Crack between extension and house

[Pinoy]

Hello.
Bought house 18 months ago.
House was empty for two years before purchase.
Located in South of France…summer 35C…winter 5C worse case.
To the left of crack is the extension built twenty years ago.
Original house to right built 1970.
It *may* be water ingress but pretty sure it isn’t…
Has not got worse in last eighteen months.

How to remedy ?

Thanks.

 

[tell80]

Why not post pics of the outside - ground level to eaves?
Is the location on a hillside or slope?
Is the width of the overhead soffit the width of the original outside wall?