Advice please on a joint party wall

I am looking for some advice please

We are well advanced in the purchase of a large semi-detached house but have not yet exchanged. The house is large as they have extended up to a newly built a joint party wall that sits equally on theirs and next doors property.
The current sellers and their neighbours essentially did not object to the respective planning permissions as they had mutually agreed with each other their houses would be adjoining once the works were done and both would benefit.
The house we are buying has completed the extension, the neighbour however has her planning permission approved in 2010 but not yet commenced any work, still has a drive way and the house we are buying appears semi-detached, but in essence could and probably will become 'terraced' imminently.

Several questions
1) what are your thoughts on how this will affect the value of the property we are buying

2) Should the neighbour NOT do the work, planning expired, would we have any recourse or ability to prevent the planning/extension up to the party wall in the future?

thank you for any thoughts

The house you are buying has been improved with the benefit of an agreement between neighbours. Now you are considering trying to withdraw the benefit of that agreement from the other household.

Walk away now, because you seem to be lining yourself up for a bad relationship with your neighbours.

Agreed. The party wall is joint property. If you make things akward for them, they could do likewise if/when you go to sell.

Or, you accept that the neighbours can build on the party wall and that the house will effectively be terraced and decide to buy on that basis.