@Christine Smith
Your investment property purchase will most likely use a conventional loan. FHA is only for owner occupants, not for investment properties. You finance the house the way you financed your primary residence. You find a lender (probably your local bank) to write a loan for 80% of the purchase price. At the settlement table you bring your downpayment funds and whatever extra is needed for closing costss.
The lender will most likely not use projected rental income to qualify you for the loan. All of your current debt obligations (mortgage payement, credit card payments, car loan payments, HOA dues, and other monthly expenses) plus the PITI for the investment property will probably need to be no more than 36% of your gross income though some lenders may allow a higher percentage determined by the investor who will purchase your loan from the bank.
Rental property that you already own under annual lease but not yet reported on two years tax returns, can contribute 75% of gross rent toward your income when computing your debt service to income (DTI) ratio. If the rental property has been reported on your tax returns, the net rental income from your recent tax return (or the average rental income from the two previous tax returns) will be used instead of the 75% estimate.
Just out of curiosity, what is a "back" house. When I was growning up, in some rural areas, the house out back was called the outhouse. Not familiar with a back house.