1. Quick Summary
A mortgage is a loan used to buy property, where the property itself is collateral. If you stop paying, the lender can take it through foreclosure.
Repayment is structured as amortisation: a fixed monthly payment that gradually shifts from mostly interest to mostly principal over the term.
2. What It Means
Each payment covers the interest accrued on the outstanding balance plus a slice of principal. Early on the balance is large, so interest is large and principal reduction is slow.
Because the payment is fixed but the interest portion shrinks over time, the principal portion grows — this is what makes the balance fall slowly at first and quickly near the end.
Fixed-rate loans lock the rate for a term; variable-rate loans pass market changes through to the payment.
3. Why It Happens
Interest rates reflect the lender’s cost of funds, expected inflation, credit risk and the term length, which is why long-term rates are usually higher than short-term ones.
A down payment reduces risk for the lender, which is why smaller down payments usually require mortgage insurance.
Total cost is dominated by two variables: the interest rate and the term. Extending the term lowers the monthly payment but greatly increases total interest paid.
Because compounding applies to the outstanding balance, even a one-percentage-point rate change can add tens of thousands to the total cost of a long loan.
Escrow or bundled payments often add property tax and insurance to the monthly amount, which is why the actual payment exceeds principal and interest alone.
Equity — the value of the property minus what you owe — is what you actually own, and it builds slowly at first for exactly the same amortisation reason.
4. Real Examples
On a thirty-year loan, the balance is often still more than half the original amount after fifteen years, which surprises many borrowers.
Making one extra payment per year can cut years off the term because the extra money goes entirely to principal.
In a variable-rate mortgage, a rate rise increases the interest share of a fixed payment, which can even cause negative amortisation if payments are capped.
Refinancing makes sense when the new rate is low enough that the savings over your expected stay exceed the closing costs.
5. How It Affects Us
Mortgage availability and cost largely determine who can buy a home, which makes it central to household wealth formation.
Because housing debt is so large, changes in mortgage rates are one of the main ways central bank policy reaches ordinary households.
Household leverage also matters for financial stability: widespread mortgage defaults were the core mechanism of the 2008 crisis.
6. Key Takeaways
- A mortgage is a secured loan repaid by amortisation, with property as collateral.
- Early payments are mostly interest; principal payoff accelerates late in the term.
- Rate and term dominate total cost far more than the purchase price negotiations most people focus on.
- Equity builds slowly at first because of how amortisation works.