Planning has got very lax now, but there used to be a '45 degree rule', where anything that you were proposing building, must be designed in such a way that it did not fall within a 45 degree 'fan' drawn from any window on an adjacent property. I had to comply with this, when I put in for planning on my conservatory, and it was all detailed how to make sure of this, on the planning guides that the local authority gave me. There also used to be strict rules on the volume of any extension, compared to the volume of the main house - especially if any extension work had already been done, which would have eaten into the 'free gratis' allowance - and I think it also depended on the type of house. Again, this is part of the reason that my conservatory needed planning permission, as all of my free allowance had been used up previously with a side extension. Probably not valid any more, but might be worth looking into.
Arfa