We are planning to build in 2022; the planning begins

Sampling is part of the house and can be specified

Sure. But to what extent. I will add a buffer for that.

It's not about keeping the money. It's about putting the money elsewhere in the house, e.g. upgrades. But as already suggested, I will simply plan the upgrades much higher.

If it's foreseeable that the amount won't be enough, talk to the bank early. Basically, plan a realistic financing. My additional costs, after I thought I had calculated all the upgrades, are currently at 8k.

Edit:
insists on actively requesting bathroom, electrical, tiles, etc. from the responsible craftsmen before signing and financing. Especially with the bathroom, costs can become very high very quickly. We also had x additional sockets planned as a flat rate. Also think about ceiling spots – I had them completely off the agenda or planned an unrealistically low number before I looked into it more closely.
It was also very interesting with interior doors and tiles. We had a tile budget with which we managed well after visiting the dealer and then negotiated the installation of 60x60 tiles included in the house price.

You can insert buffers wherever and however you want. In case of doubt, the bank will respond with higher interest rates or even a rejection, so it is better to plan precisely rather than too generously.

You can include buffers however and wherever you want. In case of doubt, the bank will respond with higher interest rates or even a rejection, so it's better to plan precisely rather than too generously.

I will still stay below the 80% financing. But if I only submit the offer from the general contractor, some things are still missing, which can only be priced after the selection process. Maybe I also have a misconception. But I understand it this way:
- Offer from the general contractor -> submitted to the bank for financing
- Not all items are included in the offer, or some can still be added
- For that reason, I would, for example, specify floors and painting at €20,000, incidental construction costs, which are mostly estimated anyway, with a 10-20% markup, etc.

So I know it as you disclose the overall calculation to the bank. That means you give them

(1) the offer from the GU
(2) a statement of the expected incidental costs
(3) a statement of the expected additional costs for sampling and special requests

Ideally, all three documents are stamped by the GU/architect, which lends credibility to the matter, although it represents a rough estimate. Then the bank should accept it as valuable. Anything else ("hiding costs") can rather backfire.

Addendum: The house construction project is a collaborative effort of many participants, including, for example, the bank. I would never deceive my project team members just because I expect to gain an advantage from it. That only jeopardizes the overall project. The more open you are, the better the result will be. My opinion.