On page 4, the rules list specific rows for normal, unique, and basement cards but not empty rooms. Can they be placed in any row, or only in certain rows?
Page 7 has a couple explicit examples of placing empty rooms on the top 2 rows (where normal and unique rooms can be placed). I think page 16 has an example with an empty room in the bottom row?
The only rule for empty rooms is that they can't be placed above empty spots. Otherwise they can go anywhere, even next to each other.