Outdoor rooms in the Gallery? How??

Okay, short and sweet: how do I put my backyard on the gallery. I know its possible, because there's an Outdoor room type with 'rooms' that have no walls or fences.

The reason I really want to know is because I made an elaborate backyard and now want to change my main house to a different house that I built. It seems that if I want to change the house, I have to bulldoze my entire lot and I really don't want to lose all that work. I was thinking, hey, maybe I can just save my backyard on the gallery as a room (or several rooms) and add them in later. Unfortunatly, I cant find any information on how to do it.

Why can't this game do exactly what I want the way I want it and when I want??? 
😕mileytongue: