General procedure / schedule

Yes, that's exactly how the answers will be.

It happened to me too, a company had a contract clause that we didn't like.

A blunt answer: Our contracts are not changed.

Why should a company at this point in time deal with someone who has different wishes when there are a hundred others who simply accept it.

You can even tell when offers have been made, many companies do not ask if you do not respond.

and not pay 10-20% more for subcontractors

So individual contracting. Subcontracting is common outside of individual contracting.

What I mean is that a basement commissioned through a prefab house provider and accordingly subcontracted costs 10-20% more than a basement I would commission directly from the basement builder.

That is a superstition. On the one hand, the prefab house provider works with a batch size >100 against your batch size of 1. So they get a significantly better price from the basement builder. We had a price from Hanse including basement, listed separately back then. For that money, as a private builder, you won't get a basement on the open market.
On the other hand, I know of the handful of prefab house providers I know that none of them builds on a basement provided by the builder himself, if only because of warranty and interface issues.

And I don't believe that the prefab house provider will let us go just because I commission the basement externally.

You believe? Have you already contacted companies and outlined your project?

The planning is done for the entire house, including the basement.

If you order without basement from the prefab house provider, why should they plan including the basement then?

Then the house building industry must be doing extremely well

Yes, the house building industry is indeed doing extremely well. Thanks to low interest rates, child building grants, and throwing KfW money around. But that’s been going on for a few years now.

Basically, we’ve had a supplier market for quite some time. That means the low-hanging fruits are preferred, and you with some special requests simply do not belong to them. Prefab house construction is highly standardized both procedurally and technically, and deviations from the standard initially cost the company resources that could be more profitably used elsewhere.

So no, no one currently has to "go the extra mile" for you.

Let's think about it, it’s about a value from €350,000 to €400,000, whereby most of it is actually called off at the shell construction stage (which is quite quickly completed). Now I come and say Point A38 scaffolding not supplied by the builder! Do you really think they would say no?

Yes, but not out of arrogance or because times are so good, but because your contracting partner earns their money from the difference between construction and sales price. If a client arranges the scaffolding and it is not available at the right time and in the right quality, it eats into the margin, and significantly so. Without profit, the prefabricated house manufacturer doesn't need turnover either. It's about a risk assessment.

2. Looking for an expert who simply supports us during construction. Here rather someone from the Bauherren-Schutzbund or someone like the "Fertighausexperten"?

We had the Bauleistungsbeschreibung (Bauleistungsbeschreibung) checked by the Fertighausexperten. We have not yet decided whom we will take as a construction supervisor.

3. Obtain offers from the prefab house companies, the basement builders, the photovoltaic system, etc. For this, I have already created a list of points so that not many additional costs appear during the sample selection, but are included in the first offer (otherwise, it would be difficult to compare).

Despite attempts with identical specifications to the prefab house manufacturers and a lot of effort and talking, we could not obtain comparable offers. My impression is that the sales staff "just do their own thing." I also think that comparability of offers in this industry is fundamentally not desired.

Of course, we will carry out a preliminary sample selection to be absolutely sure.

Unfortunately, we have not found a prefab house provider that made this possible. However, it would be great if you manage it.

What order makes sense here? Meaning prefab house provider first, then basement builder, then photovoltaic company, then builder, then stair builder?

I would start with the prefab house provider.

5. Then also the price negotiation. Should you also seek professional help here, and if so, whom? Would that person also be involved?

There seem to be professionals for that. You have already mentioned the "Fertighausexperten." According to the website, they are transparent regarding costs.

Short addendum to the batch size We don't want to build with a provider who builds >1000 houses p.a. anyway, but with a smaller one around 200. Since many build without a basement or in other federal states, the batch size is definitely not >100. You may be right that we could come out more expensive but I can find out by doing a price comparison. In addition, I have also seen that my 4 favorites do not offer their own basement option. I will give feedback on that when I know more. Well, just because 3-4 FHA do not build on external basements now does not mean that it is the rule. I think the big companies rather do not do that but I don't want to go to them anyway. There, I am just a grain of sand in the glass. Besides, it somehow makes little sense because the basements are also commissioned externally. So there is 0.0 difference, only the client is different. I haven't had contact yet but as I said, I will give an update on that. That will certainly be ready in the next 1-2 months. Yes, it is certainly true that the industry is doing quite well. But as I said, I don't want to build with 1000+ providers because then my wishes still have a certain weight. And you also have to consider that right now perhaps some are hesitating with house building because of Corona. The child benefit for construction and construction subsidy are anyway not available, so I also don't believe that all order books will be full in the next few years. That is at least my opinion... or are there really evidences for that, actual evidences or just all personal opinion and theory? Thank you Now I also have a question. What came out of the construction service description and contract review? Obviously comparisons are not desired but did you have no possibility? My idea was to send a floor plan along with an exterior view, then list the many small extra wishes like e.g. 70cm roof overhang, windows & doors in RC2 optionally RC3, wood fiber insulation, venetian blinds, snow guards, "barrier-free" construction with doors (90cmx211cm) and much more. Then I should get an offer. Afterwards, then a preliminary sampling or a visit to the construction studio (which is offered without any problems). There I can note everything I want, compare it with the offer and change and add points. Then submit the whole thing again to the company and ask for an update.

You are reading many factual, good, experience- and knowledge-based hints here. Unfortunately, it is not what you want to hear. Therefore, you either ignore them or "refute" them solely with assumptions and "belief"... try to reflect on that.