What Percent of Your Income Should Actually Go Towards a Mortgage?
A quick heads-up before we dive in: This article is strictly for general informational purposes and does not constitute financial, legal, or tax advice. Every situation is entirely unique, and bank lending policies change frequently. Before making any decisions about your property journey, it is highly recommended that you seek independent advice from a qualified financial adviser.
If you spend time reading personal finance guides or talking to previous generations of property owners, you will inevitably hear a traditional rule of thumb: “Never spend more than 30% of your gross income on your housing costs.”
It sounds like safe, sensible advice. But in today’s New Zealand property market, it doesn't really work anymore.
When you are trying to figure out if you can afford a house, old rules of thumb can be misleading. Here is a look at why the old maths fails, how to figure out what you can actually afford, and exactly how the bank will view your application.
Why the 30% Gross Rule Fails
"Gross income" is the headline number on your employment contract—the money you earn before income tax, KiwiSaver, or your student loan is deducted.
The problem with basing your budget on your gross income is simple: you can't pay a mortgage with pre-tax dollars.
For example, a couple earning $120,000 gross might think they can easily spend $36,000 a year on a mortgage under the 30% rule. But once standard PAYE tax, your KiwiSaver contribution, and a student loan are taken out, the actual take-home pay is significantly lower. Suddenly, that mortgage payment is taking up a much larger, far more stressful chunk of the cash that actually hits their bank account every week.
The New Reality: Looking at Your Net Cash Flow
Instead of looking at the imaginary gross number, modern financial planning focuses purely on your Net Income—the actual take-home pay that lands in your bank account.
So, what is a fair percentage of your net income to spend on a mortgage today?
While everyone's lifestyle is different, it is incredibly common for modern buyers to see their housing costs sit between 35% and 45% of their Net Income.
A higher percentage might be perfectly fine for a couple with no other debt and low living costs, whereas a lower percentage might be necessary if you have significant childcare costs or car loans to pay off.
How the Bank Actually Measures You
It is a common misconception that banks use simple percentages to approve or decline a loan. In reality, the bank doesn't look at percentages at all. They look at a simple concept: Money In vs. Money Out.
When the bank assesses your application, they do a strict calculation:
Your Income: They look at your stable, reliable take-home pay.
Your Fixed Debts: They subtract payments for car loans, personal loans, and credit cards (even if you pay the card off every month).
Your Living Costs: They subtract your everyday expenses (groceries, power, insurance, childcare).
The Stress Test: Finally, they calculate your proposed mortgage payment. However, to make sure you are safe, they don't use the current advertised interest rate. They "stress test" you on a higher simulated rate to ensure you could still afford the house if interest rates went up in the future.
If there is money left over after all of those deductions, the computer generally says "Approved."
Why Percentages Break Down at Higher Incomes
There is another major flaw with treating percentages as a golden rule: it completely ignores the actual dollar amount left over.
Let’s look at a household earning a high combined gross (before-tax) income of $300,000. After income tax, ACC, and KiwiSaver are deducted, their actual net (after-tax) take-home pay is roughly $205,000 a year.
If they apply the 40% rule to that net income, they are dedicating $82,000 a year to their mortgage. While that is a substantial mortgage payment, it still leaves them with $123,000 in hard, post-tax cash for their living costs.
Because basic living expenses (like groceries, power, and petrol) do not double just because your income does, a high-earning household might comfortably afford to spend 50% of their net income on a mortgage without feeling any financial stress whatsoever.
Conversely, just because the bank allows you to spend 40% or 50% of a high income on a mortgage, there is absolutely no rule saying you should. That same household might choose to spend only 20% on a mortgage because they prefer to invest in shares, travel, or simply don't want a massive house.
This is why percentages should only ever be a loose starting point. Your affordability isn't dictated by fractions; it is dictated by nominal dollars—the actual, physical cash left in your hand each week.
The "Sleep at Night" Test
While the bank's calculator determines what they are willing to lend you, the final test is personal. We call it the Sleep at Night Test.
Before you sign on the dotted line, you need to look at your actual take-home pay, subtract the real mortgage payment, and ask yourself: "Does the money left over allow me to live the lifestyle I actually want?"
If the answer is yes, you have found your perfect budget.
The Time Factor: Why the First Years Are Always the Hardest
When you first calculate your mortgage budget, the percentage of your income going straight to the bank can look terrifying. But there is a hidden, long-term bonus to homeownership that spreadsheets rarely show: inflation works in your favour when you have a mortgage.
Think about a client who bought their first home 20 years ago. Back then, they might have purchased a house for $250,000. At the time, with 2006 wages, taking on a $200,000 mortgage felt exactly as scary and financially stretching as taking on an $800,000 mortgage feels today. It likely consumed 40% of their take-home pay, and they probably lost a lot of sleep over it.
Fast forward to today. What happened?
Over those two decades, the cost of living went up, but more importantly, their wages went up. However, the original amount they borrowed from the bank stayed exactly the same.
Because your mortgage debt does not increase with inflation, it gets relatively "cheaper" to pay off over time. As you progress in your career and your income naturally rises over the next 10 to 20 years, a mortgage payment that took up 40% of your income in year one might only take up 20% of your income in year ten.
The day you move in is usually the tightest your budget will ever be. You don't necessarily have to work harder to make the debt easier to manage—you just have to let time and normal wage growth do the heavy lifting for you.
Interactive Affordability & Purchase Estimator
Use the tool below to simulate your weekly cash flow. Enter your incomes (gross or net) and we will calculate your approximate New Zealand take-home pay behind the scenes. This tool provides a rough baseline estimation and does not constitute an offer of finance.
What Percent of my NET income should go towards a mortgage?
Calculate your household take-home pay and see what your mortgage budget could buy based on your chosen % of NET income spending.
Person 1 Income
Person 2 Income (Optional)
Combined Household Net Income
Estimated Weekly Budget
Indicative Property Purchase
Don't Forget the Emergency Fund
Calculators are great, but they assume your life will look exactly the same for the next 30 years. When deciding on your perfect mortgage budget, it is crucial to leave a buffer for the unexpected.
Interest rates move, houses need maintenance (roofs leak and hot water cylinders break), and your personal circumstances will inevitably change. If your mortgage payments require 100% of your leftover cash just to keep the lights on, you leave yourself with no room to breathe. Always ensure your leftover net income leaves enough room to build a comfortable emergency fund.
The Bottom Line
The old "30% of your gross income" rule is somewhat outdated for todays New Zealand home buyers. You cannot pay a mortgage with pre-tax dollars, which is why looking at your actual, weekly take-home pay is the only metric that matters.
Aim for a mortgage payment that you can comfortably service while still having enough cash left over to actually enjoy the house you just bought.
Ready to Run Your Real Numbers?
Online calculators are a fantastic starting point, but they don't know which banks currently have the most generous stress-test rates, or which lenders are offering the best cash contributions to offset your costs.
If you want to know exactly what the banks will lend you based on your unique household income, let’s have a quick, no-obligation chat. We can run your numbers through the actual lender calculators and give you a precise framework for your house hunt.
Get your preapproval underway with a Home Loan Factory adviser
