Originally posted by Daft Unikorn:

impossible to get a a perfectly level ground, most games I wouldn't care but seeing how the building works in this game, I feel like it's vital

Alternatively, you could worry less about an impeccably flat terrain as a foundation and simply raise the floor of the building and then it's a much easier task to build a flat floor ;)
https://steamcommunity.com/sharedfiles/filedetails/?id=2440186645
What is it you want to build?