As soon as you take anyone on where any of the following conditions apply, you need to register with HMRC as an employee.
- the employee already has another job
- the employee receives a pension (state or occupational)
- the employee will earn at or over the PAYE (tax) threshold
- the employee will earn at or over the lower earnings threshold for National Insurance
- you are providing the employee with employee benefits.
You can register up to 4 weeks in advance.
If you are the sole employee of your own limited company and are paying yourself a wage/salary where the above conditions apply, you will need to register as an employee. In this case, you would be both an employer and an employee!