Not necessarily - it would be useful if you could find out which trees they are and what rootstock they're on - most apple trees these days are on semi dwarfing rootstock or even dwarfing, which means the trees won't get very big anyway. If they are going to be 25 footers, then 20 feet is a little too close to the house.

One of the big problems with Apple trees even on a taller growing rootstock, is that they just do not have wide spreading roots. Our Catshead Apple is now over 25 feet tall but the roots do not even reach more than 15 feet from the trunk. We have had one blow over last year and the roots were very close to the trunk. That was on a mid size rootstock. Personally I would not give them a second thought. And worse case scenario, a bit of judicious root pruning would do the trees no harm. If they are only recently planted you will probably have a good 10 years to watch the roots grow.

Thanks to Bamboo and Owdboggy. Very quick response and very helpful answers. I am not sure what type of apple trees they are but will try to find out when I move in. I will certainly keep a close eye on how they develop. Thanks again.