Every affordability calculator gives you a number, and lenders treat that number as gospel. It's optimistic by design — and it ignores a whole category of costs that routinely blow the budget in year two.
Type your income into any mortgage affordability calculator and it hands back a confident number: this is the house you can afford. Lenders run a version of the same calculation and treat the result as a ceiling you're pre-approved to hit. The problem is that the number is optimistic by construction — it measures the narrow slice of homeownership cost that's easy to model, and it goes quiet on the parts that actually break budgets. Those parts don't show up on move-in day. They show up in year two.
This isn't an argument against buying, and it's not an argument against calculators — you should absolutely run one. It's an argument for knowing what the number leaves out before you anchor your life to it.
1. What the calculator actually measures
Almost every affordability tool models four things, and mortgage people have a tidy acronym for them: PITI. Principal, Interest, Taxes, Insurance. Add mortgage insurance when your deposit is under 20%, and that's the whole model:
"In relation to a mortgage, PITI is the sum of the monthly principal, interest, taxes, and insurance, the component costs that add up to the monthly mortgage payment in most mortgages."
— Wikipedia, "PITI" (CC BY-SA 4.0)
PITI is real and it's the biggest line item. But it's the cost of holding the loan, not the cost of owning the house. Those are different numbers, and the gap between them is where people get hurt.
2. The six costs that don't appear
None of the following show up in a standard affordability calculation, and all of them are predictable:
- Maintenance. The rule of thumb is 1–2% of the home's value per year. On a €300,000 house that's €3,000–€6,000 annually — call it €250–€500 a month that the calculator never mentioned.
- The first-year "discovery" repairs. The water heater that was fine at the inspection and dies in November. The roof detail nobody flagged. Buyers of older houses routinely spend several thousand in the first twelve months on things they didn't know they'd bought.
- Utilities. An older, larger house can run three to four times the heating and power cost of the flat you're leaving. This is a step change on move-in, not a gradual creep.
- Service charges and community fees. Where they apply, they rise faster than inflation and you don't control the vote.
- Furnishing the space you just doubled. Empty rooms have a way of demanding money.
- Moving and transaction friction. Paid once, but paid in the same month you're already stretched thinnest.
Stack maintenance and utilities alone and you've often added €400–€700 a month to the "affordable" payment — every month, for as long as you own the place.
3. The income the calculator believes
Affordability tools take your current income at face value. Lenders don't. If you're self-employed, on variable bonus, or commission-heavy, the underwriter applies an effective haircut — often averaging two or three years and discounting the good ones. A €70,000 headline income can be assessed closer to €50,000 for the purposes of what you'll actually be approved to borrow.
The trap runs the other way too. Someone with stable salaried income who's approved for the maximum has no margin left for the year-two costs above. Being approved for a number and being able to live on what's left after that number are not the same test — and only one of them is on the calculator.
4. Building a corrected number
You don't need a spreadsheet model. Take the calculator's output and adjust it:
- Start with the affordability figure from the mortgage affordability calculator.
- Subtract a real maintenance reserve — 1.5% of the target price, divided by twelve.
- Add 20% to whatever utility estimate you're carrying from your current place.
- Cap the housing payment at roughly 28% of a three-year average of your income, not last year's best number.
What comes out the other side is smaller than the headline figure, and it's the number worth trusting. The gotcha most first-time buyers hit: they treat the maximum they're approved for as the target rather than the ceiling. The approval is what a lender is willing to risk on you. The corrected number is what you can actually live inside.
5. When renting is the rational choice
Buying isn't automatically the smarter financial move, and the price-to-rent ratio is the fastest way to see it. Divide the purchase price by a year's rent for a comparable property. A ratio above roughly 20 means you're paying a heavy premium to own — and it can take seven or more years just to break even against renting and investing the difference, before a single repair bill lands.
That break-even is exactly the calculation the rent vs buy tool exists to make concrete: it weighs the deposit, the ongoing costs, and how long you plan to stay against the alternative of renting. In a high price-to-rent market with a short expected stay, the math frequently favours renting — and the affordability calculator will never tell you that, because it only knows how to price a mortgage.
Clear the runway first
One move improves affordability more reliably than shopping for a lower rate: clearing high-interest debt before you apply. Every euro of monthly debt payment reduces what you can borrow and eats the margin you'll need for the year-two costs. Model the payoff order with the debt payoff planner before you get anywhere near a mortgage application.
The affordability number is a starting point, not a verdict. Run the calculator — then correct it for the costs it was never built to see. The buyers who do that are the ones still comfortable in year two.
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