The disabled band reduction scheme aims to ensure that disabled people do not pay more Council Tax because they live in a larger property than they would have needed if they were not disabled. However, having a disability does not automatically entitle you to a reduction.
In summary, the requirements for a reduction are that the property must be the main residence of at least one disabled person and it must have at least one of:
an additional bathroom or kitchen,
any other room (not being a toilet) which is mainly used by the disabled person, or
enough space for the use of a wheelchair - if the wheelchair is for outdoor use only, this will not count
The room or the wheelchair must also be essential or of major importance to the disabled person's well-being, due to the nature and extent of their disability.
'Disabled person' in this context means a person who is substantially and permanently disabled. The disabled person can be either an adult or a child and does not have to be responsible for paying the Council Tax bill.
An extra room does not need to have been specially built, but your home will not qualify for a reduction unless the 'essential or of major importance' test above is met. Simply rearranging rooms (for example, having a bedroom on the ground floor rather than the first floor) is unlikely to make your home eligible for a reduction