Each bank has its own criteria.
A common one for rental income is to discount the rent to account for possible gaps between tenancies, expenses, tax etc. So if the actual rental income is €1,000 per month, a bank might take €750 per month into their calculations.
In addition, many banks will "stress-test" the mortgage repayments on the rented property. So even if you're actually paying interest-only at 3%, the bank will calculate what the repayments on a capital-and-interest mortgage at 5% would be.