I couldn't find a tutorial on this anywhere so I thought I'd make one.
Step 1:
Open image of a premade building in an Image Editor like Photoshop and change Canvas Size (Not Image Size).
Step 2:
Change the Canvas Size to Width: 300% and Height: 400% and make sure the Anchor is on the top left
Step 3:
Save the file with !$ before the file name.
Reminders of why:
RPG Maker moves characters up 6 pixels (so living characters look more natural), ! stops that from happening, this is a building, not a living character afterall.
Also, RPG Maker reads charsets as if they always have 8 sets or 3 across and 4 down, $ makes it know that it is only one set, so we can do +300% and +400%. Without it the files would be huge to make this work (+1200% width and +1600% height to be exact)