You can build a loft, it just takes a bit of using the games build mode, place your 1st story down, go up a floor and using the fence decking place the room as you want for your loft
http://i99.photobucket.com/albums/l313/jagwithtude/60a1dc36-b13b-4e86-a1e6-39d9b5092783_zps9bfz5vju.png
you can now knock out the floor you don't want any longer,http://i99.photobucket.com/albums/l313/jagwithtude/9eada061-2a79-4b0d-84d8-9a773d78d883_zpsxqcumj8z.png
use the room tool to draw your outer wallshttp://i99.photobucket.com/albums/l313/jagwithtude/de2bc046-3a48-42ae-bd20-99015d0a89e1_zpshyglo3ko.png
draw the upper walls around the other part of the room so the room closes
http://i99.photobucket.com/albums/l313/jagwithtude/06-15-17_3-01-06AM_zpsvwz7bfrq.png
Go up to 3rd floor and use the no wall decking tool to place over your loft so that there is a flat roof, once this is done you can now place a roof on it and then drag it to cover the whole house.
http://i99.photobucket.com/albums/l313/jagwithtude/795cb05e-4660-4a3c-85ec-6709506b937f_zpsrvojf36t.png
