KfW 124 subsequently for the construction loan

One could of course have thought a bit beforehand about whether and to what extent additional costs might arise... It is clear that one cannot know exactly down to the cent beforehand, but at least roughly.

Just got forgotten. It's a shame that the bank advisor didn't notice it either. Now the supplementary financing has to be arranged before construction begins.

Yes, the banks expect a calculation and a list of items, and as a layperson you submit these inaccurately, and then there is no indication from the knowledgeable mortgage financiers that something is still missing.

Annoying that you have to provide additional financing.

Before you make the next mistake, please start with a list of all the "additional costs" you have included in the loan amount. Post the list here, then we can tell you what else is missing. Only then can you estimate the missing volume.

I mean: Construction power is billed in installments, which is not expensive at first. What is expensive is the one-time installation of the connection and, if applicable, consumption costs for seasonal construction dryers, electric screed heaters, etc.

This, of course, raises the question of which utility connections you have even considered and which you have not. Connection fees of €1000 and more per utility are more the rule than the exception.

What about surveying, which usually consists of 3 components:
1. Surveying and creation of site and elevation plans
2. Measurement at the start of construction and setting out of formwork
3. Measurement after completion for the cadastral office.
All together easily several thousand euros!

And this continues quite easily throughout the entire construction period with additional costs, none of which are included in the house price. Therefore: what have you calculated and what not?

The property is located on sloping terrain, it must be properly filled...

That is where the biggest trap lies, if the earthworks on a hillside property were not properly calculated. Make sure to clarify this, compared to that the construction electricity is peanuts! And please follow the advice of and provide a complete list of the "additional costs" you are aware of. I alone paid 1000€ just for laying the drinking water from the development point on the property into the house, and I still had to have all the earthworks done separately – the water supplier just laid a few meters of pipe and connected it at both ends. That probably didn’t even take an hour.