If your unincorporated business was adversely affected by the pandemic and had annual trading profits of £50,000 or less, you would’ve been eligible to obtain the first three taxable grants via the SEISS in 2020/21.
These need to be included in your 2020/21 tax return, and will be subject to income tax and Class 4 NICs. The grants also form part of the small-profits threshold for Class 2 NICs.
They will be added to any other taxable income before you are assessed for income tax on the excess above the £12,500 personal allowance.
The fourth and fifth grants will also be taxable for income tax and NICs purposes, but not until 31 January 2023. This is because if your applications are successful, you will receive these payments in the 2021/22 tax year.