I think the main financial barrier is that you can't really own half a house. You can have everyone on the mortgage but then you're all 100% responsible for the all the payments which would scare the bejeezus out of me because in this scheme you're almost always buying a house that no single person/family in the group can afford.
I realize that I'm a little financially paranoid but I made sure that when we bought our house that we could afford our mortgage on a single salary and trying to scale this up sounds like a nightmare.
It helps to be in the same place as the other person(s) and time bomb the agreement. For us, it's:
Roughly similar salaries at Microsoft/Amazon
Same age
Significantly overlapping friend groups (which increases house utilization and happiness)
Ability to buy the house outright with reserves
Significant levels of trust
ARM mortgage guarantees that we're going to sell in 5-7 years, so if either of us want out we can generally wait it out in the worst case.
It's not an ideal scenario for everyone - I'm very lucky to be where I'm at. But it's very satisfying,and we're fortunate enough to not have to worry about the same financial concerns as others.
1. Always have a buffer and not land up on the street. Most likely it will be hard to find a cheaper place unless you are sharing anyway!
2. You could transfer the mortgage to a new roommate, and get out of the deal, you can liquidate your equity for the downpayment plus principal.
3. Rent it out to someone, opting for nominal cash loss while maintaining equity, assuming you found another spot more suitable to your married / new found environment.
4. While living, either your accomodation money goes into someone else's pocket, or 50% of it goes into theirs - that is the difference between renting vs. mortgage.
A contract won't pay the mortgage. It won't clear the title. Basically, in business contracts only come into play when relationships are bad. In a good relationship, discussions of the contract are "Why didn't you do that?" "Doing it is not part of the contract." "Oh, OK."
I realize that I'm a little financially paranoid but I made sure that when we bought our house that we could afford our mortgage on a single salary and trying to scale this up sounds like a nightmare.