Land available - ancillary construction costs, ancillary house costs, financing?

Consider carefully whether you pay for the building plot directly from your equity.

Why? There is equity returned once the house planning is done and the overall financing is set up. I see no reason to pay interest on a loan that is not needed. Unless the OP has magical, short-term investment opportunities that yield a lot of return.

How? If you pay the building plot "in cash" from the equity, nothing comes back, right, or am I misunderstanding?

You (and the bank) look at the overall project.

Purchase price of land including incidental costs: €120k
Construction costs: €400k
Construction incidental costs: €50k
Kitchen: €10k
Carport: €10k
= total €590k = use of funds

Source of funds is

Equity €110k (or €120k if, for example, you get the rest paid back in a year)
Bank loan: €480k (or €470k)

Of course, you can also tell the bank that you only want to use €90k equity, the rest is a reserve.
Then equity is €90k and debt capital €500k. The bank would therefore pay out €20k/30k to you as the first disbursement.

Unfortunately, that's true. We were at the kitchen studio the last few days. Nothing will work under 15k. Since that seems way too expensive to us (kitchen studios are like general contractors with glossy catalogs and model houses -> everything is reflected in the price), we will have our kitchen "tailored" in Poland (we both speak Polish). There you are very well served with 10k€. With 15k€ you have a dream kitchen

It always depends on the requirements. Some buyers spend over 15K EUR on electrical appliances alone, and I am not aware of any Polish manufacturer who delivers high-end products for a fraction of that, but I am not very familiar with the Polish household appliance scene.