What is the $400 claim?

CRA allows all employees, who worked from home because of COVID-19 in 2020, to claim up to $400 in employment expenses as a flat rate. This amount is a tax deduction and not a credit, which means you deduct it from your income to reduce your tax liability but will not result in a refund.

Temporary flat rate method

• This method simplifies your claim for home office expenses (work-space-in-the-home expenses and office supply and phone expenses).
• If you worked more than 50% of the time from home for a period of at least four consecutive weeks in 2020 due to the COVID-19 pandemic, you can claim $2 for each day you worked from home during that period.
• You can then also claim any additional days you worked at home in 2020 due to the COVID-19 pandemic.
• The maximum amount that can be claimed is $400 per individual. This method can only be used for the 2020 tax year.
• Claim the amount on line 22900 of your tax return.

Source: https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-return/completing-a-tax-return/deductions-credits-expenses/line-229-other-employment-expenses/work-space-home-expenses/how-claim.html