Only financing during the construction phase

The best option would be without a bank:
A) Sell the house, agree on a right of residence, pocket the money, pay for the new construction, move.
Or
B) Put household goods in a container, set up a motorhome near the new construction site, sell the house, move into the motorhome, build the house, move in.

I would take a close look at the timeline. You should add a certain buffer to your expected duration of the house construction, especially if variant A) involves the duration of the right of residence. Believe me, it always takes longer than planned.

I also tend to proceed without a bank. There will be a way. Regarding the house construction duration: It would be my third house construction. In addition, I would probably build with Keitel-Haus. The experience reports with Keitel-Haus show that the planning is adhered to. We will definitely set a certain buffer.

With some suggestions, it really makes your hair stand on end. I wouldn't even come up with such ideas, regardless of the fact that these paths will not be passable at all.

I see 2 possibilities: look for/select a buyer in Switzerland who buys the house, proves the purchase price, and temporarily collects rent.

The second alternative – if I wanted to save rent – is a variable loan, where money is drawn according to the construction progress and interest must be paid accordingly. This loan can then be repaid from the sales proceeds after completion.

May I ask a question about the plan? Are there really plots of land for €60,000? Or do you perhaps also have to consider that the €500,000 for the land and house are not sufficient at all?

Why start a cost discussion in this thread now? Suggestions on how to solve his problem are probably what is needed, so a solution to his question.

Just ask him via private message.

A problem can also include that the assumptions about the initial criteria are flawed. In this respect, a factual inquiry about a requirement can also influence the problem solution.
The requirements always come first. If there are already inaccuracies or errors here, everything is built on a false foundation. The earlier this is uncovered, the more favorable it is to fix it.
In other words: maybe montessalet has no idea what a property costs there and runs in the completely wrong direction, which can have immense consequences for him. A brief factual inquiry sheds light on the matter and possibly gives him something to think about. My phrasing of the question is very cautious.

By the way, this is a discussion forum and not a “you may only write what I want to hear.” If you notice something off-topic, lead by good example with your suggestion, follow your own advice, and write a private message. Your embittered post certainly does not help him. Probably not this one either.