Financing scope for construction planning - assessment and experiences?

Bridge financing is completely normal. For many people, it is exactly like that. It is absolutely manageable. The important thing is that the overall concept fits, and in this case, I believe it doesn’t work without a sale.

By the way, depending on the loan and bank, the existing loan can possibly be simply taken over for the new project. This saves the AHE.

Have you already set your sights on a plot of land? New development area that still needs to be developed or a building gap?

I'm only asking because the construction time is not the only time factor. For example, we bought our plot back in 02/2017 or had to buy it, but it can only be built on after the development is completed, at the end of this month. Similarly, you would now already be paying interest and principal on 150k€ land for at least 13 months and would not even have started building yet.

I just want to say that there are many additional things that need to be considered, which unfortunately are not apparent at the first consideration.

Exactly - keyword "overall concept"
I would also like a few suggestions from you.
Of course, the existing condominium should be sold after construction - that goes without saying.

Bridging finance, but here one should always remain realistic. Example: one hopes for a proceeds of 475k for the apartment, but the market only provides 400k, so one sells it for that, therefore here gap (is only an example)

No, we do not have any property in sight yet, but I am aware of the issue of pre-financing the property.

As for the proceeds from the apartment, the value is set conservatively, meaning it should really be achievable.

You seem to be doing your household planning with Excel. My recommendation is to make an Excel project out of it to get a feel for it. Read through the internet, especially here in the forum. Keywords: additional construction costs/kitchen/furniture etc.
You don't live too far from me... the land prices are similar here. I think if you take everything into account, you will end up rather around 600,000 euros with the above-mentioned. Then calculate (there are plenty of calculators on the internet) whether that fits you.

Keep in mind that the search for a plot alone can take time, and the development as well. This gives you the opportunity to save equity and at the same time pay off the existing financing.