Porting Mortgage Calculator

What does this porting calculator work out?

It compares two paths when you move home with an existing mortgage:

• Path A — pay the ERC, take a fresh mortgage on the combined balance at the current market rate.
• Path B — port your existing rate on the original balance and add a separate top-up at the current rate.

The calculator estimates your total saving from porting over the remaining fixed-rate period, including the ERC avoided and the lower monthly payments on the portion kept on your existing rate.

Where does the saving actually come from?

Two places:

• The ERC you avoid — typically 1-5% of the existing balance, paid only on Path A.
• The lower monthly payments on the portion you keep at your existing rate. The top-up is at the market rate either way, so it doesn't affect the comparison.

The further your existing rate is below current market rates, and the higher your ERC, the bigger the saving from porting.

What does this calculator assume?

• Both options use a repayment mortgage on the same remaining term.
• Affordability and lending criteria are assumed to be met.
• ERC is calculated as a percentage of the existing balance — the most common UK structure.
• The saving is shown over the remaining years on your current fix. Beyond that, both options re-fix at whatever market rates apply.
• Lender-specific quirks (NatWest's £10,000 threshold, max LTV caps, etc.) aren't modelled — see our porting insight for the lender-by-lender breakdown.

Will every lender let me port and borrow more?

All six major UK lenders (Halifax, Nationwide, Santander, NatWest, Barclays, HSBC) allow porting with additional borrowing. But each runs a full affordability check on the combined new balance, and policies differ on time windows, maximum LTV, and how the top-up is priced.

Approval isn't guaranteed — circumstances change, and what comfortably passed three years ago may not pass today.

What if my additional borrowing is declined?

You usually 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 that's more accommodating.

Independent advice helps — the right answer depends on the ERC size, the rate gap, and which other lenders you qualify with.