Retaining wall issue - house under contract - real estate lawyer?

[Drew Denham]

Hi all,

I have a house under contract in which there is a large retaining wall that seperates two properties (one being mine). When I go to fix I would have to extend 5 feet onto the neighbors property (GEO Grid) and dig alot out of his yard. My questions:

1) Would a basic contract work with the neighbor stating he agrees I can work on his property and install the grid? If not, should I get a real estate lawyer involed?
2) Should I ask the neighbor to help pay since the wall is between both properties?

Looking for some feedback