Property development - basement yes or no?

One knows the usual suspects when one has read quite a bit there oneself.

Why are you actually fixated on a [Fertighaus]? Fast, simple, cheap, all from a single source?
These are just advertising promises, the reality will differ significantly.

We are not completely decided yet. We are also having discussions with architects. But the biggest point in favor of a prefabricated house for us is the price guarantee. We are afraid that if we build with an architect, the prices for the components will rise so much that our planning will no longer be feasible. We have already spoken with an architect who was highly recommended to us and is known for being very precise with his cost estimates. But even he told us in the preliminary discussion that he can no longer guarantee this in the current situation, as they themselves are repeatedly surprised by price increases – especially in their magnitude. With prefabricated house providers, we believe we can plan better with the stated price. We make sure that the price guarantee is longer than the construction period guarantee. But as I said – solid construction is not completely off the table.

If you want a basement and the prefabricated house provider does not offer it - would that be a deal-breaker in the selection?

EVERY provider who does not deliver what YOU want should be eliminated from the selection The question is at most how budget, requirements in the development plan, and wish list can best be reconciled ...

I think you are mixing up some topics. There are basically two ways to build:
1. with a general contractor/general constructor and a fixed price. In this case, a builder takes on everything and guarantees you a price according to the construction service description.
2. with individual contracts: here you (or the architect) make a tender according to the wishes and then award the individual trades to the craftsmen with the best offer.

Prefab house manufacturers are also just a special variant of general contractor; the basic principle is the same in solid construction. However, the decisive factor is that on the one hand the usual standard meets your desired standard, and on the other hand that you later make few upgrades. These are relatively expensive with the GC.

With many prefab house manufacturers, price control is also just an illusion. It starts in the offer phase; for us, the first offer had differences of sometimes well over €100,000 compared to the catalog price even though we did not add any luxury upgrades. Then there are things in the offers where you can only shake your head. €100/sqm surcharge (!) for vinyl flooring instead of carpet from the standard, it must lie on a base of gold leaf. That’s when the customer is milked. Sure, you can remove that and contract it yourself, but then you also become a private construction manager.
Earthworks are usually never included in prefab house providers. Often the costs for things as basic as the architect or waste disposal are also missing. The latter is a particular fun; an acquaintance then spent evenings sorting the garbage containers because mixed waste is the most expensive.

Long story short, just because something has "Prefab" in the name doesn’t mean it actually is. And a price guarantee always depends on what exactly is included.

Sorry, I actually used prefab house providers synonymously with GU :).
So our favorite is to build with GU/GÜ (what is the difference there). We are quite open about whether it is timber frame construction or solid construction.
So far, we have only had initial discussions with various providers. We understand that the additional features can be a pitfall, although there are also providers who, for example, allow preliminary selection or provide exact cost information for additional feature components in advance.
100,000 euros is of course brutal... May I ask who you built with and whether you were satisfied apart from the additional features?
Also happy to receive it via private message if this is not desired here.
We always appreciate recommendations ;)

We then decided against one of the major providers and chose a smaller [GU] from the region. So far, I am quite satisfied; we had a few delays during the development and therefore lost the price guarantee while prices simultaneously skyrocketed. However, the [GU] had mostly secured fixed prices on their side, and we managed with a small surcharge (€1500) for the carpenter. It would certainly have looked different with a prefabricated house provider. It was also important to me that the [GU] takes care of almost everything: earthworks, energy consultant, architectural services, construction electricity.