They travel quite a way from the main plant, David, and very often suckers (new plants) grow from these, and from the base of the bush, so you need to give them plenty of space. There is a miniature lilac on the market now, though that you can keep in a pot. Annie (Cumbria)

In practical terms, expect a root spread 50% greater than the width of the top of the tree. So, a tree 10 ft. across will have finger-thick roots out to 15 ft. across. The really fine roots could be twice as wide as the crown, but they are less of a suckering hazard when damaged.

My mum had a lilac tree, and the roots traveled some 40ft from the main plant and they were HUGE. This was after 10yrs from being planted. We found this out when we were digging up her garden to section it off, she had a lovely 120ft garden. The tree did look lovely when in flower but it's only in flower a short time so you need to think carefully before you decide to plant one.