Had a good mess around with the building tool and have been able to create a few buildings with it, both big and small. So here is my feedback based on what I have experienced.
The building tool is well laid out I like the fact that it works in logical steps as you move between tabs to construct your buildings.
The ability to choose between manually deciding how walls will be built and automatically having it done is great love having the choice and both seem to work well. Maybe a 3rd option to automatically have it done for you but only on one side of a building would be nice to have, especially when creating buildings with multiple rooms that require multiple floor plans.
The undo button didn’t work too well for me 3 out of 4 times that I used it the game crashed to desktop.
Having the option to add doors and windows in the planning before the building is constructed is really nice especially as workers will leave holes where these objects will be placed.
My final suggestion is would it be possible for there to be the addition of a scale showing the length of the floor whilst placing it to help with making things the right shape and symmetrical, this way you can see at a glance if your floor plan is 5x5 or 10x5 without having to count voxels.