I have a 3x3 column of up-down stairs that I've dug out, and accidentally broke through to the caverns before I was ready. In trying to block the bottom, I dug out the bottom set of up-down stairs and tried to replace them with an up stairs construction.
DF won't let me build a 3x3 up stair in the space; it says it's blocked. However, it does let me build a 2x3 stair and a 1x3 stair to fill the same space. Why?
z+1 is the level above the build issue where the last up/down stair (designation) is.. z is where the 2 up-stair constructions are (2x3 on top, 1x3 is the row on the bottom) and z-1 is cavern.
z+1 z z-1 ----- --££- .-- -XXX% £<<<% ----- %XXX% £<<<% ,,',- %XXX% £<<<% '𝓇-'- %%%%% ☼☼☼%% ---.`