Muscle mortgage - Is the calculation realistic? From when with proof?

Of course, as a private person you buy much cheaper and at the same time of higher quality,.... You can talk everything up. The craftsman usually has a mixed calculation, what exactly you pay there for material / hours is usually not shown to you precisely. He may earn a little on the material, but also has the work of procurement, quality control, and so on. You do all that in your spare time.

On the subject of material: With the electrician who would have done the installation, I would have gotten the meter cabinet for about €1800, online the same for about €1100. The same applies to all electrical materials, even larger craft businesses have worse purchasing conditions than the prices at big mail-order companies. And since the craft business also wants to make some profit, you often pay 50 - 100% more for identical material. Our "bathroom fixtures" (i.e. bathtub, showers, WCs, washbasins, etc.) should have cost over €12,000 at the specialist company, about €7,000 online!!! In this respect, Sebastian is absolutely right, of course you have to do something for it, which I gladly do if savings in the five-figure range are possible...