I bet it’s not structural. I would imagine is either heat from a chimney or overdue decorating

What is the going rate to do a specification on Common parts on 4 floors with 2 flats per floor in an Victorian Terrace property.

Jonathan, I have no problem admitting when I'm wrong. And Yes I agree that their original question was very straightforward, to whom to entrust with a structural engineer. I just wanted to offer some words of caution since they didn't provide enough information about the builder. I didn't make an assumption on who they are or their qualifications and building practices, and while I agree that a good builder would of course get everything done correctly, unfortunately, you do get bad actors in the industry, which you can't deny. They can do with my information as they wish, if they don't think it applies to their situation they can completely disregard it. However, I don't understand why this has struck such a nerve with you, that you took it personally and started to berate and attack me. Yes English is my second language if you must know, but I have been speaking English since was 9 years old, I also speak 3 more other languages in case you are wondering and currently learning French as well. English is by far the easiest language and the not-so-subtle nuances of your insults do not escape me. Since you brought it up like it's something that I should be embarrassed by. For the record I never berated you but only pointed out your incivility towards me, which again you took personally and lashed out. I see a good night's sleep didn't help to improve your temper, which I do regret, as I do find myself often agreeing with your answers on Houzz, which I'm always happy to state if that is the case. However, I don't remember seeing you giving that courtesy to anyone. I do want to say thank you for taking the time and finding out my name, not entirely sure what the motivation was, but I will just say thank you and leave it at that. Hope you have a nice day and a good easter holiday and in the spirit of holidays let go of this silly grudge against me. Let's be respectful to each other and help the Houzz community with our own opinions and advice. If you disagree with something that I write and feel compared to say something my DM is always open for a friendly debate.

I would contact a few structural engineers first to see if any are interested or available to come and survey the problem and give you a quote. If it's only correcting the problem then I don't think you need an architect for this. Try and find a SE that does a lot of residential work as it's not that easy to find one. Agree about not getting involved in underpinning. This should have been done before either of you started renovation work. Your instincts are spot on. I would never use an SE recommended by a builder although I have done structural work on a previous house with great builders recommended by my SE. If the SE finds that the party wall needs underpinning it's worth having a conversation with your neighbours about it so at least they are aware of the building's movement.