Construction project, estimate construction costs?

No, the children will only come once the house is built and everything is livable.

That means if I have the house built in a "package" by a company, do I pay the real estate transfer tax on the entire amount?

The state needs money, and it collects some of it through the real estate transfer tax. In recent years, this has led the tax authorities to increasingly focus on the connection between land acquisition and the construction of the building project. If a provider sells land and construction as a package, the real estate transfer tax is calculated on the total amount in any case. However, this can also be the case in other scenarios: For example, if a plot of land is reserved for a general contractor and the buyer acquires the land directly from a municipality, the tax office will still collect the real estate transfer tax on the land and construction costs if (which is usually the case) the builder contracts with the general contractor. Even if a reservation in favor of the general contractor does not formally exist, but the general contractor only establishes the connection between the builder and the landowner if the builder also builds with this general contractor, the real estate transfer tax can similarly be charged on the total amount. The tax office is also keen to fully apply the tax in comparable cases with architects (instead of general contractors).
If you want to avoid this, you must first and only buy the land and then - in a temporally verifiable and completely independent manner - subsequently make the building and building contract decision without any connection being constructed between the two transactions.
This issue should also be checked in advance if, for example, a municipality imposes a building obligation on you to be fulfilled within a certain deadline when buying land.

Okay! Thanks, that is good to know! We were planning to buy a plot of land as soon as we found a suitable one. That can be quite difficult at the moment. After that, the detailed planning phase should begin.

No problem. If - now or later - you have any questions about financing, just feel free to ask. There are a number of models that a "new" builder doesn't necessarily think of right away.

Hello! The 300€ are not living expenses. They are groceries and daily purchases, or the average over the last 7 months. We were both students and know how to manage efficiently.

Wow! I was also a student and know how to manage efficiently – but that has nothing to do with saving and "living"! That statement is a bit arrogant. Are you going to take over your parents' company? How is the situation there? One thing is to get the financing (possibly before starting as self-employed), the other is the actual income that will come then. Is the company doing well? Are there fluctuations, etc.? You have to decide that for yourself and be sure that you can afford to pay yourself that much money monthly (4000 net is about 8000 gross for the company) as salary.

Belief with the student only referred to the fact that there is no money for anything other than toast, not that students manage their finances better than others...