Hello I am a solicitor with over 15 years experience. I will try to help you with this.
Can you give me a little more information please. What is he planning to build and how does it have an adverse effect on you?
Will his foundations project under your land? If so the Party Walls etc Act will be triggered and he must serve notice on you seeking your consent.
If the foundations do not extend under your land and the wall does not cross the boundary then the Act will not be engaged.
I don't understand how this proposed extension will stop you accessing windows and gutters etc. Can you explain a little more please. As far as the light is concerned you may be able to do something about that if you have enjoyed light in the room that is about to be blocked for 20 years or more then you may well have acquired a right to light in that room. If your neighbour is building an extension as a permitted development o/s planning permission he may not have taken this into account. He should have.
AS far as access is concerned I assume that in fact if he comes rigth up to the boundary he just leaves such a narrow atrip that you can get ino
it. Is that right?
If you do not want this building to go ahead you should ask about the foundations as above as this would make it a party wall matter and more difficult for him to go ahead without your permission. Also you can enforce your right to light as well to prevent it or to seek compensation. What kind of a neighbour behaves in the way he is behaving? here is link to the RICS guide re Right to Light.