Google "water runoff laws in [your state, county]"

Water runoff is regulated, so you definitely have a case here. These regulations may vary depending on your location, so I'm afraid you'll have to do some research, or hire a lawyer. But the law is on your side.

I'm surprised the neighbor is not aware of that, given his construction background.

I know it's not easy to drag your neighbor to court, but your city/county may give you some advice on what other options you have. These issues come up often (I had a neighbor like that, he destroyed my fence by using it as a retaining wall and attaching stuff on his side, even though the fence was a foot into my property... )

...and have you talked to the neighbor?

tj

No matter how you look at it, your neighbor is to blame in my opinion. Talk to them and if you can’t resolve the situation by talking, then talk to a lawyer!

debra

The fence question depends on whose property it resides. Usually changing grade elevations that alter the natural flow of the land requires a permit. As others said, the runoff might have local AHJ regulations, but when lacking and the AHJ is "hands off," there may be defined case law precedent in your state.

Your neighbor is 100% in the wrong on multiple levels. If you approach the neighbor and they offer a solution I don't expect it will be what needs to happen. At this point only a civil engineer working off a current survey will know what that solution needs to be. You shouldn't have to pay for the engineer or any remediation. Call a lawyer. They knew what they were doing and they were counting on you not saying anything.

If the fence is built on the actual property line, in many states both neighbors share responsibility for the cost of upkeep and repair (surprising, I know!). Check the fence laws in your state/community.

I honestly had the same thing happen to me last summer with a not-so-neighborly landscape project that my next door neighbors undertook, where they fully regraded their backyard and built up their property by 2-4 feet, piling compacted soil up against my fence, and in order to do so, they ripped out a retaining wall on my property (I know because I put in a fence a couple years prior and hand the property line staked) because, as you probably guessed, all of this was done without a land survey, the required building permits, or "neighborly" transparency (they told me they were taking out a couple stumps and putting in a fence).

Anyways, I agree with others that the drainage laws will vary by jurisdiction, so start by first looking at you municipal and watershed ordinances, then check out your state laws. In my state, damage caused by a neighbor's negligence (i.e., failing to get building permit and land surveys are good indications of negligence) is eligible for damages up to three times the cost of repairs.

Also, consider installing some sort of drainage to divert the water flowing in from the other property - this homeowners drainage solutions map from NDS is really helpful for recommending solutions depending on the drainage source and decree of severity https://www.ndspro.com/home-drainage 

Call your city hall and ask to talk to the city engineer. Request that he/she visit your site to see what the situation is. There should be no charge for this. Many cities have ordinances against altering drainage.

OR, you could talk to my brother! He has successfully sued for just this kind of thing!

OTOH, where I used to live, no more than 5 yards of soil was allowed to be dumped on a property (out in the country), but the COUNTY filled a 1 1/2 acre field with 18" of fill, then denied us a permit to dig a swale to drain our property (which flooded due to their fill!) Good luck with that!