Who pays when a roommate's guest stays for weeks

There is no universal line, and pretending there is one is what makes this conversation go badly. What households actually do is set a threshold in advance — a number of nights a month — and agree that past it, the guest counts towards the bills that scale with people: power, water, heating, internet if it is capped. Rent almost never moves, because the rooms have not changed.

Why it is rarely about the money

A household that is genuinely fine with somebody's partner being there four nights a week does not usually mind the extra water. What builds is the sense of a change nobody agreed to: the flat has a new resident, and the process for that was somebody simply not leaving.

Which means raising it as a bills question is often the easiest way in. It is concrete, it is not a judgement about anybody's relationship, and it gives the conversation somewhere to land.

Agree the threshold before there is a guest

Any number decided while somebody's partner is asleep in the next room is a number about that person. The same number agreed in week one is a house rule, and it holds far better because nobody had to be the reason for it.

What a fair adjustment looks like

Most households that do adjust keep it simple: the person whose guest it is picks up a larger share of the usage bills for that month, by an agreed round figure rather than a calculation. Nobody wants to meter a shower, and a household that starts trying to has already lost the thing it was protecting.

💡If the guest is becoming a resident, say that instead. A quiet fifth of the rent is a worse arrangement for everybody than an honest conversation about somebody moving in.