Understanding your employees' tax codes
Updating for the new tax year
HM Revenue and Customs (HMRC) will tell you between January and March about any new tax codes to use for employees in the new tax year. This starts on 6 April.
If an employee’s tax code is not changing, HMRC will not contact you and you should carry forward the employee’s tax code to the new tax year.
If your employee’s tax code ends with ‘M1’ or ‘W1’ (‘month 1’ or ‘week 1’), do not carry this part of the code into the new tax year.