This intrigued me so I put some numbers in to a spreadsheet, keeps the grey matter ticking....
For a 0% cash transfer of £5k from a credit card with a fee of 3.1% that's £155 in what is technically still interest. The balance of the CC account is then £5,155, over 17 months that's £303.24 per month for 17 months to clear the balance in time. Remember the fee of £155 is not interest fee and if not cleared in full interest will be charged on all of it.
You'll have to keep paying the minimum of that balance which for some credit cards is 3%. The first 3% payment (£154.65) won't cover the fee so additional payment is required as mentioned before. In this example I'll assume additional payments on top of the minimum 3% will be made anyway, like this:
Assuming a mortgage of £170k over 20 years at 3.5%, so £986.26pm with no additional payments would look like this, our baseline:
Taking the £5k borrowed earlier and putting in to the mortgage, keeping the monthly payments as before, would look like this:
Only an interest saving of £84.18 over the first 17 months because the £155 fee eats in to the saving. There is a substantial saving of £4,747.99 over the full term of the mortgage, plus it would be paid off 10 months early. This assumes that the interest rate stayed the same, never remortgage or altered in any way.
What if instead of taking the credit card cash transfer we increased the mortgage monthly payments to what would have been paid to the cc balance for the same 17 months?
Over the 17 month period there is a saving of £122.40 compared to the baseline which is better than the credit card cash transfer method as there is no fee to pay. A nice saving of £4,819.15 over the full mortgage term and it's 10 months shorter compared to the baseline too.
In summary, making regular mortgage overpayments each month is better than a fee inducing cash transfer from a credit card. It's less risky, you can decide not to overpay should you need to one month or more and you don't have to calculate additional payments should you use the credit card for purchases too.
There are a few ways I could see a cash transfer from a CC being better. Instead of making regular CC payments above the 3% minimum required you put those in to a savings account and pull it at the 17 month mark to clear the balance of the CC. I haven't calculated what interest you'd obtain from such an approach but I reckon the savings account interest would need to be at least 3% or higher to see any benefit, plus it's added hassle and complexity. Another factor for consideration is if you are about to end a low fixed rate and enter a higher rate mortgage then paying something in the present will save in the future, increasing the savings made.
I've tried my best with my numbers and calculations but if you see any errors or omissions please let me know
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