The 5 factors that dictate your home buying budget
Lenders use a strict math equation to determine your maximum purchase price. Learn how your income, debts, interest rates, down payment, and local taxes impact your borrowing power.
Jul 2, 2026 5 min read
Ask a mortgage lender how much house you can afford, and they will not ask about your lifestyle, your grocery budget, or how much you spend on summer vacations. They run a math equation based on a few specific numbers.
To figure out your maximum purchase price, conventional lenders usually apply the 28/43 rule. This standard establishes two separate debt-to-income, or DTI, limits. Your actual borrowing power is capped by whichever limit is stricter, combined with the realities of your local housing market. Here is exactly how lenders look at your finances and what actually moves the needle.
Income and the front-end limit
Your gross annual income is what you earn before taxes, retirement contributions, and payroll deductions. This number sets the absolute ceiling for your housing budget.
Under the 28% front-end rule, your total monthly housing payment cannot exceed 28% of your gross monthly income. In the mortgage business, this payment is called PITI: Principal, Interest, Taxes, and Insurance. It also includes any Homeowners Association, or HOA, fees.
If you earn $100,000 a year, your gross monthly income is $8,333. Multiply that by 0.28, and your maximum allowed housing payment is $2,333 per month. A higher income directly increases this cap. Get a raise to $110,000, and your front-end limit rises to $2,566 per month, giving you slightly more room to borrow.
Existing debts and the back-end limit
Income sets the ceiling, but your existing debts pull it down. That brings us to the 43% back-end rule. Lenders require that your new housing payment plus all your recurring monthly debts—car loans, student loans, minimum credit card payments, alimony, child support—do not exceed 43% of your gross income.
For a borrower making $100,000 a year, 43% of their monthly income is $3,583.
If they have zero debt, their housing limit is still capped by the 28% front-end rule at $2,333. But if they carry $1,500 in monthly auto and student loan payments, the math shifts:
$3,583 (total debt allowance) − $1,500 (existing debts) = $2,083 available for housing.
Because $2,083 is less than the front-end limit of $2,333, the back-end limit takes over as the binding constraint. In a scenario like this, every additional dollar of monthly debt reduces your maximum housing payment by exactly one dollar. Paying off a car loan before applying for a mortgage can significantly increase your buying power.
The cost of money: Interest rates
Interest rates translate your monthly payment into your total loan amount. The rate dictates how much of your PITI goes toward buying the actual house versus paying the bank for the privilege of borrowing.
When rates rise, borrowing becomes more expensive. A larger portion of your fixed monthly payment goes straight toward interest. Since your income locks your maximum monthly payment in place, a higher interest rate forces the principal loan amount down.
As a rough rule of thumb for a 30-year mortgage, a 1% increase in your interest rate decreases your total borrowing power by about 10%. If you can afford a $300,000 loan at a 6% rate, your buying power for the exact same monthly payment drops to roughly $270,000 if the rate rises to 7%. You are making the same payment every month, but buying less house.
Down payment
The cash you bring to the closing table affects affordability in two ways.
First, it is a one-to-one addition to your purchasing power. If the bank says you can afford a $250,000 loan based on your income and debts, and you have $50,000 in cash ready to go, your maximum home price is $300,000.
Second, if your down payment is less than 20% of the home price, conventional lenders typically require Private Mortgage Insurance, or PMI. PMI protects the lender in case you default on the loan, not you. It usually costs between 0.5% and 1.5% of the total loan amount annually. This insurance premium adds a new monthly expense to your PITI. That extra cost eats directly into your 28/43 limits and reduces the base loan amount you can qualify for.
Hidden monthly costs: Taxes, insurance, and HOA fees
People often focus so heavily on the principal and interest that they forget property taxes, homeowners insurance, and HOA fees are baked right into the DTI formula. These local costs vary wildly from town to town and have a massive impact on your final number.
Every dollar you pay toward taxes or an HOA is a dollar you cannot spend on the mortgage principal.
Say you have a 7% interest rate on a 30-year loan. Every $100 per month in HOA fees wipes out about $15,000 in mortgage borrowing power. If you compare two identical $300,000 houses, but one sits in a neighborhood with a $300 monthly HOA, you will need to qualify for a payment equivalent to a $345,000 home without an HOA.
Property taxes act the same way. A house in a county with a 2.5% property tax rate will be significantly harder to qualify for than a house of the exact same price in a county with a 0.8% tax rate. The higher tax burden simply consumes your allowable front-end DTI.
Summary: What moves the needle?
| Factor | Change | Impact on affordability |
|---|---|---|
| Gross Income | Increases | Goes up (raises 28% and 43% limits) |
| Monthly Debts | Increases | Goes down (lowers available back-end limit) |
| Down Payment | Increases | Goes up (adds direct cash, avoids PMI) |
| Interest Rate | Increases | Goes down (higher cost per dollar borrowed) |
| HOA and Taxes | Increases | Goes down (consumes more of your PITI limit) |
To find your exact limits, you need to run your own numbers through the formulas. You can find your maximum purchase price and see exactly which DTI rule is constraining your budget using the Home Affordability Calculator.