Seattle Stormwater Rules for Driveways and Patios: 2026 Guide
Last Updated: 10/12/2026Planning to tear out an old driveway, pour a bigger patio, or add a walkway in Seattle? Check the stormwater rules before you price the concrete. Seattle's Stormwater Code treats driveways and patios as hard surface. When a project's total new plus replaced hard surface passes certain square-footage marks, you need a permit, drainage review, and possibly on-site stormwater management. This guide explains how the city counts area, what each threshold triggers, and how to design compliant concrete.
How Seattle counts "new plus replaced hard surface"
The Stormwater Code is Title 22, Subtitle VIII of the Seattle Municipal Code (Chapters 22.800 through 22.808). The Seattle Department of Construction and Inspections (SDCI) reviews it as part of construction and grading permits, and SDCI and Seattle Public Utilities (SPU) publish a joint Stormwater Manual. The 2026 code took effect July 1, 2026, and applies to applications completed on or after that date.
- Hard surface means an impervious surface, a permeable pavement, or a vegetated roof. Driveways, patios, and walkways are listed as impervious.
- New hard surface covers paving over lawn, dirt, or gravel, or modifying an existing hard surface, such as regrading and repaving.
- Replaced hard surface means removing a surface down to the base course or subgrade and replacing it. If the base or subgrade elevation is changed beyond what repair requires, it counts as new.
Overlaying existing paving without exposing the base course, or crack sealing, is a "pavement maintenance practice." Unless they are integral to a larger parcel-based project, these practices are exempt from the on-site stormwater management, flow control, and treatment requirements.
Project size and the likely requirement
For parcel-based projects on private property, the total includes every new or replaced hard surface, roof area included.
| Total new plus replaced hard surface | Likely requirement |
|---|---|
| Under 750 sq ft | The paving area alone doesn't trigger an SDCI grading permit. Baseline rules still apply: preserve natural drainage patterns and control erosion during construction. |
| 750 sq ft or more | Grading permit (or grading review within a building permit) plus standard drainage review |
| 750 sq ft or more on a lot created or resized by a document recorded on or after January 1, 2016 | On-site stormwater management |
| 1,500 sq ft or more on other lots, or 7,000 sq ft of land disturbance | On-site stormwater management |
| 5,000 sq ft or more | A drainage report from a licensed civil engineer. Flow control and treatment may apply depending on the drainage basin. |
Some sites trigger drainage review at any size, including potentially hazardous locations and Category I peat settlement-prone areas. For larger projects, rules vary by drainage basin (creeks, wetlands, small lakes, combined sewer, capacity-constrained systems), so confirm your parcel with SDCI.
What on-site stormwater management means for concrete
On-site stormwater management (OSM) was formerly called "green stormwater infrastructure" in the code. For each surface, you work down a ranked list of practices and use the first feasible one. The parcel-based list includes full dispersion, infiltration trenches, drywells, rain gardens, bioretention, permeable pavement, and sheet flow dispersion. Projects in drainage review must use SDCI's OSM List Approach Calculator.
Permeable paving still counts as hard surface, so it won't get you under a threshold. It is on the OSM list, though, so a pervious concrete or paver section can manage its own runoff. For conventional concrete driveways, patios, and walkways, the practical move is to slope the slab toward a dispersion area or rain garden, not the street or a neighbor's lot.
How it ties into permits, SDOT, and ECAs
Stormwater review happens inside the SDCI construction or grading permit. If grading is already covered by a building permit, you don't need a separate grading permit. SDCI requires a pre-application site visit when disturbing more than 750 square feet of land. Work past your property line is handled by SDOT through Street Use permits. For details, see the driveway approach permit guide, the permits and inspections overview, and ECA review for concrete projects.
How to plan a replacement that meets the rules
- Measure everything. Total all hard surface you will remove and rebuild, plus any new paving.
- Keep base elevations where you can. Changing the grade beyond what repair requires turns replaced surface into new surface.
- Consider resurfacing. If the slab is sound, an overlay that leaves the base course in place counts as pavement maintenance, and SDCI says resurfacing needs no grading permit.
- Check your lot's history. Lots created or resized on or after January 1, 2016 reach the OSM threshold at 750 sq ft.
- Pick the drainage destination early. Choose a dispersion area, rain garden, or permeable section before setting slab slope.
- Confirm with SDCI. Ask about your drainage basin, ECA status, and discharge point before you sign a contract.
Pacific Northwest rain and your slab
Seattle's long wet season means a new slab sheds a lot of water, and the code says that runoff must not cause significant adverse impact to neighboring properties. The Stormwater Manual describes till as less permeable and recommends placing bioretention and permeable pavement over more porous soils, so infiltration testing may be needed. During the pour, every project must also follow construction stormwater pollution prevention requirements.
Frequently asked questions
Does replacing my driveway count toward Seattle's stormwater thresholds?
Yes. Removing a driveway down to the base course or subgrade and repouring it counts as replaced hard surface, and Seattle's thresholds are based on new plus replaced hard surface.
How much paving triggers a permit in Seattle?
SDCI requires a grading permit when you add or replace 750 square feet or more of paving, unless the grading is part of a building permit. Standard drainage review applies at that size.
Does permeable paving get me out of stormwater requirements?
No. Seattle counts permeable pavement as hard surface. It is, however, an on-site stormwater management option, so a permeable section can help meet the requirement.
When do I need a civil engineer?
SDCI requires a licensed civil engineer's drainage report for projects with 5,000 square feet or more of new plus replaced hard surface or 10,000 square feet or more of land disturbance.
Is resurfacing a driveway subject to the Stormwater Code?
An overlay that does not expose the base course is a pavement maintenance practice. It is exempt from on-site stormwater management, flow control, and treatment unless it is integral to a larger project.
Get a stormwater-ready concrete estimate
Call (206) 552-9998 for a free on-site estimate, or visit the Seattle Concrete home page to connect with a local contractor. SDCI makes the final call on your parcel.
