I did some digging in CK to find where the markers were as you cannot access the basement from the normal location
BUT
You can from outside the house - As you look at the main doors, go around the right hand side, right up against the house itself, theres a trapdoor to the basement
You are able to use the ladder in the cellar to go up to the main house but have to access the basement from the outside
Something to do with the Fishing CC from what Ive read but I dont know for sure.
Lakeview Manor - As It Should Be (Bugs/Glitches)
in Skyrim SE
Posted
CELLAR ACCESS BUG - For those it may help
I did some digging in CK to find where the markers were as you cannot access the basement from the normal location
BUT
You can from outside the house - As you look at the main doors, go around the right hand side, right up against the house itself, theres a trapdoor to the basement
You are able to use the ladder in the cellar to go up to the main house but have to access the basement from the outside
Something to do with the Fishing CC from what Ive read but I dont know for sure.
Hopefully this is of use to someone out there :)