I live in a town home complex where neighbors share a party wall. Our next door neighbors had their entire french drain system redone and now half of their backyard floods every time it rains (several inches of water). All this water now floods our finished basement. We have asked them several times to grade their backyard to help the water flow away from the property, but they have not acted. Could we sue and legally force them to remedy the situation or is it our responsibility to fix the problem on our property?