Why building a house almost always costs more than calculated

Maybe someone can help me in this thread, no idea where else I should place the question

We have an offer from the 1st GU, which will also be compared.

However, I can't make anything of the following "optional" items provided by him:

- Antenna, network, and telephone cables in empty conduit system (€2,600.00) --> Isn't that "standard" included in the house price? Or is that maybe a separate trade that I would have to calculate myself in the additional construction costs? What exactly is meant by that?

- Heating system filling block (€800.00) --> No idea what is meant by that.

For refilling the heating system, a system separator (prevents backflow of heating water into the drinking water network) along with a pressure reducer must be installed. According to the state of the art, a cartridge for demineralizing the refill water should also be integrated.

- Antenna, network and telephone cables in empty conduit system (€2,600.00) --> Isn't that "standard" included in the house price? Or is that perhaps done by a separate trade, which I would have to calculate myself in the ancillary construction costs? What exactly is meant by that?

It depends on what is included in your standard scope of work description. This is also installed by the electrician. Usually, only the lowest standard is included in the telecommunications section of the scope of work description. Something very important is missing in your text, the quantity and execution!!! Are there 5 or 25 network outlets included? Connections for 1 or 4 TVs and where will the wiring go? You should also do that with the sockets in the normal scope of work description. The number is usually well below the standard that is installed nowadays.

We have an offer from the 1st general contractor available, which will also be compared then.

If a corresponding construction service description is posted here, many helpful forum members will point out any potential pitfalls. Just be brave!

... especially nasty: You basically do everything right and inform yourself in advance about the standard of the GU at the respective companies (in our case: stairs) and there you are given "wrong" numbers for the surcharges. Very, very annoying, that. But you see: A proper buffer is really worth a lot

I am slowly realizing that my planned buffer of 5% is unfortunately much too small. Is it allowed to post the construction service description from the BU here in the forum and get advice? You never know who is reading along and if they see their offer here 1:1, I think there will be trouble, right?