The mortgage lender considers your debt-to-income ratio, which is a comparison of your gross (pre-tax) income to housing and non-housing expenses. Non-housing expenses include such long-term debts as car or student loan payments, alimony, or child support.
According to the FHA, monthly mortgage payments should be no more than 29% of gross income, while the mortgage payment, combined with non-housing expenses, 4 should total no more than 41% of income.
The lender also considers cash available for down payment and closing costs, credit history, etc. when determining your maximum loan amount.