Like most of our relationship, we blundered into something that worked.
Although we initially started in separate residences, we moved into a large college apartment that had three bedrooms, so everyone had their own room. This was fortuitous. Everyone had their own space, there was enough common space we had to manage chores, and things went (relatively) smoothly.
Even in monogamous relationships, the people often have 'their' private spaces. (It's common enough we've all seen the cartoons, gags, etc. about men's workshops, women's kitchens, i.e. places that 'belong' to one person in the relationship.) I think this is a very common need in any long-term intimate relationship; to have a place where you can 'be yourself' without having to make any concessions or compromises.