Was A renting out a room in the house before asking B to move in? If B moving in caused A to lose that income it seems fair to charge some amount of rent (although it should have been decided when B moved in, not 4 years later). BUT If A was paying the mortgage beforehand without planning to rent part of it out, maybe it's a little opportunistic to try to gain monetarily from living together? In that case, I would aim for a contribution that feels like an amazing deal on housing to B and a small perk for A instead of going for "fair" since this isn't the same as just being roommates. Maybe B pays all or most of the utilities? That way if you break up, B won't feel resentful for contributing to equity they won't retain and anything that B did pay is just gravy for A. If you do get married, it might not matter practically anymore, but things like this can cause lingering hurt feelings.