access to light and air for plants. The Green Factor score shall be calculated as follows: 1. Identify all proposed landscape elements, sorted into the categories presented in Table A for 23.86.019. 2. Multiply the square feet, or equivalent square footage where applicable, of each landscape element by the multiplier provided for that element in Table A for 23.86.019, according to the following provisions: a. If multiple elements listed on Table A for 23.86.019 occupy the same area (for example, groundcover under a tree), count the full square footage or equivalent square footage of each element. b. Landscaping elements in the right-of-way between the lot line and the roadway may be counted, provided that they are approved by the Director of the Department of Transportation. c. Elements listed in Table A for 23.86.019 that are provided to satisfy any other requirements of this Title 23 may be counted. d. For trees, large shrubs, and large perennials, use the equivalent square footage of each tree or shrub according to Table B for 23.86.019. e. For vegetated walls, use the square footage of the portion of the wall covered by vegetation. All vegetated wall structures, including fences counted as vegetated walls, shall be constructed of durable materials, provide adequate planting area for plant health, and provide appropriate surfaces or structures that enable plant coverage. f. For all elements other than trees, large shrubs, large perennials, and vegetated walls, square footage is determined by the area of the portion of a horizontal plane that lies over or under the element. g. All permeable paving and structural soil credits together may not count for more than one third of the lot's Green Factor score. 3. Add together all the products calculated under subsection 23.86.019.A.2 to determine the Green Factor numerator. 4. Divide the Green Factor numerator by the lot area to determine the Green Factor score. Table A for 23.86.019 Green Factor landscape elements Green Factor landscape elements Multiplier A. Planted areas (choose one of the following for each planting area) 1. Planted areas with a soil depth of 24 inches or more: 0.6 2. Bioretention facilities meeting standards of the Stormwater Code, Title 22 , Subtitle VIII 1.0 B. Plants 1. Mulch, ground covers, or other plants normally expected to be less than 2 feet tall at maturity 0.1 2. Medium shrubs or other perennials at least 2 feet tall, but less than 4 feet tall, at maturity 0.3 3. Large shrubs or other perennials at least 4 feet tall at maturity 0.3 4. Small trees 0.3 5. Small/medium trees 0.5 6. Medium/large trees 0.7 7. Large trees 0.9 8. Preservation of existing trees at least 6 inches in diameter at breast height 1.0 C. Green roofs 1. Planted over at least 2 inches but less than 4 inches of growth medium 0.4 2. Planted over at least 4 inches but less than 8 inches of growth medium 0.6 3. Planted over at least 8 inches of growth medium 0.8 D. Vegetated walls in C and NC zones only 0.4 E. Permeable paving 1. Installed over at least 6 inches and less than 24 inches of soil and/or gravel 0.2 2. Installed over at least 24 inches of soil and/or gravel 0.5 F. Structural soil 0.5 G. Bonuses applied to Green Factor landscape elements: 1. Landscaping that consists entirely of drought- tolerant or native plant species 0.1 2. Landscaping that receives at least 50 percent of its irrigation through the use of harvested rainwater 0.2 3. Landscaping visible from adjacent rights-of-way or public open space 0.2 4. Landscaping in food cultivation 0.1 Table B for 23.86.019 Equivalent square footage of trees and large shrubs Landscape elements Equivalent square feet Medium shrubs or other perennials at least 2 feet tall, but less than 4 feet tall, at maturity 9 per plant Large shrubs or other perennials at least 4 feet tall at maturity 36 per plant Small trees 75 per tree Small/medium trees 150 per tree Medium/large trees 250 per tree Large trees 350 per tree Existing trees 20 per inch of trunk diameter 4.5 feet above grade (Ord. 125791 , § 116, 2019; Ord. 123495, § 101, 2011; Ord. No. 123209, § 67, 2009; Ord. 122935, § 24, 2009.)
Source: Seattle Municipal Code (Municode library)URL: https://library.municode.com/wa/seattle/codes/municipal_codeFetched: 7/25/2026