Tree with flowers

This is a bit vague, but next door but one, is a tree which is starting to grow about 25 feet now.   It is covered in white flower pannacles, which are now shedding and it looks like a layer of snow.   Cannot put the washing out and have had to close doors and pull windows in.  I cannot identify it from books or google.  I wonder if you may have a tree that is doing this and can tell me what it is ?  (It has never had flowers before)   thank you   Mrs Bouquet