There is no single UK-wide deadline for Council Tax Support, as each council sets its own scheme and application rules. You should apply as soon as you become liable for Council Tax or your income drops, because support may usually start from the application date. Some councils can backdate an award, but this is normally limited and may require a good reason or evidence of why you did not apply earlier. Check your local council’s website or GOV.UK for the relevant rules and application process.