Every new roof, patio and path changes how rain moves across a lot. When you add a backyard cottage, the water that once soaked into lawn now runs off hard surfaces. DADU stormwater management is how a project handles that runoff, and Seattle’s Stormwater Code sets the rules.

This guide explains the thresholds, the review steps and the on-site options the code describes. It reflects the 2026 Stormwater Code, which took effect on July 1, 2026.

Why Runoff Matters on a Cottage Lot

Soil and plants absorb rain and release it slowly. Roofs and paving do not.

So a cottage, a new walkway and a patio all send more water off the site, and faster. That water has to go somewhere safe. Otherwise, it can affect nearby structures or properties downhill.

The Stormwater Code addresses this in several ways. It sets basic rules for all projects, adds steps for larger ones and lists on-site tools to manage flow. Our guide to DADU site drainage covers the temporary side during construction, while this post focuses on the permanent system.

The 2026 Seattle Stormwater Code

Seattle updated its Stormwater Code through Ordinance 127444, passed as CB 121190. The new version took effect on July 1, 2026. SDCI and SPU share authority under the code, with SDCI reviewing drainage control plans for projects outside the public right of way.

The code applies to all projects, though the steps grow with project size. For a cottage, that means the basic rules plus any added steps the project’s size triggers. For the full text and companion manuals, see the SDCI Stormwater Code page.

Rules That Apply to Every Project

First, some rules apply no matter how small the job is. Under SMC 22.805.020, natural drainage patterns must stay in place, and discharges should occur at the natural location to the maximum extent feasible. Also, water leaving the site must not cause a significant adverse impact to receiving waters or properties downhill.

The code also covers the discharge point. Criteria for choosing it include preserving natural drainage patterns and confirming that the drainage system has enough capacity. For projects that meet the drainage review threshold, the drainage control plan must identify that point for SDCI review.

Thresholds for DADU Stormwater Management

Beyond that, a cottage project may reach the next level of review. Under SMC 22.807.020, standard drainage review applies to a building permit that authorizes 750 square feet or more of new plus replaced hard surface. It also applies to land disturbing activity of 5,000 square feet or more, including demolition permits.

On-site stormwater management has its own trigger in SMC 22.805.050. For a lot created or altered by a document recorded on or after January 1, 2016, the trigger is 750 square feet of new plus replaced hard surface. For other lots, it is 1,500 square feet. In either case, 7,000 square feet of land disturbing activity also triggers it.

So the lot’s history matters. Fortunately, a survey and title review can reveal the most recent recorded change to the lot. Our post on DADU property survey work explains how that research fits into early planning.

How the On Site Lists Work

When on-site management applies, SMC 22.805.070 offers two paths. A project can meet a performance standard based on modeled flows, or it can follow the on-site lists. By comparison, the lists offer a step by step path. In contrast, the performance standard relies on modeled flow durations.

The lists sort tools, called best management practices, into categories. For each surface, you consider every tool in the first category and use any that is feasible. If none works, you move to the next category. Once one tool serves a surface, no other is necessary for that surface.

DADU Stormwater Management Tools on a Residential Lot

In fact, the parcel list includes a wide range of options. For example, it names full dispersion, infiltration trenches, drywells, rain gardens, bioretention and permeable pavement. It also lists splashblock and trench downspout dispersion, vegetated roofs, residential cisterns, perforated stub out connections and trees.

However, feasibility depends on the site. Soil type, slopes, setbacks and nearby structures all play a role. That is why soil information helps here, and our guide to DADU soil testing explains how teams gather it.

Soil Amendment and Planted Areas

The code also cares about the ground that stays soft. Under SMC 22.805.050, projects must retain and protect undisturbed soil outside the areas under development. Before the project is complete, crews must amend new, replaced and disturbed topsoil with organic matter, as the Director’s rules require.

Notably, this step includes construction lay down areas. In other words, the spot where materials sat during the build needs attention before final approval. As a result, planted areas can absorb rain again. This step also ties the landscape plan to the drainage plan, so it helps to design both at once.

Planning DADU Stormwater Management Early

Stormwater choices affect the site plan, the budget and the landscape. First, tally the new and replaced hard surfaces, including the roof, paths and patios. Next, confirm the lot’s recording history and the soil conditions. Then ask the designer which on-site tools look feasible before the layout is final.

Code thresholds and rules can change, so check the current edition before you submit. Overall, DADU stormwater management works best when the team plans drainage, soil and surfaces together from the start.