Neighbours Extension

[Jackoregan]

I'm the middle of a 3 house terrace and the houses are staggered so my house is about 1 metre behind the one on the left and the one on the right extends 1 metre out from me, so we all get the sun if you get my drift.  My Neighbour on the left has just buillt a ground floor extension, not only blocking out my light, but it appears to be attached to 1 metre of my side of the house.  Are they allowed to use my house or should there be a definate gap?  Also how do I find out who owns the fences in my garden