Since April 2013 social housing properties have been subject to the under occupation charge. This is based on a tenant having more bedrooms than they are eligible for. The charge can be either a 14% reduction based on having one additional bedroom or 25% based on having two or more bedrooms over their requirement. If you receive help with your rent through benefits and you have additional rooms above your eligibility then you would need to make up the difference.
For example: a lone parent with one child lives in a 3 bedroom property and the rent is £100 per week, she receives maximum help with her rent. Her bedroom eligibility is two bedrooms. She would need to find 14% of her rent due to the under occupation charge as she has an additional bedroom beyond her eligibility.
The benefit cap was brought in to ensure the amount of benefit available to claimants had a limit. This ensured a maximum amount of benefit was available for most households. This can affect the amount of help you can receive towards your rent. For further information visit direct gov.