How long does a mortgage in principle last?

Most mortgage in principle certificates last somewhere between 30 and 90 days, depending on the lender, though the exact window varies and some effectively expire in practice sooner if your details or the market change (Pepper Money, Tembo, unbiased.co.uk guidance, checked September 2026). It's not a fixed calendar date so much as a working assumption: once it lapses, or once the rates and criteria behind it move, the figure needs to be reissued using whatever is live on that day. If you're house-hunting for more than two or three months, which is common given how often UK chains stall, treat your original AIP as expired in practice even if the paperwork has not technically run out, and ask your broker to rerun it before you make an offer on a property, since the number you saw when you started may no longer be the number a lender will actually agree today.

Does a mortgage in principle lock in my interest rate?

No, a mortgage in principle doesn't lock in a rate or reserve any money for you, it's simply an estimate based on the rate and criteria available on the day you applied. Between February and September 2026 alone the average two-year fixed mortgage rate rose from 4.83% to 5.59% (Moneyfacts, via HomeOwners Alliance), exactly the kind of shift that can change your AIP figure without a single thing about your own finances changing. Locking an actual rate normally only happens once you have a property and make a formal mortgage application, sometimes for a fee, though a growing number of buyers now ask their broker about reserving a rate ahead of exchange specifically because rates have been this unpredictable. If keeping your monthly payment protected matters to you, ask whether your lender offers a rate reservation option once you're close to an offer, rather than assuming your AIP has already done that job.

Why has my mortgage in principle amount gone down?

Your AIP amount almost always falls because the interest rate a lender is using has risen since you first applied, not because anything about your income, deposit or credit history has got worse. A higher rate means a bigger share of the same monthly payment goes on interest rather than capital, so the maximum loan a lender will offer for that payment shrinks, sometimes by 7% to 9% over just a few months in a market like 2026's (Zoopla House Price Index, August 2026). It can also fall if a lender tightens its own affordability stress test, adjusts its income multiples, or if your outgoings have genuinely increased, for example through a new car finance agreement or a rise in existing debt repayments. The fix is the same either way: ask your broker to show you exactly which assumption changed, then decide whether to adjust your budget, your deposit or your target area rather than assuming the original figure will come back.

Do I need a new mortgage in principle if interest rates change?

Yes, if you're still actively searching and rates have moved meaningfully since your last AIP, it's worth getting it rerun, because the figure you're working from may no longer reflect what a lender would actually offer. There's no formal rule forcing this, which is exactly why so many buyers keep house-hunting against a stale number for months without realising it. As a working guide, refresh your AIP every four to six weeks during an active search, and always before you make an offer on a specific property, since finding out your real budget only after a seller has accepted your offer is the worst possible moment to discover a shortfall. Most brokers can rerun an AIP in a day or two with no cost and only a soft credit check, so there's little reason not to keep it current once rates are moving this much.

Your mortgage in principle already cost you 9% of your budget

You got your mortgage in principle back in February, printed it off, and treated the number at the top like a fact: this is what I can borrow. Since then you have been looking at flats and terraces within that figure, mentally subtracting Stamp Duty and solicitors' fees, picturing move-in day. Now your broker is asking you to redo it, and the number that comes back is smaller. Nothing about your job, your savings or your credit file has changed. The house-hunt has not moved. The mortgage market underneath it has.

The average two-year fixed mortgage rate has climbed from 4.83% at the end of February to 5.59% today (Moneyfacts, via HomeOwners Alliance, 27 February and 7 September 2026), and Zoopla puts the practical effect at a 9% cut in what the same monthly budget will actually borrow (Zoopla House Price Index, August 2026). A mortgage in principle was never a promise that number would still be true months later. It was a snapshot, and the market has moved on without telling you.

A mortgage in principle isn't a locked rate, it's a guess with an expiry date

A decision in principle, sometimes called an agreement in principle or AIP, is a lender's estimate of how much it might lend you, based on a soft credit check and the income, outgoings and rate available on the day you applied. It's not underwritten, it's not guaranteed, and it doesn't reserve a rate or a sum of money in your name. Most lenders treat an AIP as valid for somewhere between 30 and 90 days, depending on the provider (Pepper Money, Tembo, unbiased.co.uk guidance, checked September 2026), and once it lapses, or once your details or the market change, it has to be reissued from scratch.

That reissue uses today's rates and today's lending criteria, not the ones printed on the copy sitting in your desk drawer. If you have been house-hunting for four, five or six months, which isn't unusual given how often a chain stalls somewhere along the line, you're very likely relying on an AIP that quietly stopped reflecting reality weeks ago.

So if your AIP is more than a couple of months old, treat the figure on it as history rather than your actual budget, and ask your broker to rerun it before you make an offer, not after.

Why the number has actually fallen

The whole-market average two-year fixed mortgage rate stood at 4.83% on 27 February 2026, before mortgage pricing was pushed up sharply by the escalation of the conflict between the US and Iran that spring. By 7 September 2026 that average had climbed to 5.59% (Moneyfacts, via HomeOwners Alliance), a rise of 0.76 percentage points that has stuck, with Bank Rate itself unmoved at 3.75% since 30 July and the next Monetary Policy Committee decision not due until 17 September 2026 (Bank of England, checked September 2026).

Run the numbers on a fairly typical first-time buyer AIP for a £220,000 mortgage over 25 years. At 4.83%, the monthly repayment comes to roughly £1,264. Reissue that same AIP today at 5.59% and a lender working back from the same £1,264 monthly budget can only support a loan of about £204,000, some £15,900 (7%) less, for the exact same job, income and outgoings you had in February.

Zoopla's own analysis of the wider market found an even sharper effect once affordability stress-testing is factored in: a buyer who could borrow £200,000 for a given monthly payment at the start of the year can now borrow only around £182,000 for that same payment, a 9% cut in buying power (Zoopla House Price Index, August 2026). The gap isn't evenly spread either. A London buyer now needs to find an extra £35,500 in deposit to bridge it, against roughly £10,200 for a buyer in the North East (Zoopla House Price Index, August 2026).

So if you set your search filters to a number from February, you have probably been looking at homes that are no longer within reach on the mortgage you would actually be offered today, which wastes viewings and risks a fall-through at the worst possible moment.

The maths, in one place:

£220,000 AIP at 4.83% (27 February 2026): roughly £1,264/month over 25 years. Same £1,264/month reissued at 5.59% (7 September 2026): supports only about £204,000, a cut of roughly £15,900, or 7%. Zoopla's wider market estimate: a £200,000 January AIP now supports roughly £182,000 for the same payment, a 9% cut (Zoopla House Price Index, August 2026).

What actually happens when your AIP goes stale mid house-hunt

None of this means your original AIP was wrong when you got it in February. It means a mortgage in principle was only ever a live snapshot, the same way a 5% deposit mortgage or a reading from one of the credit score myths buyers chase only tells you something true about the day it was pulled. When you finally apply for a real mortgage against a specific property, the lender runs a full assessment, a hard credit search, payslips, bank statements, against whatever criteria and rates are live on that day, not the ones behind your original AIP. If the figure you have been house-hunting against was stale, you often only find out once you're under offer, sometimes with a completion date already agreed with the seller, which is the worst possible moment to discover a shortfall.

If you're still actively searching, refresh the AIP every four to six weeks rather than treating the original number as fixed, and ask your broker whether reserving or locking in a mortgage rate early, the way a rising number of homeowners are already doing, makes sense once you have found a property, so the number you offer against is one you can actually still get. Anyone still budgeting against an old Stamp Duty threshold from a previous cycle should run the same fresh check, since assumptions from months ago rarely survive contact with today's market unchanged.

So treat your AIP as a moving target rather than a fixed ceiling, and rerun it every few weeks you're actively searching, because the alternative is finding out the real number only after you're under offer.

What this means for you

If you got your mortgage in principle more than eight to ten weeks ago and you're still actively viewing property, the maths points toward getting it refreshed this week, not after you have found somewhere and made an offer. Most people who run this check discover their real budget has moved, usually down, and it's far cheaper to adjust your search now than to lose a deposit-worthy property, or a survey fee, to a mortgage that no longer clears at the number you were expecting. If you're within a few weeks of finding somewhere, it's also worth asking a whole-of-market broker whether you can reserve or lock a rate ahead of a formal offer, since that at least fixes one side of the equation while you're still searching. Frankly, an AIP was only ever meant to tell you roughly where you stood on the day you asked. Six months on, in a market that has moved this much, it's not telling you that any more.