The roots will spread about as far as the branches do. It will depend what rootstock the tree is on. It might be a dwarfing rootstock which can keep it to a manageable size. You can keep the top part pruned to fit a space - in a fan shape or espalier (in lines out each side) it should be pruned in summer, as it is a stone fruit, they can get rain-borne disease else.

Thank you. Espalier sounds interesting solution to above ground. Will check root stock.

One of the rare fruits that does well on a north wall, where espalier training would be ideal.

Thank you. Sorry I suffered from "fat fingers" and pressed "flag"! I have told organisers it was an error.