Seattle’s Land Use Code gives accessory dwelling units (ADUs) their own gross-floor-area limits. The applicable limit depends on the number of bedrooms and, for a limited larger-size route, the lot’s zoning and other conditions. It is a floor-area limit, not a complete building envelope or a guarantee that a design will fit a particular property.
The standard size tiers
As of September 25, 2026, Seattle Municipal Code (SMC) 23.42.022 limits an ADU with up to two bedrooms to 1,000 square feet of gross floor area. An ADU with three or more bedrooms may have up to 1,200 square feet, subject to the section’s other provisions. The section allows attached, detached, or stacked ADUs, with no more than two ADUs on a lot with a principal dwelling unit.
The section excludes certain space from the gross-floor-area cap: up to 250 square feet in an attached garage, all stories or portions of stories that are underground, and up to 35 square feet dedicated to long-term bicycle parking. These are exclusions from this size calculation; they do not remove other zoning or building requirements.
Check the limited 1,500-square-foot route
SMC 23.42.022 also provides a maximum of 1,500 square feet regardless of bedroom count when all three conditions are met: the lot is in a Lowrise (LR) zone, it is in a frequent-transit-service area, and it has not been purchased for more than $1,000 in the past 20 years. This is a narrow code provision. Confirm the parcel’s current zone, transit-area status, and purchase history before relying on it; transit proximity alone does not establish eligibility.
The code separately allows an attached ADU to exceed 1,000 square feet when the portion of the existing principal structure containing it existed as of July 23, 2023. Check the complete current section and the facts about the structure to determine whether this provision applies. Do not assume it removes other size limits or development standards.
Apply the limit to a property
The city’s size cap does not determine the maximum footprint or establish allowable setbacks, height, lot coverage, or an approval. Unless SMC 23.42.022 provides otherwise, the underlying zone and applicable overlays still supply development standards. Identify the zoning and project form, use the current code definitions for gross floor area, and confirm any relevant exception with Seattle’s ADU permit guidance and the authority reviewing the application.
The permit review also addresses current land-use, building, fire, energy, environmental, shoreline, and other applicable requirements. A zoning size limit does not confirm that an existing structure is lawful or that a proposed design can be permitted.