Porting means taking your existing mortgage deal to a new property instead of paying it off and starting again. Most fixed-rate deals are portable, and it is often sold as a simple way to keep a good rate and avoid an early repayment charge. Sometimes it is exactly that. Often it is more complicated than the word suggests.
It is not really one move
The part people miss is that porting is not a transfer, it is a fresh application against a new property, at the same time as you repay the old loan and draw the new one. The lender re-checks affordability, re-values, and can say no, even though you already hold the mortgage. A change of job, a new car loan, or tighter lending rules since you last applied can all get in the way.
When it clearly helps
Porting earns its keep when you are mid-way through a fixed deal with a meaningful early repayment charge and a rate better than anything available now. Keeping that rate and avoiding the charge can save real money, especially on a large balance early in the term when the charge is at its highest.
When it quietly does not
If you are borrowing more for the new home, the extra is a separate loan at today's rates, often on a different end date, and you end up managing two parts at once. If your deal is near its end, or the early repayment charge is small, the freedom to choose from a broad range of the market usually beats the saving from porting. And if the rate you are protecting is no better than what is on offer, there is nothing to protect.
The timing that catches people out
Most lenders give you a short window, often thirty days, to complete the new purchase and the sale together for a port to work. Chains rarely line up that neatly. If the sale and purchase fall on different days outside the window, the port can fail and the charge can apply after all. This is the detail worth pinning down in writing before you rely on it, not after the offer is accepted.
Porting is a useful tool and sometimes the right one. It is just not automatic, and treating it as a formality is how people get surprised at the worst moment. Check the affordability, the timing, and whether the rate is genuinely worth keeping, and the decision makes itself.