Tricky.
She'll be sharing communal areas (living room, kitchen, bathroom), but they'll share a bedroom. I think they should pay more than half the rent between the two of them, but I don't think they should pay 1/3 each - they won't have quite as much space as you, and therefore shouldn't have to pay and equal amount to you.

You could say that yes, they pay half, you pay half, she pays 1/3 of the bills, but they also do something slightly extra every month that you don't have to do (buy a couple of communal bottles of wine? Cook a storming Sunday roast once a month?)

However, will this cause friction? Could it possibly lead to an end to your friendship? If so, it's not worth quarreling over what is possibly a not huge amount of money.
(The Empress it seems you CAN polish a turd!, Thu 15 Apr 2010, 10:06, Reply)