Complete Home Purchasing Power & Underwriting Guide
Master Fannie Mae 28/36 DTI rules, gross monthly income calculations, recurring debt impacts, and maximum mortgage borrowing limits.
1Anatomy of Mortgage Underwriting & DTI Ratios
Mortgage underwriting uses your Debt-to-Income (DTI) ratio to establish the absolute maximum home purchase price you can qualify for. DTI Ratio Formula: DTI = (Total Monthly Debt Obligations / Gross Monthly Income) × 100 Lenders split DTI into two critical benchmarks: • Front-End DTI Ratio (Housing Cap): Caps total monthly PITI expenses at no more than 28% of gross household income. • Back-End DTI Ratio (Total Debt Cap): Caps total combined monthly debt payments (housing PITI + student loans + car payments + credit cards) at no more than 36% to 43% of gross income.
- •Gross Monthly Income is calculated BEFORE tax withholding and payroll deductions
- •Every $100/month increase in recurring debt reduces home purchasing power by ~$15,000
- •FHA underwriting permits Back-End DTI ratios up to 43% - 50% for qualified applicants
2Down Payments & Equity Ceilings
Your available down payment cash directly elevates your total home price ceiling. Adding cash down increases your home purchasing power dollar-for-dollar above your maximum loan amount, while simultaneously lowering your Loan-to-Value (LTV) ratio below 80% to eliminate mandatory PMI insurance.
- •Putting 20% down avoids PMI premiums, freeing up monthly cash flow to qualify for a larger loan balance
- •Reserve Funds: Lenders require 2 to 6 months of cash reserves remaining after closing costs