Cost of a single-family house with a full basement, 1.5 stories

Yes. In the Rhein Main area, the square meter costs from 480. Up to 600 is all included, and you still have 30 km to ffm.

Rent is paid as interest to strangers, the monthly payment is interest paid to yourself, because this house in Berlin will easily double its value in 25 years. That alone makes it worth building.

OFFTOPIC: this house in Berlin will be exactly twice as valuable in 25 years, as a standard stock portfolio will be worth 3 times as much in 25 years! Notice something? Find the mistake :)

And no... I pay the rent interest to the bank or the landlord. That makes NO difference... or are you not quite clear on the difference between rent, interest, and repayment?

Yes. In the Rhein Main area, the price per sqm starts at 480. Up to 600 is all possible and there you still have 30 km to FFM

Strange, we pay 220€/sqm in Rhein-Main and are in FFM South in 25 minutes... so it's a bit too general.

And I consider the price for the house too low.
Ours is slightly bigger and now after selection we pay 323k (without sanitary objects, without floor coverings).

I'll take the stock comparison as a side remark.
Regarding rent versus repayment. Example rent 800,-, annuity 1000,- as of 2016. The interest portion in the annuity is currently low. In 12 years the annuity is still 1000,-. Rent perhaps already 1500,- And the repayment is included in the annuity, which is constantly increasing. The house becomes more and more mine. It will never be that way with the rent. Karsten

I recommend reading "Kaufen oder mieten?: Wie Sie für sich die richtige Entscheidung treffen" by Gerd Kommer.

denn dieses Haus in Berlin wird in 25 Jahren seinen Wert locker verdoppeln. Schon das ist das Bauen wert.

And I would also like to have the crystal ball you are using.

On the subject of Berlin "The value of Berlin real estate will hardly increase until 2030" 4 days ago in the Berliner Morgenpost; this refers to the Postbank Study Wohnatlas 2017.

So, it doesn't look so rosy everywhere. Because the only thing that counts is location, location, location.

Seems a bit tight to me. Not much... depending on what the basement costs. Definitely plan a buffer. 30k would certainly not be bad. Outdoor area is also completely missing.

It doesn't seem too expensive anyway. If I subtract the basement with 50k... that leaves 214k for the house. That would be 1860 EUR per sqm. Should be normal standard..

Objection, not quite right. Less would remain for the house, since otherwise, if I haven't overlooked something, no further incidental construction costs, such as earthworks (quite complex with a basement), are indicated. If these are included, that's certainly a "value" of at least 15-20 k€.