You can declare your monthly income and expenses in the self declaration to see how much your monthly disposable income is. This is necessary to understand how much of your income can be allocated to monthly mortgage payments. A general rule of thumb is that no more than 40% of your income should be allocated to your mortgage payments. If your monthly mortgage payments exceed this, you can either pay back the mortgage over a longer period of time by decreasing the annual repayment or you can choose to take on more risk by taking a shorted fixed interest period. Banks can require such a self declaration.