Why do i have to pay half my rent when i claim ESA?

If you are on Income Support, income-based Jobseeker’s Allowance, income-related Employment and Support Allowance (ESA) or the guarantee credit of Pension Credit (whether you get it on its own or with the savings credit), Housing Benefit will cover all of your eligible rent. But you need to remember that not all of your rent or housing costs may be covered.

If you pay rent to a private landlord, the rent your Housing Benefit can cover will normally be restricted to an amount set by a rent officer.
If you rent from a social housing landlord, such as the council or a housing association, and you are working-age, the rent that can be covered by housing benefit will be restricted if your accommodation is considered to be too large.

Also, the Housing Benefit you can get may be reduced if another person lives with you who could be expected to pay towards their accommodation, even if they do not.