Seattle's permit department is putting builders on notice: if a Neighborhood Residential lot touches a wetland, creek corridor, shoreline or steep slope, figure out how much of it counts before you sketch a unit count. A short guidance post published Nov. 3 spells out which environmentally critical areas (ECAs) get subtracted when calculating density and lot coverage under the city's new NR rules (via Seattle Department of Construction and Inspections).
The timing makes sense. Seattle's updated NR standards, passed as Council Bill 120993 to implement Washington's middle-housing law, House Bill 1110, took effect in mid-January 2026, according to the city's Office of Planning and Community Development. Those rules let a lot carry the greatest of one unit per 1,250 square feet (one per 600 square feet for stacked flats), four units, or six units near a major transit stop or when at least two units are affordable. Maximum lot coverage is 50%.
City summaries of the new code have said both the density and lot coverage allowances shrink in proportion to the share of a lot covered by ECAs. The Nov. 3 post fills in which ground actually comes out.
According to SDCI, the full area of these features is excluded from lot area for density and lot coverage: wetlands and their buffers, riparian corridors, submerged lands and anything inside the shoreline setback. SDCI's exemptions page describes riparian corridors as land within 100 feet of a river, stream or water body, so a creek at the back of a lot can take a real bite.
Steep slopes are treated differently. For steep slope erosion hazard areas, SDCI says only the no-build portions have to come out of the lot area. That distinction matters, because the city defines a steep slope as a rise of at least 10 feet over 25 horizontal feet or less, with a buffer that's typically 15 feet wide, and some of that ground can be built on with city approval.
Here's a simple illustration of why it matters. On a 10,000-square-foot lot, the one-per-1,250 formula supports eight units and 50% coverage allows 5,000 square feet of building footprint. Pull 3,000 square feet of wetland buffer out of the lot area and the same formulas are working from 7,000 square feet, which means 3,500 square feet of coverage and a smaller unit yield.
SDCI points applicants to two tools for getting answers early.
For wetlands and riparian areas, builders can file a "Tree or ECA Related Analysis," one of the Land Use Analysis request types, to get an early delineation of ECA boundaries on the property. Those requests go through the Seattle Services Portal; the city's help article says fees show up in your cart before checkout but doesn't list amounts.
For steep slopes, SDCI says to apply for relief to pin down how far code allows you to encroach into the slope and its buffer. The city's ECA Steep Slope Development Relief program, governed by SMC 25.09.090B and covered in Tip 327A, applies to four situations: previously developed sites with no added impact to the slope, slopes created by legal grading such as road cuts, isolated steep slopes under 20 feet tall that sit at least 30 feet from other steep slope areas, and work needed to stabilize an active landslide. It doesn't apply to waterfront lots.
Relief requests start with a preliminary application in the Project Portal, typically need a pre-application site visit, and are billed hourly at the SDCI base rate, with a half hour paid up front. SDCI targets an initial review within two weeks, and you need the decision before your building permit or master use permit intake appointment.
Why it matters for Contractors
For small infill builders, the risk here is designing to a unit count the lot can't legally carry. If you're looking at an NR parcel near Seattle's creeks, wetlands, Lake Washington or Puget Sound shorelines, or any hillside, pull the ECA maps before you run a pro forma or make an offer, and budget time and fees for a land use analysis or steep slope relief decision before permit intake.
The steep slope rule is the more forgiving one, since only no-build areas come off the lot. Getting a relief decision early can protect lot area you'd otherwise lose, and it's far cheaper to learn your real buildable area before a designer draws six townhouses than after.