Evaluation of construction project with maximum own efforts

Please leave aside the discussions regarding duration, risk, and availability of labor for now.

Spoilsport! - what are you supposed to answer if you are told to leave out precisely the essential?

no fancy bay windows, projections, or similar

And a simple roof shape, those are already good prerequisites for overall inexpensive building - but they change little specifically about the difference between "turnkey" and "kit house." If you have so many construction workers in the family, you should actually be able to answer the question yourself (because you know what modest share the shell construction has in the overall house). Don’t forget that your own labor and that of your relatives also "cost money when buying." The bricklayer relative loses his working time there, where he would otherwise sell it to outsiders. If you know so many bricklayers, then don’t let them do the masonry themselves, but supervise cheap laborers. That probably saves more ;-)

Please put aside discussions about time duration, risk, and availability of labor. We are really interested in how much savings potential could be achieved here and what overall price one must expect.

We don't need to discuss that either. The question or the assumptions are very naively thought. Theoretically, you can also build a house for €0 if the extended family provides their time free of charge and you acquire the material laboriously through barter trade, acquire it, or get it as a gift, including scaffolding and equipment, possibly selling it again for the same or more. But it is all pure theory. It doesn't work as intended; the craftsmen within the family also have to feed their families. Building your house only on weekends takes a very long time. But some savings are probably possible; maybe you can get the upgraded features instead of the standard equipment. Then there are still €3,000 per sqm living space plus ancillary construction costs. So €600,000 for the calculation plus exterior work plus ancillary construction costs, which were already mentioned. But even if it is cheaper: I rather see the financial problem because banks usually only finance realistically calculated construction projects based on preliminary calculations, cost estimates, or fixed prices. "Black constructions" in quotation marks, i.e. upscale DIY house construction, are not financed at all.

Let’s do a rough calculation.

The construction costs for your cabin will currently be around €680,000 with medium to high-end equipment. Plus incidental construction costs, outdoor facilities, etc.
Then let’s assume a labor cost and profit share of 20-25%. Savings potential thus roughly +/- €150,000-200,000 overall.

We don’t need to discuss this either. The question or the assumptions are very naive.
Theoretically, you can also build a house for €0 if the extended family provides their time for free and you acquire the materials through elaborate bartering, purchase, or gifts, including scaffolding and equipment, possibly reselling them for the same or more.
But it’s all just theory. It doesn’t work as intended; the craftsmen in the family also have to feed their own family. Building your house only on weekends takes a very long time.
But some of it might be feasible; maybe you can get the high-end equipment instead of the standard equipment.
Then there are still €3,000/sqm living space plus incidental construction costs. So €600,000 for calculation plus outdoor facilities plus incidental construction costs, as already mentioned.

But even if it’s cheaper: I rather see the financial problem because banks usually only finance realistically calculated construction projects based on preliminary calculations, cost estimates, or fixed prices.
“Black constructions” in quotation marks, i.e., upscale DIY house building, are not financed at all.

These two estimates already differ significantly:

Above, we have €3,400/sqm * 200 = €680k minus own work worth €180k (20-25%) = €500k

The estimate from seems significantly more conservative regarding own work.
Final price after deduction of own work = €600k

This €100k difference in own work is not insignificant.

These 100k difference in own contributions is already not insignificant.

The range of possible own contributions is as well. Friendship prices are not automatically zero labor costs and own contributions are a lot of work for nearly 200sqm. How much is realistic for you has to be assessed by you. A house without own contribution is around 3000-3500€/sqm, depending on equipment and region

As I am well known as an advocate of doing things yourself. Alone the saving on the value-added tax is definitely worth it.

The creation of the foundation slab can be done well alone. Only when the concrete mixer arrives should you have 2 men helping for 2-3 hours. Then you have already saved the first 10k.

When I bond with KS stones today, 2 men and a handsaw are completely sufficient. Bonding the stones with thin-bed mortar or today partially glued with foam. It's like Lego for adults. You can't work easier, cleaner, or faster.

The stones for a house also don't cost more than 10k.

You have the ceilings done like the roof frame. Roofing is also not rocket science if you don't have a special roof. Installing windows and doors is also relatively simple. He does the electrical work himself. He can also lay the heating and water pipes himself.

I would have plastering done. It goes faster. I have sometimes plastered myself and sometimes hired a plasterer.

In the end, you get a house cheaply. And building it yourself is also a lot of fun.

The creation of the base slab can be done quite well alone. Only when the concrete mixer arrives should you have 2 men to help for 2-3 hours. Then you have already saved the first 10k.

How exactly do you want to save 10k on a base slab? What prices for concrete/pump/formwork/steel (possibly also earthworks?) – and what labor savings do you assume? What square meter quantities are you talking about? If I could save that, I’d be on board immediately for my planned house construction!