DTI ratios are usually pretty generous, but if you can't qualify without the rental income, you may need to buy a 2-4 unit building and rent the other units to friends. This can be done with an FHA insured loan. It may be possible to count rental income on a 1 unit property under some programs, I am not fmailiar with all the rules. I don't think you can with an FHA loan, though.

AFAIK the rental amounts count for 75 cents on the dollar against the mortgage total, and you need to either have experience being a landlord or have a signed letter of intent to show the underwriter.

What? That's not how the qualification works. If it's no bigger than a 4-unit building and is going to be your primary residence, you can still qualify for a FHA loan at 3.5% down.

My bank required proof of income sufficient to qualify for the mortgage -- presumably if part of that proof was "4 of my friends promised to pay me $1000/month each to rent a room in the house", that would no longer qualify for a regular homeowner mortgage program.