Cladding the gap between semi-detached houses - What is the best way?

I need your ideas! We have built a prefabricated house as a semi-detached house next to an existing solid house. Unfortunately, the wall of the existing building is not straight, so we have up to 30 cm gap in the base area, while at the gutter there is no gap between them. Does anyone have an idea how we can cover this gap? The construction manager advised us to use a perforated sheet or ventilation grille so that no mold can develop behind it. Plastering will be done soon and I would like to take advantage of the opportunity. I am grateful for any suggestions.

In such cases, images significantly help to spur collaborative thinking.

I could have thought of that right away
Attached is a picture of the problem.

I unfortunately can’t help you professionally, but that already looks extremely like: built strictly according to plan, but without brains. That’s not acceptable, they could have (should have?) filled it in directly during construction.

I’m just trying to imagine what you would find there after a few years... keys, children’s toys, the neighbor’s cat,...

Unfortunately, I can't help you professionally, but that already looks extremely like: Built strictly according to plan, but without brains. Absolutely no good, that could have been backfilled directly during construction (should have?).

This is a perfectly executed building separation joint to prevent structure-borne sound bridges. Typically, nothing is backfilled in new construction; on the contrary, using cohesive fill material/Styrofoam etc. actually destroys the excellent structure-borne sound insulation of the multi-layer communal separating wall. Perforated sheet metal/ventilation grille etc. works fine, but you have to make sure that it is attached to the exterior wall with permanent elasticity. Otherwise, it's a sound bridge.

Ok, then I withdraw my professionally unqualified objection.

It is precisely the solid connection that makes a semi-detached house what it is. We have already lived in 2 different ones and never had problems with noise to the neighbor. I still don't understand the point of the gap if a semi-detached house was desired after all.