Download the big picture and just build in a block of empty space. Big blocks are floors, thinner ones are the empty space for ducts, and so long as you go from one floor to another you'll be fine. Alternately, heights of 45, 112, 179, 246, and 313 pixels are all valid. Please avoid anything over 3 floors at this point if you can help it, as they tend to make spacing more challenging. Ideally everyone would take single-floor rooms one at a time, but I know what drives people to go bigger.