When a person earns a different amount each month, lenders will average out the income over a period of time. This will always be over a minimum of 3 months, but then as a general rule the more variable the figure the longer the period they would use to assess. If you have been in the same job for over a year, they may also ask to see your previous P60. This is to help assess whether the average over a year is in line with your most recent three months earnings.