ᐅ Existing basement, a new full basement, or no basement at all?

Hello everyone,

My wife and I are increasingly planning to build a new house on my grandmother’s property.

The plot is about 770m² (8300 sq ft) and currently has a house from 1956 with a 9x9 meter (30x30 ft) footprint plus an extension and a garage.
My grandfather built the house back then. The basement is quite low and not very deep below ground level (approximately 80cm (31.5 inches)).

We have a development plan from the 1960s in Sütterlin, which allows for 1 full story with a 50cm (20 inch) knee wall.
However, the building authority has indicated that neighboring city villas with 2 full stories and a shallow hipped roof represent the upper limit of what is possible.

What concerns me somewhat is the question of the basement.
I have read the so-called 11% basement rule, but I don’t think I fully understand it.
I can follow it in the case of a hillside house.
But does it mean that if you have a (basically) level plot, you would never build a basement?
Instead, you would rather increase the footprint of the house and place the typical utility room on the ground floor?

How would it be if there is already a hole from the demolished existing house?
I actually see the old basement as unusable— it’s not deep enough and unfortunately not in great condition anymore.
The area, though, would roughly correspond to what we are envisioning.

In our case, the current plan is that the basement would have, besides the technical/ laundry room, a small storage/ pantry area.
Also important is a large workshop (20m² (215 sq ft)) and a living room (20m² (215 sq ft)) for office or guest use.

How can I calculate whether it would be more economical to level the existing basement and increase the house footprint instead?

I always thought that it gets more cost-effective the closer you get to building a cube — minimizing exposed exterior wall and roof area.
The number of floors is limited, after all.

Best regards