Basement waterproofing cost in Seattle (2026):
interior vs exterior, drains and sump pumps
HOW MUCH DOES BASEMENT WATERPROOFING COST IN SEATTLE?
Angi puts basement waterproofing in Seattle at $2,345 to $6,245, with an average of $4,257, above the national average. Interior methods cost less than exterior ones: This Old House lists interior waterproofing at $2,000 to $6,000 and exterior at $7,000 to $15,000, because exterior work usually means digging around the foundation. Washington sales tax applies to the whole job.
Basement waterproofing cost in Seattle
| Angi, Seattle | Reported cost |
|---|---|
| Typical range | $2,345–$6,245 |
| Average | $4,257 |
| Low end | $800 |
| High end | $12,500 |
Interior vs exterior waterproofing
| Interior | Exterior | |
|---|---|---|
| Typical cost (This Old House) | $2,000–$6,000 | $7,000–$15,000 |
| What it involves | Drains, a sump pump, crack sealing and coatings inside the basement | Usually digging along the outside of the foundation wall |
| Fixr's take | Cheaper, but manages water that already got in | Costs more because of excavation |
Cost by item
| Item | Fixr | This Old House |
|---|---|---|
| Waterproofing, per sq ft | $3–$10 | — |
| Interior perimeter drain, per linear ft | $40–$120 | $60–$120 |
| Exterior French drain, per linear ft | $10–$100 | $10–$100 |
| Sump pump installed | $600–$2,500 | $1,200–$2,500 |
| Epoxy crack injection, per crack | $250–$1,000 | $800–$1,500 |
| Vapor barrier | $1,500–$4,000 | — |
| Foundation inspection | $300–$1,000 | — |
Cost by basement size
| Basement size | Fixr estimate |
|---|---|
| 500 sq ft | $1,500–$5,000 |
| 750 sq ft | $2,250–$7,500 |
| 1,000 sq ft | $3,000–$10,000 |
| 1,500 sq ft | $4,500–$15,000 |
| 2,000 sq ft | $6,000–$20,000 |
Example with Seattle sales tax
| Price before tax | With Seattle's 10.55% sales tax |
|---|---|
| $2,345 (Angi Seattle low end) | $2,592 |
| $4,257 (Angi Seattle average) | $4,706 |
| $6,245 (Angi Seattle high end) | $6,904 |
Seattle permits for drains
- SDCI's side sewer guide says foundation (footing) drains are not side sewers; they are service drain connections that carry groundwater to an approved outlet.
- A side sewer permit is needed before work on side sewers and service drains, and a repair permit is needed to repair or replace a pipe or an outside fixture such as a cleanout or yard drain.
- Pipes inside the building footprint count as plumbing, not side sewer.
- Ask your contractor which permits your plan needs before work starts.
Cracks that come from settling or a bowing wall are a structural problem, not only a water problem: estimate those with the foundation repair cost calculator. Older house? See also seismic retrofit cost in Seattle.
Before you sign
- Ask where the water is coming from and whether the plan fixes the source or manages water that already got in.
- Get the drain length in linear feet, the sump pump model and the warranty in writing.
- Ask where the sump pump or drains will discharge, and who pulls any permits.
- Check that sales tax is shown on the estimate.
- Look the contractor up on L&I's Verify a Contractor tool. For jobs of $1,000 or more on a home, the contractor must give you a written disclosure statement (RCW 18.27.114).
Frequently asked questions
How much does basement waterproofing cost in Seattle?
Angi puts basement waterproofing in Seattle at $2,345 to $6,245, with an average of $4,257. With Seattle's 10.55% sales tax, the average comes to about $4,706.
Is interior or exterior basement waterproofing better?
Exterior waterproofing costs more, about $7,000 to $15,000 according to This Old House, because it usually means digging around the foundation. Interior systems cost about $2,000 to $6,000 but, as Fixr notes, manage water that has already come through the wall.
How much does a sump pump cost to install?
Fixr lists $600 to $2,500, and This Old House $1,200 to $2,500 for a professional installation.
How much does it cost to fix a leaking basement crack?
Epoxy crack injection costs $250 to $1,000 per crack according to Fixr, and $800 to $1,500 according to This Old House.
Do I need a permit for a footing drain in Seattle?
SDCI says footing and foundation drains are service drain connections, and a side sewer permit is needed before work on side sewers and service drains. Ask your contractor to confirm the permits for your plan.
Sources
- Angi — Basement waterproofing cost in Seattle, WA (October 24, 2025)
- Fixr — How Much Does It Cost to Waterproof a Basement? (updated February 6, 2026)
- This Old House — Basement Waterproofing Cost (updated March 4, 2026)
- Seattle SDCI — Tip 503, Side Sewer Permits in Seattle (August 12, 2019)
- RCW 82.04.050; DOR — Local sales tax rates, Q4 2026
- RCW 18.27.114 and L&I — Hiring a contractor
Sources accessed November 1, 2026. Angi's Seattle figures were checked on a screenshot of the page.
This guide summarizes published cost estimates and City of Seattle permit information. It is not engineering advice. Have a qualified contractor or engineer find the source of the water before you choose a method, and get written quotes from licensed, bonded and insured contractors. See our disclosure and disclaimer.