A "non-dependent" is a person who is over 18 and:
They include adult son or daughter, a relative or friend who lives with you (but not a partner). A lodger lives with you, but this is on a commercial basis and they are not regarded as non-dependents. Joint owners, tenants and professional carers are also not considered to be non-dependents.
If you do have non-dependents living with you, your benefit will be reduced by a specific amount relevant to their income/savings (if they are a "couple", only one deduction will be made based on their combined income).
There is no deduction made if the claimant is blind or in receipt of Attendance Allowance or the care component of Disability Living Allowance, full time student.
The amount of deduction varies between Council Tax Support and Housing Benefit and in some cases there is no reduction eg in Council Tax support cases where the non-dependant is on income support.
If you do not provide evidence of your non-dependent's income, the Council will presume the highest deduction is appropriate. You must remember to advise the Council of any changes of income of your non-dependents, or if they move away or go into hospital.
The responsibility to advise the Council of any changes is the claimant's and not the non dependent's.