Architect's house in Pforzheim - Offer too expensive?

We have about 1200 sqm of land. 57m deep and the house goes up to 23m. Outdoor area (including driveway, trash bins, fence, gate, terrace, platform, roll lawn, sprinkler system, paths, garden shed, small outdoor kitchen, sandbox, greenery & plants..trees).. I hope 100k-150k

But it will probably be more

Hello again,
first of all, thank you very much for the answers. But I immediately have another question, or rather an idea, and wanted to hear if my calculation could work out like this, or if I am overlooking something.

At the moment we would have to finance about 530k with 300k equity.
At an interest rate of 1.49% and 2% repayment, that makes a rate of about €1550/month.
Now the idea was that, since the development plan allows it, we could add a full floor on top and rent it out.
Estimated cold rent here would be about €900-1000 for 100 sqm living space.

Let's assume €150,000 would be enough to build a 3rd floor.
Then we would have to finance 680k and would pay about €2000/month in installments.
If I include the rent earned, I come to just under €1100/month, ignoring taxes and depreciation.

Would that be realistic? Am I making a mistake somewhere?
I mean, I pay the incidental building costs anyway. In this case, I can hardly get rentable living space any cheaper.

The only "problem" is that you basically share a house with the tenants.

What do you think about that?

Regards, Steffen