This ground "must be level?" I don't think so (well yes, but no)!
To combat the troublesome Z-axis, there should be a smoother mechanic to flatten ground for structures, other than what we had previously glimpsed in Salem.
This below image helps to demonstrate the following suggestion.
Players should be able to lay a three-dimensional foundation as the base for their construction projects.
In this mock-up, a player lays the foundation in stone—but not in the typical way. The foundation this player lays is three-dimensional, and it brings the elevation of the construction area to that of the natural elevation (natural elevation which is highest and nearest the foundation).
With the foundation finished, it gets exciting, a player can now treat that land as any other stone-layed area—perhaps even to create his or her own custom structure (see references @ bottom).
It so happens the player prepared this area to fit a straw mansion.
Prior References to "Foundation:"
Mansion Costs
...Create Own Building Designs