If you are buying a house jointly with a partner, you must put down in writing beforehand what you are agreeing to. It may not be very romantic, but relationships and friendships break down, and it is much easier to sort things out if you have an agreement done beforehand. Here are some examples...
An interesting article in the Irish Times https://www.irishtimes.com/life-and-style/homes-and-property/thinking-of-buying-a-house-jointly-with-friends-here-are-five-homes-suitable-for-sharing-1.4822546 But few lenders will deal with multiple parties. “Pre-recession there were many incidences...