Need help with a spell for copying buildings

Need help with a spell for copying buildings

I want a spell for a custom builder unit that works like this:

You will have a builder guy that has a spell that he can target a building with and then he can build that building once. It is OK if he can build that building forever until he casts the spell on a different building if there isn't a way to make it so he can't build it agian. I tried a bunch of different ways with triggers but I couldn't get it to work every way I tried. Any help would be appreciated greatly and I will give U credit in my map.

PEONMASTER

Well... the only easy solution would be to replace the unit with another similar unit, give it the same hit points and have the replacing unit have the ability to cast buildings, and yeah, only the building it targets. So practically you need a lot of units to accomplish this.

~Daelin

Well, thanks...It's not possible if that's the case...too bad I'll think about a different unit.