ᐅ Expansion joint between two terraced houses – responsibility according to DIN standard?

Hello everyone,

We have a single terraced middle house, which has recently had an extension added. The exterior wall facing the neighbor is made of calcium silicate brick, probably 17.5 cm (7 inches) thick, and is unplastered.

The neighbor has attached a prefabricated timber frame extension. Now there is an approximately 5 cm (2 inches) wide gap between our houses. According to my builder, the party adding the extension is responsible for sealing this gap.

From what I understand, this should be sealed by inserting mineral wool strips and/or compressible sealing tape along with a mounting rail. Additionally, a metalworker should install a flashing extension at the roof junction to prevent water ingress between the houses.

My neighbor seems unaware of this and claims their building is designed as a "detached house," even though their timber stud wall facing my property also has open joints where moisture could enter.

Is there any specific standard (e.g., a DIN norm) or regulation that clearly states the responsibility lies with the party building the extension?

I am concerned about the upcoming winter if the neighbor does not take action. Also, I wonder whether compressible tape and the mounting rail alone are sufficient, considering the potential for thermal bridging... I am willing to insert the mineral wool strips myself if necessary, but I am not prepared to cover the costs for the rest.