Building description for a single-family house with a basement

Where are you building?
2100€ per sqm are already very close to the lower limit for a house on a slab foundation in most regions of Germany without ancillary construction costs. And in your calculation, kitchen (10-15TE), basement (at least 50TE), earthworks, sewer works, and also the garage (?) are still included. There are serious doubts whether it will work out like this, even if you plan to keep heating costs low with gas and radiators. As a rough estimate, you should rather expect the following order of magnitude:
House 150sqm at 2000€/sqm = 300TE
Basement = 50TE
Ancillary construction costs = 50TE
plus exterior facilities, garage, etc.

I am currently in the planning phase for the construction of a single-family house.

You should also get help here with other things besides the construction service description.

Is the building description usable

Apparently, this is not yet a concrete offer to you, but only the example from the general contractor’s website with advertising (soundproofing between the rooms), all kinds of options, "for example seventy sockets," and so on. The garage is supposed to be similar – yes, clearly it’s not digital, but similar to what? For example, it does not need the same finished floor height as the living area, and exposed wiring has never bothered me in the garage or the basement. If you are planning without your own architect also overseeing the construction, you should get an expert construction supervisor – the "site managers" of the general contractors have a different task. There is a moderated section in this forum called "Experiences with Construction Companies," where you can also find experiences and suggested competitors for many regional providers if you enter the names of the general contractors or the district towns around your construction site into the forum search.

Construction performance descriptions with significant gaps and a free pass for all types of components are either
- from a small general contractor who isn’t very precise but may use subcontractors who take and install completely standard brand-name products. Then you’re lucky.
- from a general contractor who wants to earn properly from you and leaves open which products he installs in your house.

Since he apparently only offers radiators and underfloor heating at an additional cost (***do not compare an older underfloor heating system with higher temperature flow with today's standard, because you won’t notice it at all***), otherwise probably the cheapest comparable option for what you want, I assume a fairly low standard. This is not intended as a judgment.
If you budget 300,000, the house is supposed to cost 315,000, and you are advised to calculate 350,000, then you have not accounted for ancillary building costs???? o_O
The house with basement is just too cheap now!

My tip:
Before I reduce the house size (due to limited budget), I would tackle the basement: 10 sqm utility room, 6 sqm storage space, and attic as storage space are completely sufficient.
Create a room program.
Then calculate and add ancillary building costs, painting work and floor coverings, outdoor facilities, kitchen, and lighting separately.

Everything he tells you verbally is not part of the contract!

Make sure it doesn’t become a cheap shack you regret later, but that standard brand-name products are installed so that it becomes a budget house.

(***do not compare an older underfloor heating with higher temperature flow with today's standard, because you don't notice it at all***)

Yes, that is a good and important remark. As legitimate as I find it when someone chooses an outdated heating method, the note about changes in the world since the time someone formed their opinion is equally important. We also encounter this discussion repeatedly with people who want a fireplace but base their views on experiences from "pre Energy Saving Ordinance" houses. For example, one now has to rethink what shape and installation locations the radiators should have, because in niche areas under the window sills they are no longer used in Energy Saving Ordinance houses.

Gas with radiators is already OK. But you are closing off the possibility for the future to simply switch to a heat pump, as if you had installed a [FHG with low flow temperature] right away.

Strip footing for basement? I think you just copied the construction service description from the provider's site and pasted it here without it relating to your project, right? The construction cost with a basement is not feasible.