Well, if we take the average across society, we come closest to the actual conditions.
Sure, there are extreme cases where [100.000 €] of own work is contributed, and on the other hand, there are just as many who contribute no own work or at most lay the insulation or help with pulling wires. Just as there are builders who spent [2300 €/m²], there are also builders who built for [1300 €/m²].
Any work carried out by someone unskilled for that particular step is auxiliary work and can only be evaluated as such. I go to work and have others work for me; then I know the quality is right and that nothing is done haphazardly somewhere.
On top of that, professionals have the right equipment and know how to use it properly.
When I see how perfectly the painter did our walls, we would never have managed that in our lives, and the worst part is that you always look at the corners where you know it was difficult...
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I don't want to badmouth own work; in most cases, it is just overrated.