These amounts are set by the government and are applied even if the non-dependant does not contribute towards the household bills. The fixed amounts depend on the non-dependant's circumstances. The following link gives details of the levels of non dependant deductions for 2011/2012.
In the case of a non dependant couple only one deduction will be made. The rate of this deduction will be the highest deduction that would be applied to them as individuals. If one or both members of the couple are working the gross income is treated as belonging to them as a couple.
In cases where a claimant or partner is aged 65 or more and a non-dependant moves into the household or has a change in their circumstances, which would increase the level of non dependant deduction, the resulting change to Housing and/or Council Tax Support entitlement will be delayed for 26 weeks.
It is very important to advise the Housing Benefit department of any changes which may affect the level of your entitlement as they occur.
Boarders and Sub-tenants
If someone other than a non-dependant pays rent for part of your property they are classed as a sub-tenant and if that rent includes money for meals they are classed as a boarder.
If you have a boarder or sub-tenant they are treated differently for Housing and Council Tax Support purposes. Instead of taking a set deduction from your benefit for them, part of the rental income you actually receive is taken into account when assessing your entitlement.