French Drain Installation Cost in Seattle and How to Hire It Right
When water keeps finding its way into a Seattle crawl space no matter how many times it's pumped out, the real fix is often a French drain that intercepts water before it reaches the foundation. Here's what installation involves, what it costs locally, and how to hire someone who'll size it correctly for our soils.
By Jason Green · Crawl Space · Published August 31, 2026 · 8 min read
Why French drains matter in Seattle's clay soils
A French drain is a gravel-filled trench with a perforated pipe that collects groundwater and redirects it away from the foundation before it can pool under the house. In most of the country this is a nice-to-have upgrade, but in Seattle, where dense glacial till and clay lenses sit close to the surface across much of the city, it's often the single most effective fix for a chronically wet crawl space.
Neighborhoods built on hillsides or old lake terraces — parts of Magnolia, Beacon Hill, and the slopes above Lake Washington in Seward Park — are especially prone to lateral groundwater movement during the wet season, where water moving downhill through the soil hits an impermeable clay layer and travels sideways until it finds a foundation wall to seep through.
Interior French drains, installed along the inside perimeter of the crawl space and tied into a sump pump, are the most common configuration in Seattle because they don't require excavating around the exterior of the house, which is often impractical on narrow city lots with tight setbacks.
Interior versus exterior French drains
An interior French drain runs along the inside edge of the crawl space footing, collecting water that has already made it under the foundation and routing it to a sump basin for pumping out. This is the more common and less expensive option in Seattle, since it avoids digging up landscaping, walkways, or driveways.
An exterior French drain is installed outside the foundation wall, intercepting water before it ever reaches the crawl space, but it requires excavating along the foundation perimeter, which is more disruptive and expensive, especially on lots where the house sits close to a property line or a neighbor's fence.
Most Seattle contractors recommend an interior system unless there's a specific reason to go exterior, such as water actively undermining the foundation footing itself or a known high-volume groundwater source on the uphill side of the property.
What a French drain installation includes
A proper interior French drain installation includes trenching along the crawl space perimeter, installing perforated PVC pipe wrapped in filter fabric, backfilling with drainage gravel, connecting the drain to a sump basin and pump, and grading the trench slightly toward the sump to keep water moving by gravity rather than pooling in the pipe itself.
Ask whether the filter fabric wrapping the pipe is included, since skipping it allows fine soil particles to clog the perforations over a few years, which is one of the most common reasons a French drain that worked fine initially stops functioning after five or six wet seasons.
The drain should also be paired with a vapor barrier on the crawl space floor between the drain and the rest of the space, so that moisture from the exposed soil isn't still evaporating up into the crawl space air even after the standing water problem is solved.
What French drain installation costs in Seattle
An interior perimeter French drain for an average Seattle crawl space typically runs $3,500 to $9,000, depending on the crawl space's perimeter length and how much excavation the soil requires. Adding a sump pump and basin to the system, if not already installed, typically adds $1,800 to $4,200 to the total.
An exterior French drain runs considerably more, typically $8,000 to $20,000, because of the excavation involved along the foundation and the cost of restoring landscaping, walkways, or hardscaping disturbed during the dig.
Ongoing maintenance is minor but real: most contractors recommend flushing the drain line every few years to clear silt buildup, which typically costs $200 to $500 depending on access to cleanout points.
How to vet a French drain contractor
Confirm active Washington L&I registration and ask how they determined the trench depth and pipe slope for your specific crawl space, since a drain installed without adequate slope toward the sump basin won't move water effectively no matter how well everything else is done.
Ask what pipe size and gravel grade they're using, and whether the pipe will be wrapped in filter fabric to prevent clogging. Reputable contractors can explain these specifications without hesitation because they're basic to doing the job correctly.
Request references from Seattle homes with a similar water intrusion pattern, and ask how long ago that drain was installed and whether the homeowner has seen any water since, since a French drain's real test is how it performs across multiple wet seasons, not just the first storm after installation.
Permits for drainage work
Interior French drains tied into an existing sump system generally don't require a permit, but any work that changes how water discharges to the exterior, connects to the public stormwater system, or involves significant excavation near the foundation may require a permit through the Seattle Department of Construction and Inspections, located at 700 5th Avenue, Suite 2000, Seattle, WA 98104, phone (206) 684-8600. Permit applications and inspection scheduling go through the Seattle Services Portal at cosaccela.seattle.gov.
If the discharge point ties into the city's combined sewer or stormwater system, Seattle Public Utilities may also need to review the connection, so ask your contractor directly which agency's approval applies to your specific discharge plan.
Red flags to watch for
Be cautious of a bid that doesn't mention slope or grading at all, since a French drain installed flat or against the natural water flow simply won't work regardless of how much gravel or pipe goes into the trench. Also watch for contractors who propose an interior drain without a clear discharge plan for where the collected water actually goes.
If a contractor proposes an exterior French drain without first explaining why an interior system wouldn't solve the problem, ask for that reasoning directly, since exterior systems cost substantially more and aren't always necessary.
Comparing French drain quotes
Compare bids on interior versus exterior scope, pipe size and material, whether filter fabric is included, trench slope and discharge destination, and whether a sump pump and basin are bundled into the price or quoted separately. For a full overview of crawl space costs, neighborhoods, and licensed pros serving the whole metro, see the Seattle crawl space hub at https://homeownerhenry.com/crawl-space/washington/seattle, and if you're also seeing foundation cracks or settling alongside the water intrusion, the Seattle foundation repair hub at https://homeownerhenry.com/foundation/washington/seattle covers that side of the house.
Also ask each contractor how they'll protect existing landscaping or hardscaping during excavation, and whether restoration of any disturbed areas is included in the price or billed as an add-on afterward, since that detail can meaningfully change which bid is the better value.
Get Matched With Up to Three Local Pros
You don't have to vet every drainage contractor in King County on your own. Tell Homeowner Henry a bit about your project — square footage, where you're seeing water come in, and roughly when you'd like the work done — and we'll match you with up to three vetted local contractors who work on Seattle-area homes.
Getting three quotes side by side is the single best way to catch an outlier bid, whether it's suspiciously cheap or padded with add-ons you don't need. There's no obligation to hire anyone, and comparing bids costs you nothing but a few minutes.
Typical cost ranges
| Interior perimeter French drain | $3,500 – $9,000 |
| Exterior French drain | $8,000 – $20,000 |
| Sump pump + basin add-on | $1,800 – $4,200 |
| Drain line flush/maintenance | $200 – $500 |
| Landscaping/hardscape restoration (exterior jobs) | $500 – $2,500 |
| Vapor barrier add-on | $1,500 – $3,500 |
Ranges are general estimates gathered from homeowner-reported quotes, not an offer or a quote for your project.
Common questions
How much does a French drain cost in Seattle?
An interior perimeter French drain typically runs $3,500 to $9,000, while an exterior French drain runs $8,000 to $20,000 due to excavation.
Do I need a sump pump with a French drain?
Interior French drains are almost always paired with a sump pump and basin to move collected water out, adding roughly $1,800 to $4,200 if not already installed.
Is an interior or exterior French drain better for Seattle homes?
Most contractors recommend interior systems since they're less disruptive and less expensive, reserving exterior systems for cases where water is undermining the foundation footing.
Do I need a permit for a French drain in Seattle?
Interior drains tied into an existing sump usually don't need one, but exterior work or new stormwater connections often require a permit through SDCI.
How long does a French drain last?
A properly installed French drain with filter fabric can last well over a decade, though most contractors recommend flushing the line every few years to prevent silt buildup.
How do I verify a Seattle drainage contractor?
Check active registration, bond, and insurance status through Washington L&I's free Verify a Contractor tool before signing a contract.
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