Why is _no one allowed_ to install my washbasins?

What is Regiebasis?

A work order on site is an additional order given on the construction site and listed separately and additively in the billing. Here: The client comes to the construction site with a washbasin and asks the SHK employee present for help. He installs the washbasin quickly and without further questions and records the time and material required for this. The SHK company then charges for these additional services. Usually, no price agreement takes place, so work orders can sometimes be surprisingly expensive.

Usually, no price agreement takes place, which is why time and materials contracts can sometimes be surprisingly expensive.

I do not know this catch as usual, but rather the regular wage – i.e., the difference from work based on a quote is only that actual time is paid. It could also be called "unscheduled work." Typically, time and materials hours occur when a preceding trade has worked outside the tolerance, and the following craftsman adjusts or fills in the installation location to the nominal dimension.

I do not know this catch as usual, but the regular wage

That's how it is, and in addition there is a standard calculation for materials. One can be surprised about that, even if it is not unfair.