If you earn rental income from Airbnb above the relevant tax-free threshold, you need to register for Self Assessment with HMRC and declare your income. You can register online at gov.uk. The deadline for registering is 5 October following the end of the tax year in which you first received rental income. Once registered, you file a Self Assessment return each year. If HMRC considers your letting activity to be a trade rather than property income, for example because you provide significant additional services, National Insurance contributions may also apply. Consult an accountant familiar with short-let income for advice specific to your situation.