Fixed or variable mortgage? How it's actually decided

It’s one of those questions with the most noise and the least help. You search and find someone very convinced saying fixed, someone equally convinced saying variable, and both with charts.

Let’s start with the honest part: the bit everyone tries to answer is the bit nobody can. Nobody knows where rates will be in eight years. Not the person selling you the mortgage, not your brother-in-law, not an AI. If someone tells you with certainty, that itself is useful information: it tells you how much to trust them.

The good news is the decision doesn’t rest on that alone.

What you can actually decide

The useful question isn’t “what will rates do?” but “how much of a rise can I absorb without it changing my life?”. And that one has an answer, because it depends on you and not on the future.

Do the calculation before anything else:

• Your payment today on the variable deal you’re offered.
• Your payment if the rate rises three points. Not two, three. That’s a scenario that has already happened, not a fantasy.
• The monthly difference between that payment and the fixed deal you’re offered.

If the bad scenario leaves you with no margin — you can’t save, can’t take a holiday, any surprise knocks you over — the decision is made and it has nothing to do with rates: variable isn’t for you, even if it works out cheaper most years.

If the bad scenario squeezes you but you’d cope, then it is a money decision rather than a stability one, and other things start to count.

What almost nobody looks at, and weighs a lot

How long the mortgage runs. 30 years isn’t 12. The shorter it is, the less time interest has to hurt you and the more today’s payment matters.

Whether you’ll overpay. If you plan to make lump-sum payments, your exposure to rises is smaller than the table suggests.

Tied products. Insurance, current accounts, cards, plans. A great margin with three tied products can work out worse than an ordinary one with no strings. That sum is done on the total figure, not the headline.

Exit and early repayment charges. They determine whether this decision is for good or revisable in three years. That changes how much getting it right today matters.

The questions to take to the lender

• “What would my payment be at the current rate, and what if it rises three points?”
• “What does it cost me to exit or remortgage in three years?”
• “Which products are tied in and what do they add up to per year?”
• “How much interest do I pay in total under each option if I run it to the end?”

That last one gets dodged a lot, and it’s the one that puts both options into the same language.

What an AI can do here (and what it can’t)

It can: run the scenarios without slipping, explain what each clause of the offer means, and — most usefully — tell you what your reasoning is missing. Asking “what am I not taking into account?” on this decision usually surfaces one or two things you hadn’t looked at.

It can’t: tell you what rates will do. Ask it and a cautious AI will say it doesn’t know, while an incautious one will hand you a forecast written with confidence. This is where seeing several answers at once genuinely pays: the one that commits alone, when the others won’t, isn’t the best informed — it’s the least honest.

And there’s a line we don’t cross: if the question turns into personalised financial advice — what to do with your savings, whether to overpay or invest — our jury doesn’t rule, it refers. Choosing the mortgage type that fits your tolerance for risk is one thing; telling you where to put your money is another, and for that there are regulated professionals and a reason they’re regulated.

The summary

Nobody knows what rates will do. What you do know is how much you can absorb.

Start there, and half the argument disappears.