when we bought our house i think it was 6feet(i may b wrong!). they ask you for insurance purposes as the roots can damage the foundations of the house, or the tree coulld be blown over and cause damage to the building.
It depends on lots of things - soil type - foundation type etc. I used to live in a house with a 40ft siver birch three feet from one corner. We were on chalk and had no problems. I now live on clay but foundations are on 30ft deep pilings so I've planted all sorts nearby because I like greenery. The web-site Cetti has suggested will give you the ultra safe distances which are probably over the top except for Willow which you should avoid near houses at all costs.