I live in a flat shared between four people. It's in a very expensive inner city area, where parking comes at a huge premium.
The rent for the rooms in the flat are split based on the size of the rooms, on a fair formula. There is one parking place in an underground garage that comes with the property. Several of our neighbours rent their spaces out for decent amounts of money.
A flatmate just moved out, who used to use the parking space on weekends if their partner came to visit. It was understood though that if someone else had a guest, that they would take priority. So we tried to make it fair, so everyone got to use the parking spot for friends/visitors.
The new flatmate who moved in has a car and now uses the spot full time. Whilst this is fine because none of us other three flatmates have cars/need it constantly, I can't help but think that their rent should be a bit higher. Obviously not the £100 a month which is the going rate for a space, but say maybe £50, which could go towards our shared bills.
AIBU/