What are the approximate total costs of a simple house?

I can estimate almost all cost items quite reliably.

Respect! Our civil engineer, who is a developer himself and has years of experience, can't do that… But yes, luckily the market is currently very stable, in the post-corona war time.

Currently, we already have a family home from a transfer within the family. However, it is somewhat older and has many defects despite extensive renovation.

The bathroom doesn’t cost me 5,000€, that comes from the family, at least when it comes to the craftsmen.

Find the reason for my broad grin ;-)

I would like a wooden house from the company "Thule-Blockhaus".

Then I’ll give you two reference points in the form of this thread for your project: and – furthermore, I call into the discussion for you.

Have your dream house offered, for example, by Scanhaus Marlow Marlow or Town & Country: if you deduct the wages of the finishing trades from their prices (including possibly "builder-side" but necessary items), you will know your realistic potential price ceiling; I consider any deeper Limbo to be illusory.

Respect! Our civil engineer, who is himself a developer and has years of experience, can't do that... But yes, fortunately the market is currently very stable, in the post-corona war period.

Well, quite certain because I get a quote from the companies. In the end, I still have to have a buffer upwards. For example, I thought that a small sewage treatment plant would cost around 12-15,000. In the end, however, it was only 8,500€. But with the heating system, I completely misjudged at first...

Internal development?? Do you mean the interior fitting?

Who brings the electricity from the street into your house? And water? And telecommunications? ==> Developed property only means that there is a street and the corresponding things are located there. You have to pay to have it dug up and installed.
The building application should be available on your provider’s website - but it only costs a "few" thousand euros. Just like the building permit.

And suddenly 10,000 euros or more are gone again...

That’s how it will continue.

... éventuelle Versicherungen ...

Our insurances related to house construction, concluded in February 2022 (except for the home and landowner liability insurance, which we took out in 2019 when purchasing the plot):

* Regarding the builder’s risk insurance, I would like to point out the "extended subsequent liability," which in our case only cost about 60 euros. Whether this is necessary, I do not know; it was a requirement of our house manufacturer.

Home and landowner liability insurance

annually

€48.80

Residential building insurance incl. fire shell construction insurance

annually, premium-free until move-in

€329.66

Builder’s guarantee/deposit insurance	one-time	€208.95
Builder’s risk insurance with extended subsequent liability (12 months)*	one-time	€853.53
Builder’s liability insurance	one-time	€236.57

We did not look for the cheapest provider but wanted, due to age reasons, a local employee who manages all the insurances here. Certainly, all insurances are available (significantly?) cheaper.

So I buy the plot of land first. I think that makes sense. And then I give the construction order for the house and foundation slab. I can estimate almost all cost points quite reliably. However, not the incidental construction costs...

Hold on (now I'm talking) – do you buy the plot separately, or is the plot from the house provider? If you buy it from the same company, it doesn’t make sense to buy it separately beforehand as far as I know – there are already like a gazillion threads in the forum on this topic, and the consensus is always: the tax office isn’t stupid. By separately I mean you buy the plot for example from the municipality or a private person. And then you look for a construction company to build something on it. I can help you a bit with the incidental construction costs for Lower Saxony, we built here. However, I have to agree with my predecessors, I also think you are underestimating some of the costs. Where do you get the 30,000 EUR figure for the heating from? Should everything be included, including underfloor heating? Unfortunately, I have no idea about the finish levels of such a wooden house and what you have to do yourself. The incidental construction costs for our construction project included: insurance, chimney sweep (may be omitted with heat pump), building notification, utility connection costs (telecom, water, electricity, fiber optic), energy consultant / expert (if necessary), costs for wastewater / rainwater connection, surveying costs. Also, bigger items that I miss so far in your post (regardless of the fact that there is generally very little info): outdoor area / paving? earthworks? kitchen? bathroom for 5,000 EUR?! floor coverings? painting work?