P11D returns are due to be submitted by 6 July 2018. These are for the income tax year ended 5 April 2018.
If an employee was given any benefits in kind during the income tax year, then a P11D return needs to be filed. Examples of benefits in kind are a company car or private medical insurance.
In addition, if an employee has been reimbursed for any expenses, then this also needs to be reported on the P11D return.