Considerations in extending semi-D up to neighbour's house on boundary

Lumen

Asking for a friend :-)

Imagine a row of hipped roof semi-Ds, and one of them has been built out sideways to the boundary, so that the house takes the entire width of the site on two storeys, with the hipped roof extended (i.e. longer ridge line but keeping the same pitch). Let's assume that was done correctly and with permission decades ago.

The non-attached neighbour wants to extend out to the boundary, also on two storeys, thus effectively mirroring the design on the other side, but making the two existing semi-Ds terraced except with adjoining hipped roofs making a valley.

There are existing windows on what would become the party wall.

What are the considerations here? Is this a thing that is likely to get approved? Is this even possible without the consent of the party losing the windows?

The non-building neighbour would lose windows. It's not just light, they'd be losing ventilation and I'm not quite sure how valley drainage would work.

I can't remember ever seeing adjoining hipped roofs.

It's an odd situation because it seems like either:

(a) The first to build out gets to keep their windows, which seems a bit unfair to the other party who would be prevented from doing the same. There would effectively be a race to get to the boundary.

(b) Both parties have equal rights to build out, and therefore the first party's windows get nuked.

How are the conflicting rights/wants resolved in such cases?

Who is the appropriate professional to engage (by the non-building party) to advise? Planning consultant? Engineer? Architect? Solicitor?

Madeoface

You need a surveyor to establish if it's a party wall.

Permission will be given, it's a common sight to see semi detached houses joined like terraces with 2 storey extensions (i.e. over what used to be a single garage).

It is odd perhaps that the first party has permission for a window on their extension, presumably on the first floor. Might have done a deal with the neighbour but, as you say, likely to lose that window.

I don't think you can vent on to the neighbors side either.

Lumen

You need a surveyor to establish if it's a party wall.

What difference does this make? That's a genuine question, not a challenge :-)

Presumably if it's not a party wall then the new extension will need to have its own wall, i.e. there will be two walls butting against each other? With a gap? Without a gap? And then those existing windows will just face a blank wall inches from them? Seems a bit weird.