Financing of building plot and new house construction by private investors

There are wooden houses for self-assembly without land and foundation slab, for just under €45,000

Exactly my thought:

those are simply just large garden sheds

You will never get that approved. I strongly recommend that you take an architect on site and discuss it. You need one anyway for the building permit, so it’s not wasted money. And one absolutely not meant to be rude question, but as an early retiree, are you still physically fit enough to build a house by yourself?

Put bluntly, the risk is too great that as an early retiree you will never finish the new house and also cannot be evicted from the old apartment in the forced sale. Such financing is done by venture capital investors and at interest rates well above 10%.

and: With the amount of own work, one year is never, never enough. There's also the time for the building application, approval, development, etc. And a 45K wooden kit house doesn't build itself either. Without a crane or at least a few strong helpers for several weeks, nothing works there either. Someone also has to sign off the plan, either an architect or an architect from a general contractor (building submission authorization).

Own work: House will be delivered. The crane is already there (rented myself, I have a crane license) and as you can tell from my username, trained carpenter. A friend is a master carpenter and roofer, another friend is a trained electrician, his father is an electrical master, so electrical installation with acceptance is no problem. Another friend is a plumber for sanitary and gutters. Exterior work is also no problem since a mini-excavator can be rented (excavator license). The only building application is created by the wooden house company. Submission and approval in the district where I want to build is very fast... The plot is fully developed, only the lines/connections need to be laid or connected. As I said: Own work.... Sure, if you haven't worked for almost 30 years in high and civil engineering like I have, something like this will be harder.... Yes, and I am still so physically and mentally fit that I together with my friends and acquaintances can/would manage that in 1 year (including interior finishing).

Such good conditions are rarely seen. I hope your friends all have time and don’t live too far away.

Now you just have to get your [ETW] sold first, then you can get started.

Do private investors generally accept that?

I know a private investor: he lets himself be registered in the land register if everything is well-founded. With brand own construction or self-built house, I see no securities except a plot of land in the middle of nowhere worth €20,000. No one who cares about their money will go along with that. The monthly check that is demanded, I also don't see that happening if there is a lack of liquidity.

Unfortunately, I am somewhat unsettled by the large sum (€120K) including land, concrete foundation, prefabricated house kit and ancillary services like scaffolding, insurance.....?

The self-built house does not include any technology etc. for that price yet. That adds another double on top!
It was already mentioned above: sell the condominium. However, with a self-built house, there is no date in sight at all when it will be finished and when you can hand over the condominium. So I see it all as somewhat hopeless.
Edit: Your post overlapped with mine.