An update:

My building goes all the way to bedrock now. (1 layer above it technically, since I wanted a smooth floor.)

The bottom 3-4 floors are still being excavated/built out though. I just took the time to create the stairwells for all floors, and do some quick digging at each level. Linoleum's Tombstone tunnel runs through one of the floors now, and as such I have doors where windows should be for it.