Relationships With Flatmates

I'm generous normally with friends, with my partner and flatmates that I have had. If they want to borrow something, that's fine - but don't be stingy because also you never know what might happen next week. Two housemates living in the same room broke up. Yay or nay? Errrr. What do you say? Well, if you start or if you start off as just flatmates and it becomes a relationship, then obviously the boundaries totally shift. You either need to find somewhere else to live, subject to contracts. Or you sort of set more rules.