There’s more to providing proof of income than just handing over a couple of payslips. You need to be able to show your earnings are stable. When making a repayment, you may have to be able to show the source of that money as well. If you’re a recent graduate who’s landed a well-paying job, someone who recently switched careers or just started a business, you could have trouble qualifying for a conventional mortgage no matter how much you’re earning, unless you can show your earnings are stable.