From the sound of it both would have the same affect - you need to ask the mortgage provider or try yourself to calculate the figures. Might be better to make the regualar overpayment off the capital or make a oneoff yearly off the capital amount owed. What you need to consider also is the procedure, costs etc to reduce the term and how easy would it be for you to change it again if your sitauation changed. At least if you are just making voluntary overpayments you can always stop if you have an emergency/ call on the extra money one month. I'm not sure about this but depending on how much mortgage interest you pay you might be able to get a greater tax relief if you changed the term and upped your monthly payments. I'd personally contact the mortgage provider and get some figures before deciding.