Search for backyard cottage rules online and you will find advice from many different years. Much of it no longer matches the code. Recent Seattle ADU changes rewrote key standards in two steps, and some older city tip sheets have not caught up.

This guide lists the main updates with their effective dates and official sources. It also flags older documents that may mislead you. For the wider zoning picture, see our guide to Seattle middle housing.

Why Seattle ADU Changes Happened

State law drove most of the recent updates. In 2023, the Washington legislature passed House Bill 1337. It set new requirements for how cities regulate accessory dwelling units.

Seattle then had to bring its code into line. At the same time, other state laws on middle housing pushed the city to update its residential zones. As a result, ADU rules changed in two waves, first in mid 2025 and then in early 2026.

The First Wave in June 2025

The city’s ADU ordinance, Ordinance 127211, took effect on June 30, 2025. You can view the record on the Seattle City Council legislation page. An interim middle housing ordinance took effect the same day.

• New code home. The city moved common ADU standards into a new code section, SMC 23.42.022, that applies in every zone that allows single family homes.
• Two units. A lot with a principal home may have up to two ADUs, and both can sit in detached buildings.
• Height and setbacks. ADUs generally follow the height and setback rules for the main home, with special allowances for detached units near rear lot lines and alleys.
• Old section gone. The ordinance repealed SMC 23.44.041, the former Neighborhood Residential section that held most older ADU rules.

Our guides to Seattle ADU setbacks and two ADUs in Seattle explain these standards in more depth.

The Permanent Rules in January 2026

Next, the City Council adopted permanent middle housing legislation, Council Bill 120993, on December 16, 2025. It became Ordinance 127376, took effect on January 21, 2026, and replaced the interim rules. The city’s One Seattle Plan page tracks this work.

For ADUs, the permanent law made several notable changes. For example, the size cap now depends on bedroom count. A unit with up to two bedrooms may reach 1,000 square feet, while a unit with three or more may reach 1,200 square feet. In addition, the code requires no off street parking for ADUs, and ADUs count toward density, floor area and lot coverage.

Our posts on Seattle ADU size and parking cover these rules in detail.

Seattle ADU Changes Outside the Zoning Code

Not every change came from zoning. On October 1, 2025, side sewer permitting officially moved from SDCI to Seattle Public Utilities. So owners now apply to the utility for that permit.

The city also set new permit speed goals as of the same date. Meanwhile, its help article on preapproved cottage plans now limits that path to Neighborhood Residential zones. The city last updated that article on May 14, 2026.

Legacy Tip Sheets That May Mislead

Some city documents still describe older rules. They remain online, so it helps to know which parts may be out of date.

• Detached and attached ADU tips. SDCI last updated Tips 116A and 116B on August 18, 2023. They mention a minimum lot size, rear yard coverage limits, older zone names and the repealed ADU section. They also send side sewer permits to SDCI.
• Older ADUniverse guide pages. Parts of the city’s ADU website still describe rules from 2019 and the old side sewer process.
• The original preapproved plan guide. This planning guide from the launch of the program cites the old size limit and rear yard coverage rules.
• The ADU code update project page. SDCI’s project page still describes the ADU ordinance as a proposal, even though it took effect.

Many of these documents still explain process steps well. However, check any zoning number in them against the current code.

Reading Older ADU Articles With Care

Blogs and news stories age fast. Before you trust a rule, look for its date and compare it with the timeline above. Anything before June 2025 likely predates the Seattle ADU changes described here.

Watch for a few warning signs. For instance, mentions of an owner living on site, a minimum lot size, required parking or a single size cap for every unit often point to older rules. Similarly, older zone names suggest an article predates the latest updates.

What Stays the Same

Some basics did not change. Each project still needs a permit from SDCI, plus any trade and utility approvals. Each lot also still needs its own check for trees, critical areas, easements and site conditions.

Further zoning work continues, and the city has proposed more changes for some areas. Therefore, check the current code and the latest city pages before you design.

Where to Confirm Seattle ADU Changes

A few official sources carry the most current information. First, the SDCI accessory dwelling unit page summarizes the rules and links to the code. Next, the online municipal code shows the current text of each section. Finally, the planning office posts updates on new zoning work.

If a question remains, SDCI also accepts land use questions through an online form. Keep a copy of any answer with your project records.

Overall, the biggest updates arrived in June 2025 and January 2026. Current code text and recent city pages remain the most reliable sources, while older tips work best as background.