Individual measurement can never be correct.

It doesn't matter how the craftsman arrives at his quoted price; for you, only what you pay and what you get in return counts!

I do care. I want to know what I am getting and, if necessary, influence it before I receive it.

There is nothing to add to the explanation as a whole, however, it remains incomprehensible to me that the craftsmen are being protected for their "incomprehensible and/or incomplete" offers, while the customers (= laypersons) are required to engage with the subject matter.

For the majority of clients, it is clear how something like this is calculated, and the rest (as here) ask and receive the answer, everything is fine?!

And it only really becomes relevant when it comes to the billing.

For the majority of clients, it is clear,

I doubt that, but OK...

how something like this is calculated and the rest (like here) asks and gets the answer, everything fits, right?!

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

In my view, THAT is customer-oriented, further emphasizes the competence and seriousness of the provider, and saves him from further "stupid questions" from a layperson.

The problem is craftsmen these days do not have to act particularly customer-oriented to have a profitable income. It is currently a seller's market and craftsmen who are good in terms of craftsmanship and service can be found with a magnifying glass. And if you have found one, they will only have time in three months, if at all...

I don't understand your outrage. You asked the cleaner why the quantities are different. He answered.

You can just give him the finger instead of thanking him for a free offer.

What would you have done if the contractor had only offered you 100 m2?

If it is stated like this in the DIN, it may roughly be correct (and the calculation will then be created in detail), but who reads a DIN as a layperson? A note in advance or on the offer would be nice then.

The note would then not only be nice but necessary if no measurement had been taken – that is, if the original poster had requested 158 sqm: then the offer should include the note "the order volume determined according to DIN can be significantly higher than the layperson’s own measurement." If every square centimeter were then detailed in the invoice, the overmeasuring would be pointless, by the way.

By the way: a craftsman who makes you an offer based on measurements is no longer a given today. [...] Maybe that is a sign that he will also do the work well. I would see that as a gift without obligation.

But it is worthy of thanks – also by giving him the order, even if he were a little more expensive than a competitor.

It does not matter how the craftsman arrives at his offer price; what counts for you alone is what you pay and what you get for it!

What is important is the effect of the overmeasuring DIN: namely, that the effect of compensating the performance area saved through the openings by the fiddling with the reveals is represented, and the offer can approach the ideal of a bullseye. This avoids the annoyance of being completely shocked about 220 sqm on the invoice after having only planned for 158 sqm from the savings account.