I brew in my garage, and have a ferment fridge out here too so don't have to lug things too far. I don't prop the garage door open during brewing but I do have the door to the house open, with a fan going to keep the air moving. Can't keep the door closed because the critters I live with (2 dogs and 3 cats) raise holy hell if they can't come out and be part of the brewday, to the point that every brewday a "brewcat" is selected by whichever one decides my lap is the place to be. Never had an issue with CO since I keep the air circulating. Hops are in their own freezer, and grain in a closed container; also learned a while ago to mill grain the night before to keep the dust out of everything.
My garage is my woman cave; got my laptop out here, with nice speakers, and the kegerator is only a step away. Husband can get squiffy if he sees me drinking before 9am so I have a nice selection of glassware in the freezer side of the kegerator, and a bag of red solo cups for sampling. And did I mention my super cushy recliner? Oh yes I could live out here.