Interior vs Exterior French Drain: Cost and When Each Wins
Interior French drains ($40 to $85 per foot, $4,000 to $17,000 per basement) manage water that is already under or inside the foundation and always pair with a sump pump. Exterior drains ($10 to $90 per foot depending on depth) intercept water before it reaches the foundation but require major excavation. Choose exterior when the problem is water against the foundation and you can excavate; choose interior when excavation is impractical or the water is coming up through the slab.
Interior or exterior is the fundamental fork in French drain planning, and the wrong choice wastes thousands. They solve different water problems at different prices with different disruption. This guide compares them head to head so you pick the right system the first time.
What each one does
An exterior French drain sits outside the foundation wall, usually at or near the footing, and intercepts groundwater before it touches the house. It is the only option that actually keeps water away from the foundation. An interior French drain (also called a perimeter drain or drain tile system) sits under the basement slab around the inside perimeter and collects water that has already made it under or through the foundation, routing it to a sump pump that ejects it outside. Interior manages water; exterior prevents it.
Cost comparison
Exterior shallow drains are the cheapest drainage work there is: $10 to $35 per foot. Exterior drains excavated to the footing, the version that truly protects a foundation, run $30 to $90 per foot, and a full perimeter on an existing home can reach $10,000-plus with excavation, waterproofing, and restoration. Interior basement systems run $40 to $85 per foot, $4,000 to $17,000 for a typical basement including the sump pump. Interior looks more expensive per foot than a shallow exterior drain, but for foundation-level water it is usually cheaper than excavating to the footing around an existing home.
Disruption: the real differentiator
Exterior work tears up the yard: excavation equipment, removed landscaping, patios, and sometimes driveways, plus weeks of a torn-up property. On a new build, exterior drainage is cheap and obvious because the foundation is already exposed. On an existing home with mature landscaping, a deck, and a patio, exterior excavation can double the effective project cost once restoration is counted. Interior work happens inside: breaking the slab perimeter is loud and dusty for a few days, but the yard stays intact and the work proceeds in any weather.
Effectiveness for each water problem
Water seeping through foundation walls: exterior to the footing is the gold standard; interior manages the symptom well. Water coming up through slab cracks or the cove joint (where wall meets floor): interior is the right answer, because the water is under the slab and exterior drains cannot reach it. Soggy yard with a dry basement: shallow exterior, no question. Finished basement you want to keep dry: interior, paired with a battery-backup sump pump, because it protects the finished space even if exterior conditions change.
Can you do both?
Yes, and on problem properties contractors often recommend it: exterior grading and shallow drains to handle surface water, plus an interior perimeter system with a sump pump as the last line of defense for the basement. The combination costs more than either alone but addresses water at both stages. What you should not do is install an interior system and assume the foundation wall problem is solved; interior drains do nothing for water pressing against the outside of the wall.
The sump pump question
Interior systems require a sump pump ($300 to $800 installed for the pump and basin; add a battery backup for $500 to $1,000 more, which is cheap insurance in storm-prone areas). Exterior systems that drain to daylight need no pump. If your interior system quote lacks a battery backup and your area loses power in storms, which is exactly when basements flood, add it.
The bottom line
Match the system to the water: surface and yard water gets shallow exterior drainage; water against the foundation on an accessible lot gets exterior to the footing; water under or inside the basement gets an interior perimeter system with a sump pump. When in doubt, pay for a drainage assessment before you pay for a drainage system; diagnosing the water's path is cheaper than installing the wrong drain.
Figures current as of October 2026. Sources: HomeGuide and Angi 2026 cost data, contractor-published pricing guides.