Someone said :
Not really sure where you're going with that! Surely the only person who could potentially share that room would be a partner of the person who already rents that bedroom?
Secondly if said person did move in, wouldn't permission have to be sought from the rest of the people in the house, you know....as a courtesy?
Exactly. If a partner of the big-room housemate was to move it, it would not be fair to keep the rates identical.
But people often don't just "move in": they slowly do so.
And "partners", especially when new, can often change the friendly dynamic of a group!
that is my point. You cannot rely on courtesy when it comes to contractual obligations...
Also the double-roomers have an inherent potential to accommodate another person, even if only for a night or a week-end. And that's one of the reasons why setting the room rates can be trickier than just setting a price.
Now if all are happy with the set-up, happy days.
I still strongly believe in putting things in writing, as a rental agreement is a written contract, and can only be changed in writing: gentlemen agreements or courtesy or other oral or moral contracts have no impact on a written contract.