Construction costs for 120m2 excluding land

I thought so too - then I read the profession of the OP in his profile: "Banker"

Of course - the advertising photos exist only once for the entire house model, not for each equipment variant.

Am I a rogue if I involuntarily have to think of a thread from last month where someone asked if one can distinguish ground lease from purchase at first glance?

Not every banker finances houses, let alone reads a construction specification.

I was there again today for measuring. The section of the property intended for me is 15m x 36m. According to Boris NRW, the standard land value there is 180€ per m2. So even at 97,000€. And yes, there is already a house on the property which my mother lives in. The ground is not completely level; a tree and a garden shed will probably have to go.

And yes, there is already a house on the property that is inhabited by my mother.

Will you be registered in the land register?
Will your area be separated so that an independent property in terms of the land register is created for you afterwards?
Are you even allowed to build an additional house there?

I recommend this reading if you really want to build on your mother's property:
https://www.hausbau-forum.de/threads/Haus-auf-Grundstück-der-Eltern-Erbschaftsprobleme.33972/

Can only advise you to look at everything very carefully and check it thoroughly. With family, I would even double-check. If something is wrong, it can quickly lead to disputes.

Your mother would separate the above-mentioned section of 15x36m² from her property and sell it to you for 15,000€? She keeps the other part with her house on it? Is that correct?

Be prepared for the fact that the real estate transfer tax will still be due on the 100k€.

In a sale between persons who are directly related in a straight line, no real estate transfer tax is payable.