The government calls rent affordable when it is 20% less than market rent:
If a 1 bed flat costs £1000 a month, they can call a flat that costs £800 a month “affordable”. If market rent goes up, so can “affordable” rent, even if wages stay the same.
A fairer system is “Living Rent”.
Living Rent is based on local income and capped so it won’t increase beyond inflation. Heart of Hastings and White Rock Neighbourhood Ventures use a Living Rents model for their homes.