Floor plan - 135 sqm, 1.5 storeys, gable roof

If he is referring to 135 sqm? I can already see them

I would also be afraid and would clarify that first of all.

How did the whole thing actually come about? You signed a contract with the construction company (I don't think a developer) before you had the plot of land, which then didn’t materialize? That's why you are tied to the company for the new plot of land, which now wants the contractually agreed money?

If he is referring to 135 sqm? I can already see them

I am wavering - in the worst case you are right. That would mean planning 125 instead of 135 sqm - one could live with that too and if the OP thinks that is worth the peace of mind, to not have to renegotiate the financing anymore, then her will shall be her kingdom of heaven, we don't have to talk her out of it.

Where were you all day? Wonderfully summarized,

Wonderfully summarized: I was enjoying the weather.

So the DIN 277 doesn’t help here, correct?

He should clarify in the contract: either DIN 277 or Wohnflächenverordnung or a separate but then plausibly presented method of determination.

You signed a contract with the house building company (I assume not a developer) before you had the plot, which then fell through?

That’s how I understood it too. That several people – myself included – wouldn’t have done it that way, that has in my opinion already been discussed thoroughly.

How did this whole thing come about? You signed a contract with the house building company (I assume not a property developer) before you owned the plot of land, which then didn’t work out? That’s why you are bound to the company for the new plot of land, which now wants the contractually agreed money?

Not quite. We had a simple preliminary contract, from which we could have gotten out at any time (it was tied to finding the plot of land). Then came the plot, financing, etc. So as part of the financing, we also concluded the offer/the actual contract with the house builder so that we could finance both together (the bank just wanted to see/have everything in detail). We hadn’t anticipated that the plot of land wouldn’t work out. For us, everything was clear at first.

I assume that even now I could get out of the contract, maybe a penalty would be due and I would probably have to pay initial services (the planner has already started, etc., etc.)... only: I am basically not negatively inclined towards the house builder (and of course I don’t want such unforeseen costs). If it weren’t for the living area issue, I wouldn’t have a problem at all... and that’s what I now have to resolve. (btw: we found the new/current plot of land ourselves, not the house builder).

You have a contract with the house construction company (I think not a property developer)

By the way, you are right here... I didn't pay attention to the term in the heat of the moment. It is a house construction company that we deliberately chose and with whom we would basically like to build.

One more question: where do I actually get the floor area ratio if there is no development plan and nothing is stated in the statute about it? Can I then do whatever I want? Or do I now have to ask the neighbors (regarding "like neighbor construction") how much area they have built on?

Where do I actually get the floor area ratio if there is no development plan and nothing is stated in the statute?

Without local regulations, the federal legal framework applies, which you basically do not need to worry about, as long as your house will occupy less than that. That means a development plan or local statute does not create the framework here but merely defines it more narrowly if applicable.