Deep foundation semi-detached house (neighbor with basement, me without basement)

Hello community,
I have a question for the earthworks experts.

I would like to extend an existing semi-detached house. Neighbor has a basement, I don't.

From previous conversations with construction companies and so on, I actually assumed that I need a deep foundation.
So basically a wall that is built up from the neighbor's basement, and the excavation pit is then filled and compacted.

The neighbor had an excavation pit when his basement was built.

Today I had a conversation with a general contractor who said that since the existing semi-detached house was built about 8 years ago, it could be that I don't need a deep foundation.
Can that really be the case?

A soil survey has been commissioned. Results are still pending.

And my second question:
The semi-detached house is 12 m long.
What could such a deep foundation cost approximately?

Best regards

We are currently in the same planning phase, but both houses are new builds. The general contractor said that you definitely have to have a foundation, otherwise your floor slab will push down and damage the neighbor's basement wall. However, I don't know if this applies to a semi-detached house that has been there for a longer time. Our general contractor offered us a foundation of 10m: approx. €10,000.

Hello
To my knowledge, a deep foundation must always be made for a basement house.
The structural engineer would also have to determine this.