How do you start? When do you assign whom with what?

I’m going to be very mean now because I cannot stand it at all when someone takes a quick look and asks questions without following certain life rules (e.g. at least reading through for 1-2 hours, adhering to house rules or similar before asking questions that are actually not well thought out at all) I don’t see this “I have already informed myself” info at all in this question.

With the array of expressed wishes à la "One with everything", meaning fully equipped ... ...

Men like to dream about their Latte
For some, it should remain a dream

A sensible approach looks like this. What can I afford? If that’s clear, then what do I get for it?

And since some plots already cost these 300,000, not much remains. You can figure that out yourself by doing 10 minutes of homework skimming through and then asking further questions accordingly, to which there are smarter answers.

I’ll put it this way: I have made up my mind to win the lottery. I already know what I will buy with my money. However, I don’t know exactly which numbers I have to pick yet. Unfortunately, I only have 5 €.

This is quite a strange reception here.
My assumptions for the costs are based on various websites where the costs for such a house are given between 150,000 and 250,000 (Without land). Land costs are hard for me to estimate, since I don’t know how many m² I would need, and also in the area I would like to build, I have not yet found any offers without an already existing house.

I have simply included additional costs for a heat pump in the estimate based on gut feeling, since I also don’t know which model I would need, and prices for it are hard to find on the internet. I assumed that it would ultimately cost more than a gas heating system, but not much, since I can save on the entire gas connection and the chimney. I would do the smart home system myself, I’m an IT specialist. Only the cables would, of course, have to be installed beforehand. A photovoltaic system would probably cost around 10-15k, and should pay for itself after a few years.

I have now found a plot of land via gdi-sh where I could imagine that it fits. How do I now know whether it is for sale or who owns it? The information I have is a parcel number, the standard land value (115€), development status (ready-to-build land), type of use (residential building area). The size of the area is not stated. It seems to have been vacant for a few years already, which surprises me somewhat.

Ask the municipality who owns it. Ask if it is for sale if you have the owner. Many plots of land are vacant as capital investments because there is zero interest. They are mostly unsellable because the speculation period is not over. The profit would have to be taxed. The standard land values reflect the past; they are based on prices paid. If 115 is stated there and currently nothing is on the market, the seller may possibly realize 215.

This is a strange reception here.

Don’t take it the wrong way, we also found your questions "strange."
But it's nice that you don’t get discouraged and respond again.
Every beginning is hard.

My assumptions about the costs are based on various websites where the costs for such a house are stated between 150,000 and 250,000 (without land).

And how many square meters of living space do they have? 100m² is not enough for 4 people. Also, these houses have no basement, no photovoltaic system, no garage, and not even a carport. Are you familiar with the term ancillary construction costs? These are things like earthworks, which are ALWAYS extra on top. In addition to the incidental purchase costs, of course. With your wishes like a basement and garage, you will never (!) get under 400,000.

I have now found a plot of land through gdi-sh that I could imagine might fit.

That is a much too academic approach. Just look on ImmobilienScout, at local banks like Sparkasse or Volksbank, as well as in the weekly newspaper. Count on 100,000€ for a decent plot of land.

This is a strange reception here. My assumptions for the costs are based on various websites where the costs for such a house are given between 150,000 and 250,000 (without land). Land costs are hard for me to estimate because I don’t know how many square meters I would need, and I also haven’t found any offers in the area where I would like to build that don’t already have an existing house.

I just included the additional costs for a heat pump based on gut feeling in the estimate, since I don’t know which model I would need, and prices for it are hard to find online. I assumed in the end it would cost more than a gas heating system, but not significantly more since I can save on the entire gas connection and the chimney. I would do smart home installation myself; I’m an IT specialist. Only the cables need to be laid beforehand, of course. A photovoltaic system would probably cost around 10-15k and should pay for itself after a few years.

I have now found a plot of land through gdi-sh that I could imagine would fit. How do I know if it is for sale, or who owns it? The information I have is a parcel number, the standard land value (€115), development status (building-ready land), type of use (residential building area). The size of the area is not listed. It seems to have been free for a few years already, which surprises me a bit.

Then I say welcome to the forum and can already tell you that your budget will not be enough. I calculated 2,000€ per square meter of living space 1.5 years ago. For the basement, 65k for 70 sqm of living space. Plus additional construction costs of 30k€. Kitchen, furniture, outdoor facilities (for a flat plot I would estimate 20k€). A double garage costs roughly 30-40k€ depending on the design. KfW+40 will cost you another 20k€ additionally.

So now tell us what number you came up with for your square meter estimate?!

And all that still without land

You are a strange computer scientist...actually they are good with numbers..