Ask a lender to show you the same loan two ways and the monthly repayment can look completely different. The repayment type is usually what changes the number: principal and interest, or interest-only. One repayment is doing more work than the other, and it is worth knowing exactly which.
What principal and interest repayments do
A principal and interest loan, often shortened to P&I, is the default most people picture when they think about a mortgage. Every repayment is split into two parts: a portion covers the interest charged for that period, and the rest goes toward reducing the amount you actually owe, the principal. Because the balance shrinks a little with every payment, the interest charged on the next payment is calculated on a slightly smaller number. Over the life of the loan, the interest share of each repayment gets smaller and the principal share gets larger, even though the total repayment usually stays roughly the same under a fixed schedule. The upshot is straightforward: every P&I repayment moves you closer to owning the place outright.
What interest-only means, and what happens when it ends
An interest-only loan, or IO, changes what each repayment is for. During the interest-only period, your repayment covers only the interest charged on the loan, and none of it reduces the amount you originally borrowed. If you borrowed a certain sum, you'll still owe that same sum at the end of the interest-only period, assuming you haven't made extra payments along the way. The appeal is obvious: with nothing going toward principal, the repayment is lower than an equivalent P&I repayment on the same balance and rate, and for someone managing tight cash flow that lower figure can bring real relief. What's easy to lose sight of, month to month, is that the debt itself isn't moving.
Interest-only arrangements are not permanent. Lenders offer them for a set period, agreed at the start of the loan, and when that period ends the loan reverts to principal and interest repayments, calculated over whatever term remains. Because the full original balance still has to be paid off in a shorter remaining time than if you'd been making P&I repayments from the start, the new repayment lands higher than a standard P&I repayment would have on the same loan. Some borrowers experience this as a genuine shock: they've budgeted around the interest-only figure for years, and the switch-over repayment arrives well above what they were paying, sometimes only as their income or circumstances have shifted too. Planning for that transition from the start of the loan is worth far more than discovering it when the letter from your lender turns up. If a further interest-only period is something you want, you generally need to apply for it again, and approval depends on the lender's assessment of your situation at the time, not on the fact that you had one before.
Who interest-only can suit, and what it costs over the life of the loan
Interest-only isn't a mistake for everyone who uses it. It tends to suit situations where the lower repayment is doing a specific job: someone managing a temporary drop in income, a renovation where cash is tied up elsewhere for a period, or a borrower who wants to direct spare money toward a higher-priority debt or an offset account rather than paying down this particular loan faster. Investors sometimes choose interest-only for a related reason, keeping more cash on hand to cover other costs tied to the property rather than accelerating repayment on this loan. How interest is treated for tax purposes is a separate question worth raising with a professional, and it sits outside how the loan itself is structured. What interest-only doesn't suit is treating the lower repayment as extra spending money with no plan for what happens once the period ends.
Because an interest-only loan leaves the principal untouched for a stretch, you keep paying interest on the full original balance for longer than you would under P&I, where the balance starts shrinking from the very first repayment. The practical result is that an interest-only loan usually costs more in total interest over its life than an equivalent P&I loan, even once the rate and the loan amount are identical. That extra cost is the trade-off for the lower repayments during the interest-only period, and it's worth understanding upfront rather than working out after the fact.
How to decide
If you're weighing the two, ask your lender to show you both the interest-only repayment and what it becomes once it reverts to principal and interest, using your actual numbers rather than a general example. Work out whether you could comfortably manage that higher figure if it started tomorrow rather than in several years. And if the reason you're considering interest-only is to free up cash for something specific, be honest with yourself about whether that plan is real or only a way of putting off a harder conversation about what you can afford. The lower repayment is real, and so is the debt still sitting underneath it once the period runs out.