Building extension next to neighbours existing extension

[whenisayjump]

Hello

I live in a mid terrace house. A little while a go my neighbour build a full width rear extension, as far as I'm aware everything was done properly and we were happy with this. We signed a PWA at the time as we had no problem with it.

As I understand it they built up to, but not astride the boundary line on our side (astride on the other side). I'm not sure why this was decided.

We're minded to do the same at some point, and I'm wondering what people generally do in this situation. Do we also both build up to the boundary line and just have no gap?

TIA

 

[^woody^]

It's common to leave a gap to avoid boundary and responsibility issues.