Mortgage lenders usually apply a formula which is based on a multiple of the incomes of the borrowers to determine the amount you can borrow. The aim is to make sure that you are not over stretched and can afford to keep up with repayments.
Typically lenders or brokers will lend a sum equivalent to income of either 3, 4 or 5 times the first and second income.
Less commonly, other much higher multipliers are used such as 5 + or more times the main income. These are known as High Multiplier Mortgages and are normally for people with reliably growing income, or where partners income is present but being ignored (e.g. because of a bad credit history, lack of accounts etc, but where you know that you can rely on it).