My landlord is willing to sell me his house and cover the $200,000-$300,000 shortfall in my qualifying ability for three to five years -- how would that work?

What your landlord is describing is seller financing, usually documented as a second loan behind your mortgage, and the paperwork details decide whether it works. Before anything else, get the terms in writing: is he charging interest, what rate applies to that $200,000 to $300,000 balance, and what happens if you cannot pay it off at the end of the three-to-five-year window. Those details are the whole deal, and leaving them vague is where these arrangements go wrong. How a lender treats it is the deciding factor. Fannie Mae, FHA, VA, and USDA all permit seller financing behind their first mortgages, and the payment on the seller-carried balance gets counted in your debt-to-income. The lender cannot pretend an extra $200,000 to $300,000 of debt does not exist. Even if your landlord wants zero interest and no payments for a few years with a balloon due later, the lender still calculates a realistic payment on that balance and counts it against you. Each program has its own subordinate-financing rules on rate, term, balloons, and combined loan-to-value, so the structure has to satisfy the specific program you use, and your lender should confirm those rules up front. All of this traces back to ability-to-repay requirements, which exist to keep you from being loaded into a home you cannot actually carry. The one way around an institutional lender's guidelines is if your landlord carries the entire purchase himself, with no bank first mortgage involved. Then there are no outside rules to satisfy, and the two of you can agree to whatever terms you both accept. If you want, we can walk through which structure your numbers actually support in a free Roadmap conversation.