Fixed-rate mortgages
Porting your mortgage: the rules, and the catch
Porting sounds like the easy option: keep your rate, avoid the early repayment charge, just carry the mortgage over to the new place. What it actually is is a full new mortgage application, assessed against today's rules, that happens to keep yesterday's interest rate if it succeeds. Knowing the difference before you make an offer on a house is what keeps this from going wrong.
What porting actually means
When you port, your existing mortgage is redeemed on the day you sell your old property and a new one is issued on the day you complete on the new one, usually the same day. You are not transferring a loan; you are taking out a new one that happens to carry your old rate and term. That distinction matters because a new mortgage means a new underwriting decision, made against the lender's current criteria, not the criteria that applied when you first borrowed.
Most residential mortgages are portable in principle. Almost none of that is unconditional. The lender has to agree to the new property, and you have to pass their affordability check as it stands today, not as it stood when your fix started.
The affordability re-check that catches people out
This is the step people assume is a formality and treat as one right up until it isn't. Your income might be the same as it was two years ago, but the lender's income multiples, stress rates, and debt rules may not be. A change that looks trivial to you, a new car on finance, a drop in overtime, a switch from employed to self-employed income, can be enough to fail an assessment that would have sailed through under your original deal.
The lender also revalues the new property and checks it against their own lending policy. A flat above a shop, a non-standard construction, a leasehold with a short remaining term, any of these can be acceptable to one lender and not another, regardless of what they lent you against your current home. Fail either check and the port is refused; you keep the option to remortgage elsewhere, but the rate and the no-ERC benefit go with it.
If the new home costs more: the top-up
Move to a more expensive property and you will usually need to borrow more than your existing balance. Lenders handle this by keeping your ported amount at your original rate and adding a second sub-account for the extra borrowing, priced at whatever their current rate is for a new customer.
Say your existing balance is £220,000 at 4.2%, fixed, with 20 months left to run, and the new property costs £50,000 more than your current one. You port the £220,000 at 4.2% and take a top-up of £50,000 at, say, 5.63%, roughly the average two-year fixed rate on the market in August 2026. Your blended rate across the full £270,000 works out at about 4.46%, a small step up from your original rate but well below taking the whole £270,000 out fresh at 5.63%. The saving comes entirely from the £220,000 portion keeping its old price; the top-up prices at today's market regardless of how good your original deal was.
If the new home costs less: the ERC trap
Downsizing has its own catch. If you borrow less than your ported balance, most lenders treat the difference as a partial early repayment and charge the early repayment charge on the amount by which you've reduced the loan, even though the mortgage as a whole is porting. A £220,000 mortgage moving to a £180,000 loan on a cheaper property can mean an ERC on that £40,000 gap, which for a 2% charge is £800, small against a full-balance charge but easy to miss if you assumed porting meant no charge at all.
The timing trap
Porting depends on your sale and purchase completing close enough together that the lender can redeem and reissue in one move, and most lenders want this on the same day. Chains rarely cooperate on command. If your purchase completes before your sale, or the gap between them stretches past what your lender allows, you may need short-term bridging or the port falls through and you are remortgaging cold. Most lenders give you a window, often three to six months, to complete the new purchase and reclaim any ERC you were charged on redemption, so a short gap is usually recoverable even if the two dates don't land together. Check your specific lender's window before you rely on it.
What to actually do
Before you make an offer, ask your existing lender for an agreement in principle on the port, not just confirmation that porting exists as an option on your mortgage. That tells you whether you would currently pass the affordability check, before you're committed to a chain. If you're borrowing more, get a rate quote for the top-up portion so you know your blended cost, not just your ported rate. And if the numbers on the port don't work, breaking your fix and remortgaging fresh is still on the table, it's a different sum, not a dead end.
Frequently asked questions
Do I have to use the same lender if I port my mortgage?
Yes. Porting only works with your existing lender, because it's their agreement to reissue your rate on a new property, not a transferable product. If a different lender has a better deal for your situation, that's a remortgage, not a port, and you'd pay the early repayment charge to leave.
Can I port a mortgage if I'm buying before I sell?
It depends on your lender's rules on the gap between completion dates. Some allow a short window between selling and buying and treat it as a delayed port; others need both to happen on the same day. Ask your lender for their specific policy before you rely on porting to bridge a chain.
What happens to my early repayment charge if the port falls through?
If you're charged the ERC when your old mortgage is redeemed but the port doesn't complete as planned, most lenders will refund it if you complete a new mortgage with them within a set window, often three to six months. Outside that window, or if you switch lenders instead, the charge stands.
Track your port window for free
The free timeline shows your fix end date, your early repayment charge step-downs, and how much runway you have if a move is coming up, so you know your numbers before you make an offer.
Examples are illustrative and rounded. Your own mortgage offer and lender confirm your actual figures and dates. This is information, not financial advice. For advice on your circumstances, speak to a mortgage adviser regulated by the FCA.