We have built-ins in the family room that my kids are using as their "space." They have their desks with computers and their toys and so forth (we have a separate living room, so it works out).
I have a couple of shelves there for my castle themed LEGO, and my son's got a couple of shelves with his Star Wars and Bionicle LEGO on it.
Up stairs in the bonus room my wife and I use, I've got shelves loaded with stuff... they are completely full, and there is a castle MOC half finished on my desk as well as some train cars I just build... I simply have no more room for them on my shelves, I'm going to have to put some stuff in storage to build new sets... I have Cafe Corner, Green Grocer, and Millennium Falcon (4504) sitting in boxes because I simply have no room for them if I build them.
I already have a bunch of stuff (mostly trains, but other stuff too) that are languishing in boxes, sitting unappreciated while out of site.
We are looking for a new house with as much space as we have now plus a basement as a requirement. On top of that, we're downsizing and getting rid of a lot of old junk to make our lives simpler. But I have no idea when we'll be able to move... I make a good salary and can afford higher house payments, but I can't afford two payments for an extended period while trying to sell the old house. I don't know how people do it.