You should apply as soon as you need help with your rent, because Housing Benefit usually starts from the date you claim. In some circumstances, your council may backdate a claim by up to one month if you are of working age, or up to three months if you have reached State Pension age, but you must meet the relevant conditions. Most working-age tenants now claim the housing costs element of Universal Credit instead, although exceptions include some supported or temporary accommodation. Check your local council’s rules and the official GOV.UK guidance.