If it did not have it's own room(just in a open basement) do u think you could smell it if you were down there?
Whether its in its own room or just in the basement shouldnt really matter if the room is in the basement anyways and you're not filtering it outside. If you have a basement with windows and you're still worried get a carbon filter and then duct it out a window. Thats what I do, it would literally be impossible for anyone outside to smell it unless they maybe stuck their noise straight up to where youre ducting out. The only problem with this would maybe be FLIR or infared from choppers if you're in a uptight state... but again this is extremely rare and they cant just infared everyone's houses and even if they did its a small grow...
There a lot of quality Carbon filter/scrubber manufacturers just look around I've heard good things about phresh but I just make my own because I know its effective.. and like the other dude said make sure it matches the CFM of the fan or better yet just buy a good fan + scrubber combo. I also run 4x4 with 4 sometimes 9 plants