We have a pretty big house with a portion of it used only for growing. There is a section if the house that is not used by my family. It's on one end of the house. The previous owners called it the guest wing. It's two bedrooms, a bathroom, and two huge closets in the hallway of this "wing". It's part of the house, under the same roof but it seems like a house of it's own. You get to it from the den and it has it's own door. It even has it's own central air and heat. Pretty cool.
I have a whole bedroom dedicate to flowering. The other bedroom has tents in it, one for my wife and one for my special stuff and plenty of space for grow stuff like extra fans, nutes and other grow related stuff. The two huge closets are for clones, seedlings and vegging. And of corse the bathroom is for watering and stuff.
I must say i feel pretty lucky to be able to have the setup I have. I sure do love my wife, she pays the mortgage, lol.