Change floor plan after signing construction contract

We would get out of the contract if we have not found a suitable plot of land by the end of March.

So you should already start looking for a plot of land calmly now and only then go to the architect.
Let the “plot service” expire. It usually leads to you having to pay the real estate transfer tax on the house as well. You could pay many architects for that.
Oh yes, the general contractor delivers a plot in 98% of cases anyway not, because he has to search and find it on the market just like you.

What would it mean if the floor plan were changed again now? I assume that a corresponding surcharge for the changes would be applied to the fixed house price in the contract, right?

With near certainty, work will have to be done on the floor plan and house design anyway, since the house is planned according to a development plan or the surrounding buildings, not the other way around.
In any case, more costs will come my way. Just drop this approach.

Nice that after licking your wounds you have returned from the inevitable reproach :-)

That's right, it is a contract with Grundstücksservice. So far, we have also been offered various plots of land by the construction company, which we have declined because they did not meet our expectations. [...]
We would get out of the contract if we do not find a suitable plot by the end of March.

Well, then I first wish you lots of bad luck with the land search until the end of March, and otherwise recommend here reading smartly with the search terms "Grundstücksservice" and "11ant Leerverkauf".

maybe add to the list that there was a segment about it on ARD plusminus this week - for readers who are more convinced by audiovisual means ;-)

@11ant maybe add to the list that there was a segment about this on ARD plusminus this week

"Trouble with prefabricated houses," from the 17th of this month – it also features a customer who, although an architect herself, "decided on a prefabricated house because she believed there would be less construction stress" (even though she should know about the non-existent "connection"!). By the way, the title is unfortunately poorly chosen: although the phenomenon was pinned on a (single) prefabricated house provider, it also affects the sales of traditional brick-and-mortar house providers (and there, too, the criminal energy is said to have originated with the sales agents); the main focus of the segment is the bait-and-switch with the pre-sold plots. The mocking term "fridge contracts" was new to me, but I find it quite fitting ;-)

We would get out of the contract if we have not found a suitable plot by the end of March.

Do you really get out of the contract that easily? The company can't be blamed for you not liking the offered plots. I think you will be left with a not insignificant amount.

The company can't help it if you don't like the offered plots of land.

Well, insofar as they don't actually have any themselves on hand, a little bit. That is then the advantage of the customers, that the salespeople are not serious enough about it and not willing to invest time to be successful at scouting plots of land. And they are also not really skilled at it. That's why they regularly offer "replacement plots" that are significantly removed from the original bait location and also regularly do not allow or enable construction with the intended house model.