Financing options for a rather high-priced single-family house

or does it not make any difference at all?

In the end, you have to finance a sum X and invest a sum Y of equity. How the sums are reached does not matter in the end.

Does it then make more sense to take one large loan with estimated renovation costs, in which you put all your equity, or rather two smaller ones – one for the house purchase and then calmly a second for the modernization?

There is no difference whether you finance €500K all at once or €400K + €100K = €500K.

And in the latter case, put all the equity into the larger loan or split it right away?

You do not put equity into the loan; it is a financing component. And banks like to see it up front. Otherwise, the bank finances upfront and has no security that you actually put the money into the property.

I think, due to the priority of the land charge, en bloc would make more sense, or at least a temporal concentration of the components with einem lender.

Wanted to quickly give feedback:

We have since been to the bank, and as expected, the financing is no problem. In the end, he didn't care how much equity we bring due to the creditworthiness with the existing houses. What I thought was good was that he also advised us on a rate between 1500 and 2000€, above that he would first need to see what kind of property it is.

He also pointed out that there is the option of a variable loan, in the sense that you initially take out amount X and if you don't need the full amount X, amount Y can be deducted from the loan sum again. I just can't think of the term for it right now

: Can you explain again exactly what you mean by the following?

I would, in consultation with the tax advisor, load the debt-free and rented houses up to the upper edge of the lower lip with debt (repayment preferably 1%, this often works with rental).

Can you explain once again exactly what you mean by the following?

Interest on loans for rented properties is deductible as income-related expenses in the context of income from letting and leasing - however, interest on loans for owner-occupied properties is not. But since these are already unencumbered properties, you should discuss this with your tax advisor.

My problem is rather that I haven't quite understood what exactly it means to load the houses up with debt? How does that lead to more equity? Can it be understood as "putting the houses in payment" and then basically having to repay them again, which then brings the not insignificant advantage that for the duration of the repayment you virtually wouldn't have to pay taxes on the rental income?

Unfortunately, you misunderstood something. You basically deposit the houses as collateral at the bank. This minimizes the risk for the bank and you receive a better interest rate. Since you are building your private house with the money from the bank, you cannot declare any interest on your tax return.