Why Do Mortgage Rates Change? A Guide to Base Rate, Swaps and Lender Pricing

Ask most people why mortgage rates change, and you'll get the same answer nearly every time: the Bank of England must have raised the base rate. It's the obvious guess, and it's often wrong. We see it happen regularly: a client's fixed rate quote changes even though nothing's been announced, and the first question is always some version of "how is that even possible?"

That gap between what people expect and what's actually going on is where most of the confusion lives. This guide is here to close it, not with today's numbers, which will be out of date by the time you read this, but with the mechanism underneath them. Understand that, and it'll make sense whatever the Bank of England decides next, this time or any time after.

The three ingredients in every mortgage rate

Every mortgage rate you're offered comes from the same basic formula: a swap rate, plus the lender's own margin, with the base rate sitting in the background shaping both.

The swap rate is the cost a lender incurs to secure funding for a fixed period, whether that's two years, five years, or longer. The margin is what the lender adds on top to cover its costs, manage risk and turn a profit. The base rate doesn't set either of these directly for a fixed mortgage, but it shapes the wider environment they're both priced against.

That's the whole picture in one sentence, and it's worth holding onto, because most of what follows is really just unpacking each part.

What the base rate actually controls

The Bank of England's Monetary Policy Committee meets roughly every six weeks to decide whether the base rate goes up, down or stays where it is, weighing up inflation, employment, the wider economy and global events each time.

Its influence is most direct on tracker and standard variable rate mortgages. If you're on one of these, a base rate change usually reaches your payments fairly quickly, sometimes within a month. Fixed rate mortgages work differently. Once you've locked in, your rate is fixed for the whole of your deal, so the base rate can rise or fall as much as it likes and your payments stay exactly the same until that deal ends. Fixed rates do change, but that only matters when you come to take out a new one; it's just that they're reacting to something other than the base rate, which we'll come to next.

Swap rates: the part most explainers skip

This is where the real answer to "why did my rate change" usually lives, and it's the bit that gets glossed over most often.

Swap rates aren't set by the Bank of England, or by any single body at all. They come from the money markets, where banks and financial institutions trade based on where they think interest rates are heading, and the resulting rate is what lenders use to price their fixed deals. 

A swap rate reflects what those markets expect will happen to interest rates over a set period. When a lender offers you a five-year fix, it's effectively agreeing to a fixed cost of funding for five years, and it protects itself from future rate swings by entering into a swap agreement based on that expectation.

Because swap rates are forward-looking, they can move well ahead of anything the Bank of England actually decides. So, if markets start pricing in a rate rise, swap rates can climb within minutes, and lenders will often reprice their fixed deals in response, sometimes before the next MPC meeting has even happened. 

That's exactly why fixed rates can jump within a month, even with no base rate announcement at all. The swap market has already moved, and lender pricing is simply catching up.

Why a two-year and five-year fix can move in opposite directions

Swap rates aren't a single number. There's a separate swap rate for each term, roughly matching the two-year, five-year and even longer deals lenders offer.

A two-year fix is priced off the two-year swap rate, and a five-year fix off the five-year swap rate, and those two figures reflect different expectations about the future. Markets might expect rates to stay elevated in the short term but ease off further out, or the reverse. When that happens, you can genuinely see two-year and five-year pricing head in different directions on the same day, from the same lender.

What to watch each time the MPC meets

Rather than fixating on whether the base rate goes up, down or holds, there's more value in watching a few other things around each decision, especially if you've got a decision of your own coming up.

The vote split among committee members shows how close the call was and how much appetite there might be for another move next time. The commentary and minutes that come with the decision often matter more than the headline number itself, because they shape what markets expect next, and expectations are what actually move swap rates. 

The days after a decision are usually when you'll see the real effect: swap rates settling into their new level and lenders adjusting their fixed pricing to match, sometimes within days of the announcement rather than on the day.

If you're a few months out from the end of a fixed-term deal, this is the window to pay attention to. It won't tell you exactly when to lock in; nobody can promise that, but it does explain why the weeks after a decision often matter more than the decision itself.

What this means for your mortgage type

Trackers move in step with the base rate almost by design. Your rate is set as the base rate plus a fixed margin, so when the base rate changes, your payment usually adjusts accordingly, often within a month.

Standard variable rate mortgages follow a looser version of the same pattern. There's no contractual link to the base rate as there is with a tracker, so your lender decides whether, when, and by how much to move its SVR. In practice it usually tracks the base rate reasonably closely, but it isn't obliged to, and SVR tends to sit well above other options anyway, which is one reason people move off it as soon as they can.

Fixed rate mortgages sit apart from all of this while your deal runs. The rate you locked in stays exactly where it is regardless of what the base rate or swap rates do in the meantime, for better or worse. The moment that changes is when your deal ends and you're pricing a new one; at that point, it's swap rates, not the base rate, doing most of the work behind what you're offered.

Why it helps to have someone watching this for you

Nobody has time to track swap rates day to day, and you shouldn't have to. It's not the kind of thing a single announcement or a quick search will tell you either; it takes ongoing attention, and that's exactly the sort of thing we think should sit with us rather than with you.

At Fowler Smith, that's part of what happens in the background from the moment you first get in touch to the day your mortgage completes. Your Mortgage Adviser is focused on finding the right route through the market for your situation, and your Client Account Executive is keeping you posted as things move, so a rate shift isn't something we believe you should find out about after the fact.

So, why do mortgage rates change?

Pull it all together, and the answer isn't one thing; it's three working at once. The base rate sets the backdrop and directly affects trackers and SVRs. Swap rates move on what markets expect the base rate to do next, and that's what actually drives your fixed rate pricing. And your lender's own margin on top of that swap rate can shift a little too, depending on their appetite for new business at any given moment.

None of those three sits still for long, which is why mortgage rates change even in weeks when nothing's been announced at all.

Frequently asked questions

Why do mortgage rates change even when the base rate stays the same?

Fixed rates are priced off swap rates, not the base rate directly. Swap rates move on what markets expect the base rate to do in future, so they can shift even in a month when the base rate itself doesn't move.

Why did my fixed rate change before my mortgage completed?

Lenders can and do reprice their ranges at any point before completion, usually in response to swap rate movements or their own funding costs, rather than a formal Bank of England decision.

How often does the Bank of England review the base rate?

The Monetary Policy Committee meets roughly eight times a year, or about every six weeks, to decide whether to raise, lower or hold the base rate.

Should I wait for a better rate before applying?

Timing the market perfectly isn't something anyone can promise, including us. It's worth speaking to an adviser about your specific circumstances rather than trying to guess where rates are headed.

Whenever the market moves, whether that's a base rate decision or a quieter shift in swap pricing, we're always happy to talk through what it actually means for your situation. Get in touch with the team.

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