Seattle stormwater code for a remodel or addition: what triggers it and what the permit set must show

Short answer: Seattle’s stormwater code reaches a house remodel or addition when you add or replace hard surface, and it ignores work inside existing walls. At 750 square feet or more, your permit needs a drainage review. At 1,500 square feet or more (750 on lots changed by a plat recorded since 2016), you must also manage that runoff on site with green stormwater infrastructure where the site allows it. At 5,000 or more, a licensed civil engineer prepares the plan.

Piper Cole designs and administers construction; I do not build. Seattle figures here come from the 2026 Stormwater Code (SMC Chapters 22.800 to 22.808), the 2026 Stormwater Manual and SDCI’s stormwater page, read October 7, 2026. Confirm any figure with SDCI.

Which version of the code applies

The 2026 code and Manual took effect July 1, 2026. SDCI says projects with a completed application on or after that date must comply; an application is complete when intake is approved and the fees are paid. Earlier applications may choose the 2021 code. The Manual is the Director’s Rule with the technical detail: Volume 1 says which requirements apply, Volume 3 designs the practices, Appendix C lists when one is infeasible.

The code calls the requirement “on-site stormwater management.” SDCI’s 2021 implementation guide says it was formerly “Green Stormwater Infrastructure to the Maximum Extent Feasible.” I use both phrases. One 2026 change touches houses: the separate single-family section and its short on-site list are gone, and a house is now a parcel-based project (22.805.050). The 750 and 1,500 square foot lines did not move, but the flow control and treatment tests below now apply to a house as they do to any parcel-based project.

What counts as new and replaced hard surface

Hard surface is impervious surface, permeable pavement or a vegetated roof (22.801.090). Roofs, walkways, patios, driveways, compact gravel and packed earth are all impervious. It splits in two:

• New: lawn, dirt or gravel turned into a structure or paving, a structure that grows past its existing footing (Manual 2.5.2), or any surface whose foundation or subgrade is removed or regraded (22.801.150, 22.801.190).
• Replaced: a structure removed down to the top of its foundation, foundation kept, and rebuilt; or paving removed to its base course and replaced without regrading (22.801.190).

For a house the two are added together. The remodel rule is Manual section 2.5.3. The foundation is the footing, stem wall and sill plate. If the roof, walls and floor joists come off and the house is rebuilt, that area is replaced hard surface, even if a slab or basement stays. Deepen a basement under a structure you keep, or crib a house up and keep its roof, walls or joists, and those areas do not count.

• Manual Figure 2.14 labels a second story over existing first-floor walls, and a roof replacement, as not counted as new or replaced hard surface. The foundation and walls must stay, and the second story must stay within the footprint.
• Overlaying pavement without enlarging it does not count if the base course is not exposed.
• An open-board deck escapes the count only if all the ground below is pervious amended soil.
• SDCI’s checklist words its test as adding or replacing building area measured by the roof outline. I treat a second story or re-roof as no count only after SDCI confirms it on your plans.

Zoning lot coverage is a separate test; see Seattle zoning explained.

The numbers that matter

If your project has	What it triggers	Source
750 sq ft or more of new plus replaced hard surface	Standard drainage review: site plan, drainage control plan, construction stormwater plan, recorded memorandum	SMC 22.807.020.A.2.b
5,000 sq ft or more of land disturbance, demolition permits included	Standard drainage review	22.807.020.A.2.a
1,500 sq ft or more of new plus replaced hard surface, or 7,000 sq ft of land disturbance	On-site stormwater management	22.805.050.B.2
Same, on a lot created or resized by a plat or document recorded on or after January 1, 2016	On-site stormwater management at 750 sq ft of hard surface (land disturbance still 7,000)	22.805.050.B.1
5,000 sq ft or more of new plus replaced hard surface, or 10,000 sq ft of land disturbance	Comprehensive drainage review, plan by a licensed civil engineer	22.807.020.A.3
5,000 sq ft or more of new plus replaced hard surface	Flow control, depending on where the water goes; treatment at 5,000 sq ft of pollution-generating hard surface	22.805.050.C and D

SDCI’s page says “more than 750” and the code says 750 or more; I design to land under both. Even without drainage review, every project must meet the chapter’s basic rules, such as erosion control (22.805.010.A). Phasing does not reset the count: projects under one plan, including a plan phased over years, are counted as one (22.805.010.B; definitions in 22.801).

Five projects, counted

I assume a lot not changed by a plat since 2016, under 7,000 square feet of land disturbance, and no wetland, creek or capacity-constrained discharge. Rows 1 and 3 rest on Manual Figure 2.14, not on a line that names roofs or second stories.

Project	What counts	New plus replaced	What it triggers
Interior remodel, new windows, new roofing	Nothing leaves the footprint or comes down to the foundation	0 sq ft	No drainage review
Rear addition 24 x 25 ft, patio 14 x 18 ft	600 (24 x 25) plus 252 (14 x 18)	852 sq ft	Standard review (over 750). No on-site management here (under 1,500); on a post-2016 lot it applies
Second story, 1,000 sq ft of floor on the same footprint	Footprint and roof outline do not grow	0 sq ft	Nothing from this scope if walls and joists below stay
Gut a 1,800 sq ft house to its foundation, add 500 sq ft, remove a 12 x 40 ft driveway to base course and re-pour it	1,800 replaced, 500 new, 480 (12 x 40) replaced	2,780 sq ft	Standard review and on-site management (over 1,500); no engineer on thresholds alone
Rebuild with foundation removed: 3,100 house, 1,200 driveway, 900 patio and walks	All new, none kept	5,200 sq ft	Comprehensive review, civil engineer, flow control and treatment tests

The fourth row surprises owners: taking the house to its foundation puts 1,800 square feet in the count with no addition at all. So I count before I sketch. See remodel or tear down in Seattle.

On-site stormwater management, the green infrastructure part

You meet it with a modeled performance standard or with the on-site list (22.805.070). Most houses use the list, and SDCI requires its List Approach Calculator on every project needing drainage review. For each roof and each other hard surface, you go through ranked categories and use the first feasible practice (a few, such as rainwater harvesting and cisterns, are optional):

1. Full dispersion, infiltration trenches, drywells, rainwater harvesting.
2. Rain gardens and infiltrating bioretention, with or without a tree; permeable pavement.
3. Sheet flow and concentrated flow dispersion; splashblock and trench downspout dispersion.
4. Non-infiltrating bioretention, vegetated roofs, residential cisterns.
5. Perforated stub-out connections and trees.

Feasibility turns on distances. Where runoff from under 5,000 square feet of impervious surface is infiltrated, the practice sits at least 5 feet from a house without a basement and 10 feet from one with a basement, and at least 5 feet from a property line (none on the street side). Appendix C treats a splashblock with under 50 feet of vegetated flow path, or a trench with under 25, as infeasible. On a narrow lot those distances are hard to find, which is why there are five categories. If none works for a surface, you document why and stop.

Testing sets a schedule. The Manual prefers subsurface investigations from November to March. The Simple Infiltration Test needs no licensed professional; one test is enough in the wet season and two in the dry. Order it before drawings are final.

Flow control and water quality

These are the larger systems, sized by modeling. For a parcel-based project they can begin at 5,000 square feet of new plus replaced hard surface, depending on where the water goes: a wetland, creek, small lake, the combined sewer or a capacity-constrained system. Creek and wetland tests add combinations of their own (22.805.050.C). SDCI links a rule listing combined sewer basins that are exempt from flow control, with a warning that groundwater discharge or a capacity-constrained downstream system can bring it back. Treatment applies at 5,000 square feet of pollution-generating hard surface, not when water goes to the combined sewer (22.805.050.D). Driveways qualify, and so do metal roofs without a baked-on or similar inert coating (Manual 2.6). Most single-lot remodels stay under these lines. A big rebuild can cross them.

What the permit set must show, and who prepares it

Standard drainage review asks for, at a minimum (Manual Volume 1, 2.8.2):

• A site plan with limits of disturbance, creeks and critical areas, trees 6 inches or larger in diameter, every hard surface, buildings with overhangs, grades, retaining walls, utilities, easements and setbacks.
• A drainage control plan showing where water leaves the site, how it is conveyed, uphill run-on, and each practice with maintenance instructions.
• On-site management documentation: hard surfaces by type, the practice chosen for each, infeasibility checklists, and infiltration results.
• A construction stormwater and soil plan. SDCI offers a pre-approved standard plan under 5,000 square feet.

You also sign a notarized memorandum of drainage control, recorded with the King County Recorder. It gives the city inspection access and obliges you to tell future buyers about the facilities; you maintain them (22.807.020.D.2.d, 22.807.090).

On who prepares it: the code requires no engineer for standard review, and standard designs may be used. A licensed civil engineer prepares comprehensive plans, and also small-project plans where the Director finds no off-site discharge point; under 1,500 square feet, the Manual lets the applicant prepare that plan in some cases. A Small Pilot Infiltration Test report needs a licensed professional, and infiltration near a steep slope needs a geotechnical engineer. I draw the site plan with every hard surface measured and totaled, since each threshold comes from that drawing, and I bring in the engineer where the rules say to.

Common surprises

• Where the water goes. SPU’s rule DWW-210 bars stormwater and groundwater, footing drains included, from sanitary sewers, even a formerly combined one, at any project size. Approved discharge points run in order: receiving waters, piped public drainage, ditches, the public combined sewer, then infiltration. Confirm yours with the city before design. Private drainage is a side sewer and needs a permit (22.805.020.K).
• Lot history. A recorded plat or boundary change since January 1, 2016 drops the on-site management line from 1,500 to 750 square feet.
• Basement dig-outs. Permanently discharging groundwater to public drainage or the combined sewer brings peak control at 2,000 square feet of new plus replaced hard surface (22.805.050.C.7). See basement dig-outs.
• Driveways and patios. Replacing one to its base course counts as replaced; regrading under it makes it new. Both go in the total.
• Winter work. From October 1 to April 30, exposed soil may sit unworked no more than 2 days; from May 1 to September 30, 7 days (22.805.020.D.7).
• Soil amendment. The code lists it for parcel-based projects with no square foot floor. It covers cleared or compacted ground not under hard surface, except slopes over 33 percent (Manual Volume 3, 5.1).

Trees, steep slopes and critical areas

The code lets the Director reduce the on-site list where a practice conflicts with Chapter 25.11 (Tree Protection) or Chapter 25.09 (critical areas) (22.805.070.D.1.b.5). Appendix C makes that specific. A practice is infeasible if it needs removal of a healthy existing tree, at least 2 inches in trunk diameter (deciduous) or 5 feet tall (evergreen), in an area protected through construction. It is infeasible where critical area rules prohibit it, and a non-infiltrating bioretention cell that cannot reach its 2 foot minimum width without entering a tree’s critical root zone fails too. The 2026 list names rain gardens and bioretention with or without a tree, and lists trees as a practice. Tree rules are in Seattle tree ordinance for building and remodeling.

Infiltration and dispersion are limited in landslide-prone areas and within 10 times the slope height of a steep slope, up to 500 feet. A slope stability analysis by a geotechnical engineer or engineering geologist can make them feasible. Any new or replaced hard surface on a listed contaminated site, or new hard surface in a Category I peat settlement-prone area, triggers standard review (22.807.020.A.2.f and g). More in Seattle critical areas and your building project.

If your house is on the Eastside

Seattle’s numbers do not carry across city lines. What I could read on October 7, 2026:

• Kirkland. Public Works Policy D-2, last revised January 2024, sets basic drainage review at 500 to 1,999 square feet of new plus replaced impervious surface and simplified review for single-family projects from 2,000 square feet that meet a list of conditions. It says drainage plans must be prepared by a professional engineer or architect. See the Kirkland permit guide.
• Bellevue. The city’s single-family addition page lists a stormwater site plan, a storm drainage report and a construction stormwater pollution prevention plan. I could not confirm its thresholds from a current primary source; ask the city. See the Bellevue permit guide.
• Mercer Island. I could not read the code today. Ask the city; see the Mercer Island permit guide.

Where I would start

1. Total the hard surface on the survey, then what you add or replace.
2. Mark which parts of the house will come down to the foundation.
3. Check the lot for a post-2016 plat change, steep slopes, protected trees and wet areas.
4. Confirm where each downspout and footing drain can legally go.
5. Order the infiltration test in the wet season if a rain garden or trench is likely.

Questions

Does Seattle’s stormwater code apply if I only remodel inside my house?

Not in a way that adds review. The code counts new and replaced hard surface, and an interior remodel inside existing walls adds none. Every project must still meet the chapter’s basic rules, such as erosion control, even where drainage review is not required (SMC 22.805.010.A). If the roof, walls and floor joists are removed down to the foundation, that area becomes replaced hard surface and counts toward the thresholds.

At what size does a Seattle addition need stormwater review?

Standard drainage review starts at 750 square feet or more of new plus replaced hard surface, or 5,000 square feet or more of land disturbance (SMC 22.807.020.A.2). At 5,000 square feet or more of new plus replaced hard surface, or 10,000 square feet of land disturbance, comprehensive review applies and a licensed civil engineer prepares the plan. SDCI’s page words the first line as more than 750 square feet.

What is the 1,500 and 750 square foot on-site stormwater management rule?

A parcel-based project, which includes a house remodel or addition, must manage runoff with on-site stormwater management at 1,500 square feet or more of new plus replaced hard surface, or 7,000 square feet or more of land disturbance. On a lot created or resized by a plat or other document recorded on or after January 1, 2016, the hard surface line is 750 square feet (SMC 22.805.050.B).

Is green stormwater infrastructure required for a single-family remodel in Seattle?

It is required when the on-site stormwater management thresholds are met, and only where feasible. Seattle’s code calls the requirement on-site stormwater management, formerly green stormwater infrastructure to the maximum extent feasible. You evaluate practices by category, such as dispersion, rain gardens, permeable paving and trees, use the first feasible one for each surface, and document why others are infeasible.

Does a second story add hard surface under the stormwater code?

Not by itself. Manual Figure 2.14 shows a second story over existing first-floor walls as not counted as new or replaced hard surface, and the Manual counts a structure as replaced only when it is removed down to the foundation. A second story within the existing footprint, with the walls below kept, adds nothing unless the footprint grows. SDCI’s checklist measures buildings by roof outline, so confirm against your plans.

Do I need an engineer for the stormwater part of my permit?

A licensed civil engineer prepares the plan for comprehensive review, which starts at 5,000 square feet of new plus replaced hard surface or 10,000 square feet of land disturbance. Below that, the code does not require one for standard review. An engineer is also required where the Director finds no off-site discharge point for a small project, with a limited exception under 1,500 square feet. Some infiltration reports need a licensed professional.

Can I send my new downspouts and footing drain to the sewer?

Not to a sanitary sewer. SPU’s Director’s Rule DWW-210 says stormwater and groundwater, footing drains included, may not enter a sanitary sewer, even one that was formerly combined, regardless of project size. The approved discharge point is set in priority order, starting with receiving waters and the piped public drainage system, and you confirm it with the city before design.

Does replacing a driveway or patio count?

Yes. Removing the surface down to its base course and replacing it counts as replaced hard surface, and regrading the subgrade makes it new hard surface. Both count toward the same totals. Overlaying existing asphalt or concrete without enlarging it does not count if the base course is not exposed.

How do trees and steep slopes affect stormwater requirements in Seattle?

A practice is infeasible if installing it requires removing a healthy tree of the size the Manual sets, in an area protected during construction. Steep slopes and landslide-prone areas limit infiltration and dispersion, within a setback of 10 times the slope height up to 500 feet, unless a geotechnical analysis shows it is safe. Critical area rules can also prohibit a practice outright.