It isn't about square footage when it comes to smoke detectors. It is much more about layout. You can look up what codes are in your area, but generally you are supposed to have one in every bedroom (to awake anyone sleeping) one outside of every bedroom(s) or in other words the hallway to detect smoke or co if the door is closed. One in the garage since this is a common place for co and fires. One next to the kitchen area since this is also a common place for fires. One at the top of your stairs to pick up smoke rising from the basement, and one next to your utility room since the is a common place for co poisoning. If you can combine these areas and grow your square footage coverage great, like putting one at the top of the stairs right next to the kitchen. However there are building codes for a reason, your safety, so if in question, follow those.