Buying land from father - building a house yes or no?

Of course they get something to eat ... and they are also insured ... what exactly is confusing? So 100,000 are supposed to be for the house, the 45,000 for the land!?

The question is - does it make sense, at the moment there is nothing where the money increases and a stone is a stone!?

Of course they get something to eat ... and they are insured as well ...

Yes, but also plingpling... cash, money, reward! Then your calculation/your budget won't be enough. You roughly calculate 100% labor cost, but maybe you should only optimistically calculate 30% ;)

Just calculate backwards: 100K€ + 45K€ = 145K€ total budget

145K€
-30 K€ for the plot
-30 to 35 K€ for construction incidental costs
-5 to 10 K€ for materials for outdoor facilities
-5K€ material cost for painter and flooring
=70K€ for house, kitchen, furniture, carport/garage, etc.

Due to large exterior surfaces (roof, floor slab) the bungalow is also more expensive than a 1.5-story house.
The base price for our 125m² 2-story house without any fancy features like underfloor heating, shutters, etc. is about 150K€. It should be noted that the price here in the East is on the lower side. Depending on the region, it can be considerably higher.
Even if much is done by friends and acquaintances themselves, I hardly believe that you will fall far below 120K€. In addition, there is the double burden with rent, because the self-made work never finishes as quickly as if all were done by craftsmen.
Overall, this does not fit with the remaining budget of 70K€.

Perhaps a small existing property is sensible, which you can renovate gradually with friends and acquaintances.

There are no existing properties and such an old dump where there might still be dry rot inside - I consider that a bottomless pit, and with a disabled child, I can’t live on a construction site for years. From the financing side, I also know that it is harder to finance an existing property, since there are virtually no or few own contributions possible.

You have to consider several things. First of all, your budget is minimal. I would say that won't be enough. But aside from that, there are certainly other problems: What if one of your helpers drops out, or worse, makes a major mistake. Who pays for the resulting damage? Are your acquaintances/relatives really willing to work in your house for free for weeks? I mean, evenings and weekends for 2-3 weeks might be okay, but for a long time? How do the families of the helpers see that? Be careful with payment among acquaintances as well, because you can quickly end up with undeclared work. You shouldn't be too naive about that and should inform yourself beforehand on how to do it correctly. Is the plot of land even developed yet? Or will that be an additional cost?

I didn’t mention that the construction project is planned/was planned in eastern Germany and believe it or not – yes, there are still friends here who help for free sometimes, and if they are also masters or had their own company, then I do trust them to do that! They are also really satisfied with just a nice barbecue party .... ok, whatever – the plot is in the current evacuation zone due to flooding and so that’s that, because after all the measures taken, it shouldn’t be so critical 10 years after the last one :( and who knows what else will come – I’m not up for water in the house! Thanks for your answers!