Stabilize basement wall for extension

Hello everyone,

briefly outlining the situation. Single-family house from 1948, so very simply constructed. 36 cm wall with cavity. According to the neighbor, in the 70s the house was supposed to be re-plastered, but since the bricks wobbled when the plaster was removed, the walls were opened and filled with concrete, then insulated with “Sauerkrautplatten.” Unfortunately not in the basement. When I drill here, individual bricks wobble.

Now a freestanding extension is planned.
My fear is that the old building will be damaged.
1. Higher weight on the basement walls due to concrete in the upper walls.
2. Missing earth pressure when excavating the strip foundation.
3. Vibration during compaction.

I want to secure the old basement wall. My idea would also be to open the wall and fill it with concrete. However, someone was advised against completely filling a cavity wall with concrete in the Fachwerkforum. What speaks against it? If the concrete pressure is too high, I could also pour the concrete in layers. Partially uncovering the wall and gradually constructing the strip foundation in front of it seems much more complicated to me.
Thanks for your tips.

I would never tackle that without a structural engineer who is familiar with such old buildings.

I don’t believe there was a cavity wall in the basement; at that time, a 38 cm solid brick wall was more common. From the ground floor upwards, cavity walls with an air gap were more likely built. I can’t link it right now from the road, but a few years ago we had such a wall construction here from 1954, although without a basement.

I can't link it right now while on the go,

The post should have at least 5 words.

So, I have now opened the wall and yes, the wall is not hollow but alarmingly "cookie-like".
I knocked too hard on one stone and it is loose and can be pulled out of the wall.
So filling with concrete is out of the question.

Now, without structural engineer advice (I will still ask one), does anyone know of a case where a house collapsed
because a basement wall was excavated or is that an urban legend?

The exposed part looks like a base / ground floor. Get the building file, otherwise you can't compare such samples with the plan.