If you are under State Pension Credit age Housing Benefit and Council Tax Reduction can be backdated if a good reason is shown for the delay in claiming. A good reason must be shown for the whole backdate.
The maximum time a claim can be backdated is one month. This time is from the date that the application for backdating is made.
If you or your partner are over State Pension Credit age you have 3 months to claim Housing Benefit and Council Tax Reduction. You do not need to provide a good reason.