Looks to be a useful tool. Nice work. Fairly intuitive to use.
The coordinates in the cells is nice, but if you could use a smaller font it might not spill over into the adjoining cells.
The coordinate system was a bit unclear, typically x,y,z indicates x=right to left, Y equals up and down, and Z is front to back. It "seems" you are using x,z,y in the top view, and I have not quite figured out how the side views correlate back to the top view exactly. For example, i locate a wall torch on the top view. Select it with the cursor, then switch to a side view and its not on that tab until I hunt around for it. Having the other tabs reorient based on the selection made in the previous tab would be a great additional feature.
The prefab tab text is cutoff on the left side of the window.
Thanks for adding this to the communities tools!