Full financing, or rather not?

Hello.

All earthworks. From my point of view, it is not much either. The property slopes 1 meter over 25 meters. Strip foundations with frost skirts are planned. As extras, I like good floor coverings, such as parquet, granite, and marble in all rooms, but we are getting those sponsored. For example, I don't need a wood stove; I've been cured of that.

Uh,
Firstly: everything that does not need to be financed because it is given as a gift counts as equity. In this case, it is probably to be recorded as own contributions, which leads to better conditions.
Topic earthworks: No, certainly _not_ all earthworks are included. Costs for earthworks can range from 3000 to 30000. A construction service description has a limit, usually 30cm... In your case, your gradient already exceeds the limit.
But you shouldn't care about what a small home builder calculates for you.
No construction company agrees to as many own contributions as you imagine.
Here the family, there the friends... The friends surely also want to be paid?!
In your case, it makes sense to build an architect-designed house and build freely, then you can award the trades to whoever you want.

However, the additional costs for a single-family house do not become lower, after all, your insurance and property tax come in annually.

Building insurance and property tax are transferable, i.e. already included in the additional costs, if the landlord is not stupid and has not forgotten it.

Building insurance and property tax are chargeable, i.e. already included in the additional costs if the landlord is not stupid and has forgotten it.

Yes, you're right... I just looked it up again.

Nevertheless, the OP talks about 400 euros, and I think you should also calculate with these costs for a single-family house.

I think the discussion about what the house costs and the question whether the financing is feasible should first be separated. Otherwise, this won't work.

1) 400,000€ with your income, without knowing other key data like debts etc., is realistic for now. We have a household income of 4,500€, two children, and have financed 325,000€. It works perfectly. You should just make sure that the bank installment plus additional costs do not exceed 1/3 of the household income. I find that a good rule of thumb. Here it is also best to get advice from the house bank (not some financial experts).

2) We have enough experts, architects here regarding the house costs. I’ll stay out of that as a simple builder. I can only say that we are building 170m2 in the Hannover area for 249k€ including all materials. Without basement/garage. We contribute labor ourselves for painting, insulating the roof, and laying the floor. Or rather, we did.