Make different objects for each building and make a single object to use as a parent for all the buildings. This is ideal in a RTS anyway since you won't want to code collisions for all buildings when you attack them, so you just code collisions for the parent object. You can then send the harvester to the nearest lumbermill or quarry or whatever.
Open Game Maker and press F1 and read up on parents.
oh cool, thanks that works, so useful