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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, most 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 and change regularly.
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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, where lender policies typically differ and the traps to watch for, 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.
How lender policies differ
Every major lender supports porting with additional borrowing, but the detail varies more than most people expect — and policies change often enough that any list of specific criteria goes out of date quickly. What matters is knowing what varies, so you can check the right things (or have a broker check them) before you commit to a sale or an offer.
| What varies | Typical range | Why it matters |
|---|---|---|
| Porting deadline | Around 90 days up to 6 months; some allow longer for new builds | This is how long you have between paying off the old mortgage and completing on the new one. Miss it and the port is lost — your old rate goes and any ERC becomes payable. |
| Minimum top-up | None, up to several thousand pounds | Some lenders won't set up a separate additional-borrowing part below a minimum amount. |
| Pricing of small top-ups | New product vs Standard Variable Rate | With some lenders, additional borrowing below a set threshold doesn't qualify for a new fixed or tracker deal and sits on the SVR instead. |
| Maximum LTV | Commonly 85–90% including the top-up | Often lower if any of the additional borrowing is used to consolidate debt. |
The trap most worth knowing about is how small top-ups are priced. With some lenders, additional borrowing below a set threshold is placed on the Standard Variable Rate rather than a current fixed or tracker product — and the SVR is significantly more expensive. If you only need a small amount more, it's worth checking your lender's threshold before applying, or arranging the borrowing a different way. This is exactly the kind of detail that changes between lenders and over time, and it's what we check across our 90+ lender panel before recommending a route.
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. In that case the better question is whether to remortgage during the fixed term instead.
- 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.
Porting deadlines, minimum top-up amounts and LTV limits reviewed across major UK lenders' customer and intermediary sites, June 2026. Lender policies change frequently — we verify the current criteria for your lender before making any recommendation.
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. Most 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.
Most lenders will consider a small top-up, but watch how it's priced — with some lenders, additional borrowing below a set threshold doesn't qualify for a new fixed or tracker product and is placed on the Standard Variable Rate instead, which is significantly more expensive. A full affordability assessment applies however small the amount, so check your lender's policy before applying.
It depends on the lender — porting windows typically range from around 90 days to 6 months between paying off your existing mortgage and completing on the new purchase, and some lenders allow longer for new-build purchases. Miss the window and the port is lost: your old rate goes, and any Early Repayment Charge becomes payable. Check your lender's current deadline before you sell.
