I think you need to explain what you mean by reasonable? Do you mean reasonable repayments result? Do you mean reasonable as in what age you will be when you have it fully paid off? The "ideal" term depends on a number of factors. For example, if you are 25 years old, then IMO it is probably not reasonable, as it will result in quite high monthly payments. However, if you are 48, the an 18 year mortgage would suit, as you would be unlikely to get much or a longer term. Anything under 20 years seems short to me anyway. I would suggest to a 20 year mortgage and overpay it. Hence, weigh up the monthly repayments for an 18 year mortgage and see if they are comfortable or will you be watching it every month. If its the latter, lengthen the term.