Big crack in the wall

[Woollysocksneeded]

We bought our house last year, it's a 1930s end of terrace.
There is a big crack near the ceiling in our kitchen between the original house and the extension. It's easy to miss because you need to be standing on a ladder to see it (it's above the cupboards) so we didn't spot it and the surveyor didn't either. We only had a basic survey done and a separate damp survey.

The extension is quite old, the whole street has the same extension which must have been done around the same time to make the kitchens bigger. Maybe in the 1960s or 1970s. The extension is also 'shared' with a neighbour behind the wall, ie. they are part of the same structure.
It's a two story extension with a storage area underneath that you access from the outside.

My question is what to do?
Contact our building insurance or get a builder in to take a look?
Our neighbours only moved in a couple of months ago and their kitchen has been done up more recently so I suspect any crack between their original house and the extension would have been covered when the previous owners did the kitchen.

Can you see a crack on the exterior wall?

@Loofah01 It's on the side that backs to the neighbours kitchen so not against an exterior wall. The extension is shared with the nextdoor neighbour if that makes sense, the whole street must have had the same extension put in at the same time, and the extensions are always shared with two neighbouring households.

At the very least get a builder to look at it. Vertical cracks always need checking.

That's not great news. I'd remove some plaster around the crack (won't matter as it will need patching anyway) and see how far you can poke into it! Is anywhere else in the house, especially near the extension) showing similar signs? Might just be an early settlement crack that no-one bothered to patch up but if there's other indicators then yes I'd get some builders in to take a look.

How recently does it look to have been painted? It looks like it's cracked since it's been painted so I don't think it's a really old crack. Fingers crossed for you!

Thanks everyone. I will get a builder to take a look.
I think the kitchen was last painted maybe 10-15 years ago, maybe more. It's been a while for sure.

We bought the house as a fixer upper so were not expecting it to be perfect. The plan is to rip out the whole kitchen and make changes to the internal walls as well, and also to the space below the kitchen.
The problem we have is that due to some unforeseen circumstances (covid!) we probably have to hold off doing the work for at least a couple of years if not more. And of course ideally would like to do it all at once.