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Porting lets you take your existing mortgage rate to a new property when you move. If you need to borrow more, all six major UK lenders allow it alongside the port — the original balance keeps its rate, and the additional borrowing sits on a separate part at the current market rate. Affordability is reassessed in full. Policies on time windows, maximum LTV, and minimum top-up amounts differ between lenders.
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If your current mortgage rate is lower than what's on the market today, porting can save thousands when you move home — even if you also need to borrow more to fund the new purchase. The key is understanding how lenders structure these applications and where each one differs. (Our porting mortgage calculator models the saving in real time as you enter your numbers.)
This article sets out how porting plus additional borrowing works in practice, with a lender-by-lender comparison of Halifax, Nationwide, Santander, NatWest, Barclays and HSBC as of June 2026, and a worked example showing the real saving.
What porting and borrowing more actually means
What does porting a mortgage mean?
Porting a mortgage means transferring your existing mortgage product to a new property when you move home. You keep the same interest rate, term, and product features on the balance you carry across, and you don't pay an Early Repayment Charge on the portion you port — provided the new purchase completes inside your lender's porting deadline.
It is most useful when your current rate is lower than today's market rates, or when leaving your deal early would trigger an Early Repayment Charge (ERC). Mortgage porting is offered by every major UK lender, but the terms differ — particularly the porting deadline, minimum top-up amounts, and how the additional borrowing is priced.
When you also need to borrow more — because the new property is more expensive, or because you're combining moving with home improvements — most major UK lenders allow you to add this to the application. The result is a single mortgage made up of two parts.
The two-loan structure
When you port and borrow more in one application, the new mortgage is made up of two parts that sit alongside each other:
So in the example above, the original £400,000 keeps running at 2.5% until its original expiry. The new £75,000 sits on a current-rate product chosen from the lender's range, with its own end date. When the original part ends, the two parts can usually be aligned onto a single new deal.
Affordability is reassessed in full
Porting is not automatic. Every lender treats it as a new mortgage application and runs a full affordability assessment based on:
- Current income and employment
- Outgoings, credit commitments and dependants
- Stress-tested interest rates
- The total borrowing — original balance plus additional borrowing
This matters because circumstances change. A borrower who comfortably passed affordability three years ago may not pass on the larger combined balance today, especially with stress rates applied above current product rates. Self-employed borrowers, those approaching retirement, or anyone whose income has dropped should plan for this.
Want to check how much you could borrow on the new combined balance? Use our affordability calculator to get an indicative figure in under a minute.
UK lender comparison (June 2026)
The table below summarises the porting and additional-borrowing policy of the six largest UK residential lenders. All figures are taken from each lender's own customer or intermediary site as of June 2026 — policies change, so confirm before relying on them.
The porting deadline column shows how long you have between paying off your existing mortgage (usually when your sale completes) and completing on the new purchase. Miss the window and you lose the port — your old rate goes, and any ERC becomes payable.
| Lender | Porting deadline | Additional borrowing | Min top-up |
|---|---|---|---|
| Halifax | ~4 months (varies by product) | New rate on a separate sub-account | £5,000 |
| Nationwide | 180 days | New product on the additional amount | Not specified |
| Santander | 3 months (6 months for new build) | New deal on the additional amount | Not specified |
| NatWest | 4 months (ERC refund window) | £10,000+ = new business rate; under £10,000 = SVR | None (but watch the SVR cliff) |
| Barclays | 90 days from sale completion | New deal on the additional amount | Not specified |
| HSBC | 6 months | New rate, subject to assessment | Not specified |
The most important quirk to know is NatWest's £10,000 threshold. Additional borrowing of £10,000 or more gets a new business rate (a current fixed or tracker product). Anything below £10,000 is placed on NatWest's Standard Variable Rate, which is significantly higher. If you only need a small top-up, it's worth either borrowing the full £10,000 or arranging the additional borrowing separately.
The saving — a worked example
Porting is most valuable when the rate on the existing mortgage is meaningfully below current market rates. Bright Box helps clients in this position avoid two costs at once: the Early Repayment Charge for leaving the existing deal, and the higher monthly payments that would come with moving the whole balance onto a new market-rate product.
The example below uses real proportions from a recent case (figures rounded for illustration, based on a 25-year repayment term).
The client's position:
- Existing mortgage balance: £400,000 on a 2.5% fixed rate
- Time remaining on the fixed rate: 2 years
- Early Repayment Charge: 1.5% of the balance
- Additional borrowing needed for the new property: £75,000
- Current market rate at the time: 4.8%
Option A — leave the existing deal, take a new £475,000 mortgage at 4.8%
- ERC payable: 1.5% × £400,000 = £6,000
- Monthly payment on £400,000 at 4.8% (25yr term): ~£2,292
Option B — port the £400,000 at 2.5% and top up £75,000 at 4.8%
- ERC payable: £0
- Monthly payment on £400,000 at 2.5% (25yr term): ~£1,794
- (The £75,000 top-up is at 4.8% in either option, so it doesn't change the comparison)
The saving across the remaining 2 years of the fixed rate:
- ERC avoided: £6,000
- Lower monthly payments: ~£498/month × 24 months ≈ £12,000
- Total saving: approximately £18,000
The bigger the gap between your existing rate and current market rates, and the higher the ERC, the larger the saving from porting. Try our porting mortgage calculator to model the saving for your own balance, ERC and top-up.
When porting isn't the right answer
There are situations where it's worth modelling the alternative side-by-side in our porting mortgage calculator before committing to a port:
- The existing rate is at or above current market rates — there's no rate advantage to protect.
- The ERC is small or close to expiring — the saving may be marginal.
- The new property pushes you above the lender's maximum LTV for porting — the application may not get through underwriting.
- Affordability has tightened and the lender won't approve the larger combined balance — sometimes a different lender on the whole mortgage is the only option.
If your additional borrowing is declined
It is possible to have a port approved while the additional borrowing is declined — usually on affordability grounds. If this happens you have three options:
- Reduce the additional borrowing to a level that does pass affordability.
- Add a second applicant if circumstances allow.
- Move the whole mortgage to a different lender — the original deal is lost, but the new lender's affordability or maximum LTV may be more accommodating.
This is where independent advice matters: the right answer depends on the size of the ERC, the gap between your existing rate and current rates, and which other lenders you qualify with. If you'd like to talk through your own numbers, get in touch with our team.
Lender policies verified June 2026 from each lender's customer or intermediary site: Halifax, Nationwide, Santander, NatWest, Barclays, HSBC.
Last updated: 4 June 2026. Sources cited above. This article is general information, not personal advice — your home may be repossessed if you do not keep up repayments on your mortgage.
Frequently asked questions
Yes. All six major UK lenders allow additional borrowing alongside a port, subject to affordability and lending criteria. The original balance keeps its existing rate; the additional borrowing sits on a separate part of the mortgage at a current rate, often with its own end date.
You don't pay an ERC on the portion you carry across, provided the new mortgage completes inside the lender's porting window (commonly 90 days to 6 months). If you borrow less than your current balance, you may pay a partial ERC on the difference.
No. The additional borrowing is placed on a current product chosen at the time of application — typically with its own end date, independent of your original product.
Watch the NatWest £10,000 threshold — additional borrowing under £10,000 with NatWest goes onto the Standard Variable Rate. With other lenders the additional amount can be small, but a full affordability assessment still applies.
It depends on the lender. Barclays gives 90 days from sale completion, NatWest and Halifax around 4 months, Nationwide 180 days, HSBC 6 months, and Santander 3 months (6 months for new build purchases).
Halifax lets you port your mortgage when you move, with around 4 months between paying off the old mortgage and completing on the new one (the exact window varies by product). You can request additional borrowing alongside the port — minimum £5,000 — which sits on a separate sub-account at a current rate. There is currently no arrangement fee to set up the additional borrowing.
Nationwide allows porting with up to 180 days between paying off your existing mortgage and completing the new purchase. If you need to borrow more than your current balance, the ported rate covers the existing amount and you pick a new product for the additional borrowing. Both are subject to current lending criteria and a full affordability assessment.
Santander gives you 3 months between paying off the old mortgage and completing the new one — extended to 6 months for new-build purchases. The ERC is refunded if you complete in time. You can borrow more alongside the port, with a new deal applied to the additional amount, and the maximum LTV including the top-up is 90%.
NatWest offers a 4-month window from redemption to complete on the new purchase for an ERC refund. Watch the £10,000 threshold for additional borrowing: anything £10,000 or more gets a new business rate (a current fixed or tracker product), while amounts under £10,000 are placed on NatWest's Standard Variable Rate, which is significantly higher.
Barclays gives you 90 days from sale completion to complete on the new mortgage. You can choose from current mortgage deals for any additional borrowing while keeping your existing rate on the original balance. The maximum LTV is 85%, dropping to 80% if any of the additional borrowing is used to consolidate unsecured debt.
HSBC allows up to 6 months between repaying your existing mortgage and completing the new purchase — the longest porting deadline of the six major lenders. Any additional borrowing is placed on a new rate and is subject to a fresh affordability assessment, with the new property also needing to meet HSBC's lending and valuation criteria.
