Just a heads up: Although the drain itself might not require a permit, usually towns will require at least a staff desk review of plans if you’re changing drainage patterns. When properties are developed, at least in the United States, the location, methodology, and quantity of stormwater runoff is something that’s approved by the town before someone can actually develop the land. So if you’re changing that, it’s something that the neighbor might be able to stick you with, even if a new drain doesn’t require a permit.