Individual measurement can never be correct.

So in our 100-year-old house not even all the interior doors are the same, but something like that. I wouldn’t expect anyone to measure each door individually, but at least one and then note the quantity, for the windows a rough estimate would be enough for me too. But just rounding up the wall surface somehow, I don’t know, I would have been surprised by that too. He explained it then and that’s okay, but I still find it strange.

Our tiler, the electrician, and the plumbers all measured quite thoroughly, even though you could probably estimate roughly in these trades too. For larger jobs like here for the whole house, I would expect diligence, because the bigger the job, the greater the deviation from the actual price later (and for that it will also be measured exactly and not roughly estimated ;) ).

I doubt it, but OK...

Every architect, site manager, etc. knows it, and a considerable portion of construction contracts probably come from them.

Why make it simple when it can be complicated?
Yes, it works. But in this case, it would be more appropriate, when taking the measurements anyway, to represent, for example, the 158 sqm and transparently use the corresponding factor according to DIN for extrapolation for the customer. A one-liner as an explanation and that’s it!

If the customer is willing to pay in advance for the pure creation of the offer with measurements, it can of course be done that way. Measuring roughly requires significantly less effort and can thus be done more easily and free of charge for the customer.

Every architect, site manager, etc. knows this, and a significant portion of construction contracts probably come from them

Especially when the customer is not an architect, site manager, etc., and the craftsman is still willing to offer his service to the layman, he should take 3 minutes to describe the items clearly! He would even only need to take 3 minutes once and include the corresponding reference to the DIN standard or whatever in his calculation as a lump sum.
Then it won’t hurt the experts to overlook it, and the layman will be happy to have understood the price breakdown for the item.

If the customer is willing to pay in advance for the pure preparation of the offer with measurement, it can of course also be done that way. Measuring over requires significantly less effort and can thus be done more easily and free of charge for the customer.

How should the customer/layman know that a measurement involves significantly more effort than measuring over!? I also don't believe that the OP asked/urged the craftsman to take a measurement!?
In my view, this is part of customer- and service-oriented behavior: either to inform the customer about the situation beforehand or to make it clear in the offer afterwards.

The real reason, however, is probably that most craftsmen these days have fully booked order books and obviously don’t feel the need to perceive the CUSTOMER as such.
It works that way too, as can be seen from the comments in this thread.

Overmeasuring is neither about an imprecision nor about generating a tip surcharge through a dropped subtraction. Rather, it is a fact that 1. the lost area of the openings is offset by the areas of their reveals, which 2. are compensated against each other sometimes more, sometimes less, and 3. after decades of practical experience, but quite by chance, are reasonably accurate when setting the threshold of ignored openings at 2.5 sqm. What value would it have for the customer if you made the measurement pedantically accurate down to the square centimeter as the basis for the offer? – none! (except for the special satisfaction of their millimeter mania – which the craftsman does not assume). Of course, the customer could inform the craftsman beforehand: "I am a millimeter maniac, you will make me happy with a most detailed calculation." But then he must not be surprised if the craftsman replies that he is surprised and has so far learned as a plasterer under the belief that people want nicer apartments. A measurement that is done in twenty minutes and explained for a whole hour (which would by no means increase the accuracy of the cost estimate!) would be priceless. Any prostitute in such a situation would (rightly so) ask her client: "Do you want to f*** or talk?" Painters paint, plasterers plaster little ornaments, and psychiatrists can help millimeter maniacs much better than any craftsman. And then people wonder about the shortage of skilled workers in the trades when they motivate the craftsmen with strange questions to tell their children: better don’t learn the same as your father, rather go to customs and chase down spirits tax evaders – at least nobody there wants to have a precise explanation why openings under two and a half square meters are simply overmeasured. Such experience flat rates are not fraud but administrative simplifications. They make an important contribution so that people interested in apprenticeship in trades are not driven away by weird questions. Otherwise, our national economy will suffer an excess of spirits tax inspectors while simultaneously facing a shortage of plasterers! Is it really SO H-A-R-D to understand???

I'll do it sometime soon, I'll roughly estimate for my clients and see what they think :cool:. On the other hand, I can't understand how someone can't understand that as a customer, you want a reasonably decent cost estimate. For example, I can't afford that and it's not common for us either. I have to look at exactly what the customer wants and make the most accurate offer possible, and that for each new customer, that's part of the business.

But I'll keep that in mind. We should have our ground floor replastered sometime in the next 2-3 years. Otherwise, we'll plaster it ourselves, but for the living rooms, I'd prefer to have a company do it for time reasons. I'll measure it myself beforehand and then ask 2-3 companies with these figures. Otherwise, everyone calculates their own thing and I can't compare.