https://www.gov.uk/government/news/54800-customers-claim-tax-relief-for-working-from-home
Once the application has been approved, the online portal will adjust an individual’s tax code for the 2020 to 2021 tax year. The employee will receive the tax relief directly through their salary and will continue to receive the adjustment until March 2021.
Employees could alternatively receive the tax relief as a lump sum payment if they wait and apply once the 2020 to 2021 tax year has ended.