say you make $5,000 a month and your only debt is your current mortgage which is $2,000 a month. Your debt to income ratio is 40%. You want to buy an investment property which will cost you an additional $2,000 a month for that mortgage, would bring your debt/income ratio to 80% which is pretty high and banks don't usually take that risk.
Now lets say that investment property will bring in $3,000 a month in rent, the bank may allow 75% of that to be used as income 3,000 * .75 = 2,250 that would bring your monthly income to $7,250 with your debt at $4,000 a month.
That would now put you at a debt/income ratio of 55% which is a little more reasonable.
Rental income can be used if seller has disclosed it on his/her tax returns for the past 2 years and has rental/lease agreements. Only 75% pf the total income from currently occupied units may be considered. However, this latter requirement utlizing income from occupied units only may vary from lender to lender. Good luck.