From couch to co-tenant: how to ask to stay for real
Choose a calm moment well before the planned departure. Say that you are interested in a longer arrangement, then ask whether the host and other occupants would be open to it. Make clear that no is an acceptable answer.
If they are open, discuss the actual housing terms: bedroom or sleeping space, rent, deposit, utilities, chores, guests, lease status, and a move-in date. Confirm whether the landlord must approve the change.
Put the agreement in writing and do not assume that past generosity settles future obligations. A co-tenant relationship needs the same clarity as any other shared home.