ᐅ Installation of vertical joint in insulation and plaster between semi-detached houses

Hello everyone,
there is probably already a thread on this topic. Unfortunately, I don’t seem to know the right term.

I want to buy a newly built semi-detached house. Both units are mostly finished, except for some remaining interior work.
What caught my attention on the outside is a joint at the boundary between the two units—a gap about 2-3cm (1 inch) wide (expansion joint?) in the plaster and insulation. This gap is open. In the past, I’ve seen similar houses where this joint was sealed with acrylic, Sikaflex, or similar materials.

Is sealing this joint mandatory, or can it also be correct to leave it open depending on the construction? The houses don’t have basements, feature porous clay block (Poroton) walls with 16cm (6 inches) insulation, and, roughly speaking, “standard plaster,” not brick cladding.

It is indeed a building expansion joint, which is structurally necessary. Closing it makes sense to prevent pests from entering. However, you need to coordinate with your neighbor. Acrylic sealant is not suitable; Sikaflex or an equivalent product is recommended. If the gap is wide, you can stuff some mineral wool in first.