Application phase land - How do I finance everything?

That is too late. You don’t have to and cannot plan the house in advance, but a rough calculation should definitely be noted down. Also, basic knowledge such as KfW and the Energy Saving Ordinance should be included. Then you have a rough idea of the house and costs, which can be aligned with salary, equity, and bank.

He has done all that. The rough calculation here should be enough to set the traffic light to green or red. They have enough equity and, in the best case (which unfortunately is the case nowadays with point systems & Co.), can buy the land with variable financing. Framework conditions like a wedding, etc., should of course also be clarified for such a project.

He did all that anyway.

True. I thought it wasn't the OP with the numbers, since they are old numbers. You can't calculate 3 years with €2000/sqm if the house construction will only take place in 2 years. The incidental construction costs were also not there. But somehow I was probably distracted while reading ops:

2. Currently, we only pay the interest, do not amortize, which means we have more equity available when the financing of the house begins (whether that brings any benefit always also depends on the amount of equity itself and on the bank)

Isn't that a zero-sum game? In house financing, it doesn't matter whether the equity contributed is available as cash (as in your case) or as a debt-free part of a property (as it would be if you were amortizing) – right? (Or from my perspective, your model could even be more expensive because, due to the lack of amortization, you have a higher remaining debt and pay more interest on it.)

A question of strategy - generally, repaying debt is certainly more sensible. However, it may be that more equity is needed for the kitchen, etc., and one wants to keep this "available." Certainly a case-by-case consideration.

A matter of strategy – generally, paying off debt is certainly more sensible. However, it may be the case that more equity is needed for the kitchen, etc., and one wants to "keep this available."
Certainly a case-by-case consideration.

That is of course true. However, in my view, the statement that by not paying off debt you have more equity available is still incorrect. You only have different equity available, namely cash.

Please correct me if my assumption is wrong. I also only have a layperson’s perspective on this, but I am very interested, as I am aiming for a similar model (financing the land with a variable loan and then repaying it with the home loan).

Yes, exactly. The equity in the property is of course like equity in home construction financing for the bank. You obviously can't get a kitchen, a car, or a stroller with it. If you have enough equity buffer outside of the financing for the kitchen, repaying the loan will certainly bring more benefit than simply saving without interest. The variable interest rate will be higher than the interest rate on the later loan.