Sycamore trees are a pain, they self-seed and then grow so damn quick. They are also very thirsty and will quickly find sewer lines. In my street there are quite a few rented properties, sycamores seed in the gardens then grow because the landlords never bother to sort the back gardens out. Before you know it, 4 30ft+ trees in the space of 5 or 6 years. We also have a superb specimen of a eucalyptus growing in someone's back garden, must be near 70 feet and still going. That's the thing about trees in city gardens, they simply aren't suitable, they end up too big for the garden and they affect everyone else. My next door neighbours had a trembling popular in a garden 10ft' x 40 ft, sold to them by woolworths, it was at 35ft after 7 years, should have seen the shock when I told them it would continue to 70 or 80ft. It came down, still having fun with the suckers though.
On topic, if I had a large sycamore with a tree preservation order on it on somewhere I'd be thinking of buying, I wouldn't buy simple as. The tree will cause enough problems now, and those problems will only increase as time passes.