ᐅ Deep Foundation for Semi-Detached House (Neighbor with Basement, I without Basement)

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

I would like to extend an existing semi-detached house. The neighbor has a basement, but I do not.

From previous discussions with construction companies and others, I assumed that I would need a deep foundation.
Basically, a wall would be built up from the neighbor’s basement, and the excavation pit would then be filled and compacted.

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

Today, I spoke with a general contractor who said that since the existing semi-detached house was built about 8 years ago, it might be possible that I do not need a deep foundation.
Could that really be the case?

A soil report has been commissioned. The results are still pending.

And my second question:
The semi-detached house is 12m (40 feet) long.
What would be an estimated cost for such a deep foundation?

Best regards

We are currently working on the same plan, but both houses are new builds. The general contractor said that a foundation is definitely necessary, otherwise your slab-on-grade could push down and damage the neighbor’s basement wall. However, I’m not sure if this applies to an existing semi-detached house. Our general contractor offered us a foundation with a depth of 10m (33 feet) at a cost of about €10,000.

Hello,
to my knowledge, a deep foundation is always required for a basement house.
This should also be determined by the structural engineer.