Lenders need to know what your outgoings are as well as your income - including elements such as costs of living, utilities and existing debt repayments.
If you have debts, the lender will usually look at how much you owe in total and the monthly repayment costs.
For example, if you pay £250 per month against a loan, that means an expense of £3,000 per annum. This figure is deducted from your annual income to calculate the maximum mortgage offered to you.
Some other outgoings will be included by some lenders but ignored by others, such as:
Pension contributions
Insurance costs
School fees
Charity donations
After assessing your income and outgoings, a lender will also stress test the application to check that you'd still be able to keep up with the repayments if your circumstances changed, or interest rates increased.