Building ground investigation report (ram core soundings) suitable or excavation?

Hello,
Background (can be skipped):
We (a family with 3 children) are planning to build a new house. We have already purchased a plot of land. We were quite naïve and bought a property with a rear house. It has 2 floors and up to 140sqm of living space. For various reasons (very poor floor plan options, no basement/cellar/storage space, uneconomical renovation) we have now been advised against it. The new plan is to build new on the other side of the plot. Currently, we are leaning towards a 1.5-story house with a gable roof (the knee wall is to be set very high in the upper floor to have the option to expand the attic). With a house footprint of about 9 by 9 m, we come to about 130-140 sqm of living space with 4 rooms and an expansion reserve of about 40 sqm and 2 rooms (3 rooms and 50 sqm if we plan a gable or a dormer). We might also want to have a basement built additionally.
Plot:
It is located in the city center. Very precisely on the planned footprint of the house there stood a corner house with a basement from 1897 until World War II (the basement was about 150 cm below ground). Around 1955 it was dismantled/reduced due to fire and explosion damage. Apparently, it was caused by an air bomb in WWII. The building materials were reused (in the GDR) and the assignment was then to fill the area 30 cm higher than the curb. We assume that the basement was not removed but filled in – but we do not know for sure.
Since we are currently looking at house providers, we would now like to have a soil survey carried out in order to determine whether:

[*]the old basement is still underneath
[*]how it was filled in
[*]whether we could build a house without a basement on it without replacement, or whether an excavation pit would have to be dug and then refilled if we build without a basement (then we would very likely decide on having the basement)
[*]all other usual results of a soil survey

All the house providers we have considered so far always base their cost calculations on the following values:

[*]soil class 1-4
[*]soil pressure of at least 200 kN/m²
[*]modulus of elasticity Es=35 MN/m²
[*]level, slope-free plot without pressing, aggressive groundwater or stratum water

Does a normal soil survey give information about this? The usual dynamic probing tests would probably quickly encounter the old basement and would not provide any information?

Since I know quite exactly where the old foundations stood, I am already considering digging there. But I am unsure what it would tell me if the walls are still there or cannot be found?

A bit off-topic: do you already have a room layout or a floor plan in mind? I honestly wouldn’t know how exterior dimensions of 9*9 and 180-190 m2 of living space are supposed to fit in a 1 1/2-storey house with a converted attic...

yes we do.
Exterior dimensions are 9*9.25m
according to WohnflächenDIN
Ground floor: vestibule, shower/WC, living/dining/kitchen, hallway (about 65sqm)
Upper floor with sloping ceiling from 2.25m: 3 rooms, bathroom, hallway (about 65sqm)
Attic (only as expansion reserve): almost 40sqm
optional basement: 3 rooms, utility room (floor plan similar to upper floor) (about 65sqm)

what is there to doubt? interior dimensions are about 8.25*8.50m=70.125, the bit lost to stairs and wall surely adds up?!

Attic with roof slope from 2.25m

Is that still a one-and-a-half-story building according to SächsBO? Depending on the state building code, anything with >75% floor area >2.3m clear height is a full story.

that is an important note. I will check that. although in the end it doesn’t really matter whether the thing is called a 1.5 or 2 or even 2.5 story building. We are allowed to build all variants there, since all variants already occur in the settlement area.

where in the end it doesn’t really matter whether the thing is called a 1.5 or 2 or even 2.5 storey.

The 1 1/2-storey surprised me. With 2 or 2 1/2 storeys, of course no problem.