New single-family house with basement (on a slope)

What does the property look like? Dimensions of the property and dimensions of the building envelope are helpful. It is best to include a section from the [Bebauungsplan].

Again to your other questions

What budget should one generally calculate for this? Is ~400k (house + additional costs) realistic?

One calculates €3000/sqm of living space for a standard house (square and practical, without bay windows and extras). A hillside is more expensive due to terrain modeling, Town & Country is cheaper because it is stripped down and therefore somewhat "below standard." This is not meant disparagingly.

Which other construction companies should we look at, also considering self-work?

For a shell house, which probably involves a bit more, the company Massa Haus comes to mind. However, to my knowledge, they are also not flexible with the basement.
Regional companies offering simple but good construction are recommended. They are more willing to negotiate when it comes to waiving trades.

Self-work usually includes flooring and painting with most general contractors. That alone can consume a year's vacation. Especially if one person is unavailable due to childcare. So be careful with self-work that prevents you from moving into the house. Because the double burden (rent and loan) must also be managed.

since the plot is on a slope

I would advise against a hillside plot if you don’t have €50,000 or more in addition to the necessary equity.
A hillside can be complicated, is simply not very enjoyable if you can’t develop it, and is often unpredictable.
In addition to the house price mentioned above, there are ancillary building costs, then outdoor facilities and kitchen/lighting.
Converting a utility basement into living space is, in my opinion, not the same nowadays as having a basement within the living area. A utility basement in living quality is usually not a living room (namely utility basement). Living rooms are clearly defined (ventilation option, natural light, 2nd escape route, ceiling height, heating). A utility basement does not have all or some of these things, even if you make it cozy with wallpaper and carpet. And these extra rooms must be paid for expensively. A children's room does not come out of such a room.

A flat plot will usually be more expensive, but it can be calculated. A simple standard house with a gable roof often (not always, because of statics) offers the possibility to convert the attic.
Keep searching or look at young used houses, possibly a townhouse. Those are usually more affordable anyway.

GU's have no static design adapted to the slope. The trusty local one might have already made adjustments there – from experience.
With us, back then, 20k was added for more reinforcement, more concrete, thicker slab.
For acquaintances, the removal of the excavation unexpectedly cost 20k more as well. So much for that being needed to level the property.
On top of that, all the retaining walls, topsoil, etc.

With good planning, you can manage to build a hillside house with no more square meters than you would plan for a slab-on-grade house. That doesn't sound like the case with you.

Maybe I should try to provide a few more details about the plot / house.
The total height difference within the buildable area is about 1.5 m. The plot is about 420 sqm.

The still unknown plot, mind you.

We also looked at the Flair 113 as a show house with a utility basement, which was converted into a living basement. However, the house was not located on a slope. From our point of view, the layout made sense and would generally fit well with the plot.

A basement not required by the plot makes little sense. And now what is it: utility basement OR living basement – do you mean a basement in BW living room height instead of 250 cm or more, and with light wells because of windows below terrain?

The opinions that Town & Country are not optimal for this case seem to agree here. For what exact reason, because of own contributions or to optimize the hillside location?

This provider calculates offers with full scope of services, but you need a shell construction general contractor (GU). This is more suitable both in terms of process and because the profit margins for "deselected" trades are never removed, which nullifies the possible cost advantage of the substituted craftsman / own contribution.

We have already had contact with a local GU, but the first price offer put us off a bit. However, we had planned something a bit different (granny flat, living basement height in the basement, ...). We have reached out again.

A granny flat is economically always nonsense.

An architect sounds much more expensive than a GU or something similar at first, or am I mistaken?

I expressly speak of a freelance architect for Module A. You can also commission a GU with your own architect (and you should never do it without consultation). You only build more expensively with " warns" architects (= floor plan painters performance phases 1 to 4 without practical construction supervision and billing experience). Read my "A house building roadmap, also for you: the HOAI phase model!" (my signature has been helping you find the way for a few weeks now).

So you would recommend towards a bungalow (like e.g. Aura 125), where the attic represents the basement? One floor above the ground floor would therefore completely fall away, correct? And the attic would serve as storage space?

Correct: (assuming a hillside location with access from the uphill side) the ground floor as "bungalow" ground floor, the upper floor ported into the basement. Above the ground floor hardly any more storage space is needed, insofar as the 12 extra sqm of the Aura compared to the Flair at the uphill side of the basement will still provide storage space.

The scope of services description by Town & Country is rather stripped down and "cheap" in many opinions. This is also currently being discussed in this forum.

No, it is complete for people who shop at Kaufland instead of Käfer. Here someone is just venting on behalf of the provider instead of on his own bad decision to go to a discounter with private insured claims – and is apparently proud that this provider does not interpret the term high quality through Gucci glasses:

Town & Country builds homes for statutory insured, and they do it properly in my opinion. "High-quality equipment" here means "only" impeccable specialist trade including hardware store goods – it is by no means meant to indicate top market leadership / benchmark setting. But this is really no secret, so you must be living somewhat behind the moon to complain about it. Town & Country is Aldi in the sense of Choceur, not in the sense of Moser Roth.

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Whoever decides to go with an architect plans a house explicitly tailored to this plot, without unnecessary slab rooms that you don’t need. The result is a house optimized to budget and plot. All this is done through consultation. Ultimately, it costs about the same or even less!

The essential difference is to complete Module A with the freelance architect, whereby the house is designed to fit the plot. I do not recommend the classic full scope "performance phases 1 to 8" in four portions (Module A – proofing with decision making – Module B – Module C) in this case, but "Module A – proofing with decision making – performance phase 3 and then with the construction advisor or architect to owner-managed regional shell construction GU". Please get in touch quickly if interested, vacation time is coming soon and afterwards the calendar for 2025 will already be full!

That may be more expensive per square meter of living space, but if in the end you manage with 140 sqm instead of 170 sqm, you will probably be much better off.

Caution: If the price is the same, then take the 170 sqm. Needs change, and what is well planned and fits today will be a "doesn't fit" tomorrow. Adapted from a well-known saying: Living space cannot be replaced by anything except more living space.

Caution: If the price is the same, then take the 170m2. Needs change and what is well planned and fits today will be a "doesn't fit" tomorrow.

I always do that too. If I have heated too much water, I freeze the rest, you can always use it.

A variation of a well-known saying: Living space cannot be replaced by anything, except by even more living space

Correct, houses with more displacement require less RPM and are therefore quieter, at least seemingly.