How Much House Can You Afford, Beyond What a Lender Will Approve

There are two different questions hiding inside "how much house can I afford", and conflating them is how people end up owning something that quietly consumes their life. The first is what a lender will approve. The second is what you can pay every month for a decade without abandoning everything else you want to do.

The first question has a formula. Underwriting is a rule-based process, and you can reproduce it on paper. The second question has no formula, because it depends on how secure your income is, what else you are funding, how much repair risk you can absorb, and how long you expect to stay.

This guide works through both, using explicitly illustrative rates and rules throughout. Nothing here is a current market rate, and mortgage pricing changes constantly; the point is the arithmetic, which does not.

#What a lender approves is not an affordability opinion

A mortgage underwriter is assessing the probability that you default, not whether the payment leaves you room to save, replace a car, or take a year of reduced income. Those are different questions with different answers, and the lender's answer is almost always the larger number.

Three things systematically make an approval amount look generous. The lender uses your gross income, before tax and before retirement contributions. It counts only the debts that appear on your credit report, so childcare, tuition, medical plans and support payments are invisible to it. And it excludes the maintenance reserve entirely, because a roof failing in year six is not the lender's exposure -- the loan is secured on the property regardless.

The result is a number that is arithmetically correct and behaviorally misleading. Treat it as a ceiling, not a target.

#The 28/36 ratios and how debt-to-income is calculated

The two traditional ratios are a front-end and a back-end test, both computed on gross monthly income.

Front-end ratio (28%). Your monthly housing payment -- principal, interest, property tax, homeowners insurance, mortgage insurance and any homeowners association dues -- divided by gross monthly income.

Back-end ratio (36%). Housing payment plus all other monthly debt obligations, divided by gross monthly income. Other debts means minimum credit card payments, auto loans, student loans, personal loans, and court-ordered payments such as child support or alimony.

Actual underwriting guidelines vary by loan program and frequently allow back-end ratios above 36% when compensating factors exist, such as substantial cash reserves or a strong credit profile. But 28/36 is a reasonable conservative frame, and the arithmetic is identical whatever the limits are.

Worked example: Gross income of $96,000 a year is $8,000 a month. The front-end test allows 0.28 x 8,000 = $2,240 of housing payment. Existing debts are a $410 car payment, a $185 student loan payment and a $60 credit card minimum, totalling $655. The back-end test allows 0.36 x 8,000 = $2,880 for everything, so housing can be 2,880 - 655 = $2,225. The binding constraint is the back-end test at $2,225, which is $15 below the front-end figure.

That $15 gap illustrates something worth noticing. Every dollar of non-housing debt reduces your borrowing capacity by roughly $2.78 of monthly housing payment capacity once the back-end test binds -- and at illustrative rates, roughly $140 of purchase price per dollar of monthly debt. Clearing a $410 car payment before applying moves the ceiling by tens of thousands of dollars.

#Why lenders use gross and why you must budget with net

Gross income is the number in your offer letter. Net income is the number that reaches your account after federal withholding, Social Security and Medicare tax, state and local tax, health premiums and retirement contributions. For many households that gap is 25% to 35%.

A payment that is 28% of gross can easily be 38% of net. If that is not obvious from your own pay stub, the mechanics are set out in a line-by-line decode of a pay stub -- the two taxable wage figures and the pre-tax deduction ordering explain most of the difference.

The practical translation is simple: run the lender's ratio to find the ceiling, then rerun the same payment against your take-home pay to see what it actually feels like. Many people find that a housing payment above roughly a third of net pay starts crowding out saving rather than merely feeling tight.

#The full monthly cost, including the parts nobody quotes

Mortgage advertising quotes principal and interest. Nobody lives in principal and interest. Here is a complete monthly cost for a $300,000 purchase with 10% down at an illustrative 6.5% fixed rate over 30 years, with property tax assumed at 1.1% of value, insurance at $1,400 a year and mortgage insurance at 0.50% of the loan.

The gap between the two bold rows is $370 a month, or $4,440 a year. That is the amount most affordability calculations omit, and it is very close to the amount by which people find themselves short.

The maintenance reserve deserves defending, because it is the line people delete first. A common planning heuristic is 1% to 2% of property value per year, weighted toward the higher end for older properties. It does not mean spending $3,000 every year; it means that a $9,000 roof in year eight, an $8,000 HVAC system in year twelve and a $6,000 sewer line in year fifteen average out to something close to that figure. Holding it in a dedicated account, in the manner of a sinking fund, is what converts a household emergency into a scheduled expense.

The calculator below runs this arithmetic for any numbers you enter, including the ratio tests and the full monthly cost.

#The price ceiling at different incomes and debt loads

Using the same assumptions -- 10% down, an illustrative 6.5% rate over 30 years, 1.1% property tax, $1,400 insurance and 0.50% mortgage insurance -- the table below shows where the 28/36 tests land at several combinations. The binding test is noted, because it changes as debt rises.

Two patterns stand out. Below roughly $750 of other monthly debt at these income levels, the front-end test binds and additional debt costs nothing in borrowing capacity. Above that, every dollar of monthly debt removes roughly $140 of purchase price. And the whole table shifts materially with the rate assumption, which is why a pre-approval issued three months ago may no longer describe reality.

#The cash required to close

The down payment is the visible number and rarely the largest obstacle. On the $300,000 example:

• Down payment at 10%: $30,000
• Closing costs at an assumed 3% of the loan: $8,100 (origination, appraisal, title, recording, credit report, survey)
• Prepaid items and escrow funding: $3,000 (several months of tax and insurance held in advance, plus prepaid interest to month end)
• Moving, immediate repairs and essential furnishing: $4,000

That is $45,100, not $30,000. And it should sit alongside, not instead of, the emergency fund you already hold. Closing with an empty reserve account is one of the most reliable predictors of credit card debt in the first year of ownership, because early repairs cluster.

Seller concessions and lender credits can reduce the closing cost line, and lender credits usually cost you a higher rate. Down payment assistance programs exist in many states and localities with their own eligibility rules. The strategy for accumulating the cash is a separate exercise, covered in saving for a down payment.

#How the down payment interacts with mortgage insurance and rate

Private mortgage insurance is protection for the lender against your default, paid by you, and it is generally required on conventional loans when the down payment is below 20%. Its cost varies with loan-to-value ratio and credit profile.

Under the Homeowners Protection Act, a borrower can generally request cancellation of borrower-paid PMI once the principal balance reaches 80% of the original value, and the servicer must terminate it automatically at 78% if payments are current. Some government-backed loan programs carry mortgage insurance for the life of the loan regardless of equity, which is a materially different structure and worth confirming before choosing a program.

Down payment size also affects the interest rate through loan-level pricing, and credit profile affects it more. A meaningfully better credit position can be worth more than an extra few percentage points of down payment, which is why improving a credit profile before applying often has a larger effect than most borrowers expect. When comparing offers, the APR captures rate plus most financing costs, and what APR actually includes explains why two loans with the same rate can differ.

#What term and rate do to the payment and the total

Rate changes the payment. Term changes the total far more than most people expect. The table below uses the same $270,000 loan.

Moving from 30 years to 15 at the same rate raises the payment by $645 a month and reduces total interest by $190,965. Moving from 5.5% to 7.5% on a 30-year loan raises the payment by $355 and adds $127,728 of interest over the term.

The trade-off is not purely arithmetic. A 30-year loan with voluntary extra payments produces a similar outcome to a 15-year loan while retaining the option to stop, which has real value if income is variable. A 15-year loan enforces the discipline and usually prices slightly better, but the higher required payment is permanent.

#The opportunity cost of the down payment

Money used for a down payment is money not doing something else, and that cost is genuine even though nobody sends an invoice for it.

Worked example: Suppose $30,000 is used as a down payment, and the alternative was an investment returning an assumed 5% a year. After seven years that $30,000 would have become 30,000 x 1.05^7 = $42,213, a difference of $12,213. Against that, the down payment reduces the loan by $30,000, saving roughly $1,950 a year of interest at an illustrative 6.5% early in the loan, and it removes the PMI requirement at 20%. Over seven years the interest saving alone is in the same order of magnitude, which is why the comparison is genuinely close rather than obvious.

The conclusion most people reach is that the down payment decision is not primarily about return. It is about whether a larger payment or a larger cash reserve is more valuable to you, and about whether removing PMI is achievable at the threshold.

#Transaction costs and the length-of-stay question

Buying and selling a property is expensive in a way that renting is not. Purchase closing costs commonly run 2% to 5% of the loan. Selling costs -- agent commissions, transfer taxes, title work, and any concessions to the buyer -- commonly run 6% to 9% of the sale price. The round trip is therefore frequently 9% to 13% of the property value.

Worked example: Buy at $300,000 with 4% purchase costs, so $312,000 is invested. Assume 3% annual appreciation and 8% selling costs. After n years the net proceeds are 0.92 x 300,000 x 1.03^n. Setting that equal to $312,000 gives 1.03^n = 1.1304, so n = ln(1.1304) / ln(1.03) = 4.15 years. Below roughly four years, the transaction alone puts you behind, before considering whether the monthly cost exceeded rent.

That calculation is why expected length of stay is the single most decisive variable in the rent-versus-buy question. Under about five years, the arithmetic usually favors renting unless the monthly cost of owning is well below the cost of renting an equivalent property, or appreciation is unusually strong -- and appreciation is exactly the variable nobody can promise.

#The specific ways people end up house-poor

The failure mode is rarely a single bad decision. It is a sequence of individually defensible ones.

• Buying at the pre-approval number. The approval is a ceiling computed on gross income with an incomplete debt picture. Treating it as a target guarantees the payment consumes the maximum the formula allows.
• Omitting the maintenance reserve. Roughly $250 a month on a $300,000 property. Skipping it works until the first significant failure, which then goes on a credit card at a much higher rate.
• Emptying the emergency fund at closing. Early ownership generates unplanned costs precisely when the reserve is at its lowest.
• Two incomes, no margin. A payment that requires both incomes converts any job loss, illness or parental leave into an immediate crisis.
• Ignoring escalation. Property tax assessments and insurance premiums rise. A payment that is comfortable at 33% of net can drift toward 40% within a few years with no change in the loan.
• Buying near a likely move. Job changes, relationship changes and school decisions inside the four-year transaction-cost window are expensive.
• Furnishing on credit. A larger property invites $10,000 to $20,000 of furniture and equipment, often financed, which then loads the back-end ratio for years.
• Counting on refinancing. Refinancing depends on future rates, future appraisal values and future qualification. It is an option, not a plan.

#What the usual advice gets wrong

"Buy the most house you can qualify for." Underwriting measures default probability, not quality of life. The lender is indifferent to whether you save for retirement.

"Renting is throwing money away." Renting buys shelter and optionality, and it avoids property tax, insurance, maintenance, and a round trip that costs around a tenth of the property value. Owning also has a substantial non-equity component: interest, tax and insurance are all consumed, not saved.

"Always put 20% down." Twenty percent removes PMI and lowers the payment, but it also delays purchase by years and depletes liquid reserves. The variable that decides it is whether reaching 20% costs you more in rent and lost time than the PMI would have cost.

"A 15-year mortgage is always better." It saves large amounts of interest and permanently raises the required payment. For a household with variable income, a 30-year loan with voluntary overpayments achieves much of the benefit while retaining flexibility.

"Your payment is fixed, so housing costs are locked." Principal and interest are fixed on a fixed-rate loan. Property tax, insurance, dues and repairs are not, and they compound over time.

"You will build equity automatically." Early payments on a 30-year loan are mostly interest. On the $270,000 example at 6.5%, the first payment applies about $244 to principal and about $1,463 to interest. Equity builds slowly at first and accelerates later.

The honest summary is that affordability is a household budgeting question wearing a lending costume. Once you have the lender's ceiling, the useful work is running the true monthly cost against take-home pay inside the plan you already use, which is what a working budget is for.