If its in a small back garden it must be quite close to the house? and this would worry me as the roots spread up to 20 metres. I would get expert advice to see if its ok to leave it as it sounds as if it needs removing?

Don't waste time and energy even thinking about it - in a small garden, a sycamore is a disaster. You won't be able to pile soil up around it, no tree will put up with that, and anything you try to grow will not work. Have it summarily removed and if you feel really bad about it, choose another tree, maybe Prunus amanogowa, instead - but only if you have more than 40 feet between your house and the tree. This also applies to the sycamore, which is much more of a danger to house foundations because its so much larger. In all likelihood, it grew on its own there - sycamores drop millions of seeds, and loads of them germinate - and that's something you will have to watch for over the next few years, loads of saplings popping up.
You can satisfy your desire to improve the lot of wildlife by using other planting - it doesn't have to be a tree, a couple of large shrubs with lots of low underplanting will be extremely useful from that point of view.

Hi Hazelman and welcome to GoY. Be aware you will not be able to create a raised be around the tree unless you leave a gap of a good foot between the raised bed and the trunk of the tree. If you cover up part of the existing trunk you will stress and possibly kill the tree.

I'd consider removing the tree having the stump ground out and replacing with one or two trees that wont get so large. Perhaps a Sorbus cashmiriana which will only grow to about 9 metres, in fact the one in our front garden has been in for 15 years + and is approx 2 metres in height. This has large white berries in autumn and looks very structural in winter when the leaves have shed.

For a time we lived in a house with a large sycamore too near, which had had a small raised bed put round it. It had not done any harm to the tree but nothing much grew there - mainly vinca minor and a few hyacinths and it really hadn't been worth doing.It remained dull and gloomy. We would gladly have had it cut down but a kind neighbour had had a tree preservation order put on it before the house changed hands. I would get rid if you can, and perhaps replace it with a more bird friendly smaller tree, perhaps a crab apple that will give both flowers and fruit. You could plant a few shrubs such as pyracantha or cotoneaster that the birds will love too. Somewhere to hide and something to eat is what they want. (If you want to burn the sycamore for fuel, leave it a year to dry out first)

If you are determined to keep it you could try euonymus fortunii varieties and some bluebells to brighten up underneath it but forget the raised bed.

If its damaging the house a TPO can be changed. You just have to present the evidence. You may be making things worse with the soil around it as they are known for going over Steragram.