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

The reason why we are approaching it this way is because there is already a model house at a construction company that we quite like. It is a classic 1.5-story house with a cold roof. We want to build on that and have the knee wall significantly raised. We want to insulate the cold roof ourselves at some point, have a proper staircase installed (this should already be planned), and use the roof as expansion space. But whether it will still be a 1.5-story house then, I don’t know. In the Saxon building code, I don’t see anything about that classification. It only generally describes whether it is a story or not. And according to the description, it is two stories that can eventually be expanded to a 3-story building.

EDIT: now I got it:
90
Transition provisions
(2) As long as § 20 paragraph 1 of the Baunutzungsverordnung refers to state law for the definition of a full story, stories whose ceiling surface extends on average more than 1.40 m above the established ground level and which have a clear height of at least 2.30 m over at least two-thirds of their floor area are considered full stories.

Accordingly, I believe that the model with about a 25 cm knee wall could already be a full story. In any case, we are planning 2 full stories with the possibility of expanding to a half or even a full story.

We want to insulate the cold roof ourselves at some point, install a proper staircase (which should already be planned), and use the roof as expansion reserve.

Wouldn't it be more sensible to have a warm roof built right away and leave the attic only in its rough state?

If you expand later, you wouldn't just have one insulation layer too few in the roof, but also one insulation layer too many in the house. Not to mention heating, water, etc.

Yes. We will discuss that with the house construction company when we get to that point. However, it might be that they do not do that. Would additional insulation in the floor be harmful?

This has 2 floors

we tend towards a 1.5-storey building with a gable roof

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, 2 or even 2.5-storey building. We should be allowed to build all variants there, because all variants already occur in that residential area.

In the Saxon building code, I find nothing about this classification. It only generally describes whether it is a storey or not. And according to the description, it is 2 storeys that can be expanded to a 3-storey building at some point.

Accordingly, I believe that even the model with about 25 cm knee wall could be a full storey. In any case, we are planning 2 full storeys with the option to expand by half or even a full storey

Now, please check what is stated or permitted in the development plan.
You are throwing the half-storeys around as if they exist, although they don't.
And even if one is not petty and defines the "attic floors as half-storeys or variants," it is still quite strange if there is no limit upwards.
For you, every floor may be a storey and "all variants allowed" in the building area, because you approach it visually or somewhat amateurishly, but simply planning vaguely what you want to build because "it doesn't matter anyway, everything is allowed" does not show that you have learned anything from your mistake.
see:

We were very naive in our approach

However, if you say that your new building comes close to the house that is there, then you are probably right.
Has the floor area ratio/plot ratio been checked?

There is no development plan there. According to the building permit office, there is no problem with 1.5 and 2.5 storeys with a gable roof and 2 storeys with a hipped roof if the old building lines are adhered to because all of this already exists in the settlement development. 2.5 storeys with a gable roof would be desirable as the main old development consists of 3.5 storeys. Floor area ratio and site coverage ratio have not been checked by me. The plot is 710 sqm. A house with an 80 sqm footprint should not be a problem, right?

On the topic: "in case of suspected underground structures," every geotechnical engineer will suggest an excavation that will be really damn expensive. The result will be 99% that cavities due to improper backfilling cannot be ruled out. No geotechnical engineer takes that risk.

Usually, back then it was deliberately backfilled loosely so that it could be excavated again later and a house could be built on the old cellar. I think the cheapest option for you would be: get a rubble container, rent an excavator and a Pavel/Vladimir/… who can handle it and excavate the old cellar again. At the bottom of the pit, your geotechnical report for the foundation of the new house will then be created, and you have your new house including cellar built into the self-excavated foundation pit.