http://www.planningpo...l_explain_booklet.pdf
That's a very useful and informative booklet explaining the Party Wall Act.
Part wall doesn't only mean a "shared" wall common to both properties. It also applies in your case where you intend to build right up to an existing (neighbour's) wall.
In my experience........... you have no "right", as far as I know, to physically connect into his building ............. if he doesn't want you to. Of course, friendly agreement is the preferred way to go, as I'm sure you know.
Assuming you build parallel to his wall without "connecting" to it...... it's usual to cap the small gap between the buildings. A simple weatherproofing procedure.............. but it does involve connection to his building, so agreement would be needed. Otherwise, a ridiculous situation arises where you build a few inches away from him, leaving a pointless void. I've had to do that a couple of times......... much against my "common sense".
In practice .......... roof overhang and guttering usually keep the masonry anything up to 2' apart........... since building a wall on a boundary means that there can be no overhang or guttering at all. It would be on the neighbour's property.
To cut a long story short ................. as is usual, common sense takes over where the Law ends.