Can a hot water tank be put in the loft?

I have a system boiler with a hot water tank in an airing cupboard which is in a bedroom. I want to move the hot water tankl and put it in the loft. My cold water tank is there but is suspended about 80cm above the flooring in the loft. Is it possible to put a hot water tank of some description underneath the cold water tank? Does there have to be a certain space between them? Our boiler is on the ground floor and we live in a town house (3 floors).