Dream of financing your own 132 sqm house

Additional costs in the area of furniture/interior fittings should definitely be paid in cash. I would not include these in the financing.

The incidental construction costs including your aforementioned notary fees etc. are certainly between 50-60k. You can certainly include these costs in the financing, but you have also saved some equity. Therefore, I would try to calculate the complete incidental construction costs accurately and pay them entirely yourself.

Hmm... you are now mixing incidental construction costs with incidental purchase costs.
Incidental construction costs increase value. The bank finances these relatively easily.
Incidental purchase costs (e.g., notary fees, real estate transfer tax) are not gladly financed by banks.

In the end, all costs are listed and added. Equity is deducted from this and the financing requirement is determined.
At least the incidental purchase costs should always be covered by equity.

I would not "pay in cash" bypassing the bank either. Make everything transparent.

That means: at the beginning you have to use up your equity anyway. And the first bills are usually the incidental purchase costs of the land. You transfer those from your equity and show the bank continuously that you have paid them. Eventually your equity is used up and the bank begins to transfer the amounts.

Everything transparent and clean.

For furniture and other movable things I deliberately do not use the word incidental costs. In the vast majority of cases, these have no place in bank financing. For that, hold back equity or save it during the construction phase.

Hmm...you're mixing up incidental construction costs with incidental purchase costs. Incidental construction costs increase value. The bank finances them relatively easily. Incidental purchase costs (e.g. notary fees, real estate transfer tax) are however not gladly financed by banks. In the end, all costs are listed and added up. Equity is then deducted and the financing requirement is determined. At least the incidental purchase costs should always be covered by equity. I wouldn't "pay in cash" bypassing the bank either. Make everything transparent. That means: at the beginning you have to use up your equity anyway. And the first bills are usually the incidental purchase costs of the land. You transfer those from your equity and continuously show the bank that you have paid them. Eventually your equity is used up and the bank begins to transfer the amounts. Everything transparent and clean. For the furniture and other movable items, I intentionally do not use the term incidental costs. In most cases they have no place in bank financing. Hold back equity or save during the construction phase for those.

By "pay in cash" I meant: The incidental purchase costs should be paid from equity :)

Have I considered everything so far or underestimated anything in your opinion? How do you think the Corona crisis will affect construction prices? Currently, the prices for existing properties in our region (AC / HS / MG / DN) are skyrocketing, so in my opinion it hardly makes sense to buy an old property (at least not if something still needs to be done).

I see it similarly - if you can get a plot of land, building new is still feasible today. Buying existing properties - the prices are heading in a direction that is hardly justifiable anymore. At least with many asking prices. Calling for a shoebox from 2019 + 100,000 after one year is interesting. Since rents aren't keeping up, the whole thing is not very suitable as a capital investment, and more and more people are also priced out as self-users. Apart from tulip bulbs, I can't think of anything else.