Every housing system on earth has one. No regulator publishes it. Everyone who has lived below it knows exactly where it is. Above the line, a home keeps out the weather, keeps the family safe, gives a child a place to study, and stays put from one year to the next. Below it, none of that can be relied upon — and everything else in life gets harder as a result. What sits on the line is not complicated: → Shelter that holds, for decades not seasons → Safety, and a door that lo