How do I prevent neighbors water from destroying my house foundation?

It is the location of your neighbour's evestrough downspout that is directing water on to your property?

Have you approached your neighbour to see if you can cooperate together in redesigning the spout so that the water is diverted?

While it is not their fault where the builder placed the eavestrough drains or how the land slopes, maybe they are not aware of the damage the situation is causing to your home.

Perhaps an underground downspout extension can be installed?

Sounds like you need to build a barrier that will direct their water away from your house. Likw a concrete wall. Also talk to them to see if they could redirect their down spouts in the mean time.

Excellent suggestions here! I fault the builder as they know about foundation issues and water intrusion. Chances are the statute has run on that. Your best bet is to try and work it out with your neighbor. If that isn't possible, get your local municipality involved. Let them be the heavy. Technically run off from a neighboring property is against the law. Or if funds permit, call in your own contractor or landscape architect and see what remedies they suggest. And then implement them. Absolute worst case scenario, hire an attorney and document every rainfall and resulting damage with time and date stamped photos. Foundation water intrusion is extremely expensive to repair, remedy the situation before it gets that bad.

water can be diverted with dry creek bed landscaping as in the photos. It could start with the neighbor and continue through your yard.

https://plantcaretoday.com/make-dry-creek-bed.html