The 35 degrees are of course the maximum. A calculation with 32 or 30 degrees is not prohibited, but requires more effort and accordingly most construction companies will implement it with 35 degrees.
You always have the option not to approve an execution plan until all questions or requests or corrections have been answered / incorporated.
However, flow30 will hardly ever be implemented for you voluntarily and free of charge, as it involves somewhat more effort in planning and may also cause additional costs during installation. You must then accept these additional costs – prefabricated house providers usually charge well for this (unless you have contractually clarified everything in advance).
It is also possible that they refuse and say it cannot be done. Then you are at a disadvantage, because if you want to fight expert opinions with counter-expert opinions, you will eventually completely blow your timeline (price guarantee, etc.).
Sorry, I expressed myself unclearly. I meant whether the 35° degrees are defined anywhere as a maximum.
Unfortunately, I do not have the primary source, but this should serve as a good starting point
Maximum Supply Temperature of a Underfloor Heating System
According to DIN EN 1264, an underfloor heating system must not exceed a maximum surface temperature of 35°C. The average surface temperature is 29°C. To achieve this temperature in an underfloor heating system executed according to DIN 4724 and 4725, the design of the heating surface is calculated with the following system temperatures (supply/return temperature):
Depending on whether the underfloor heating can be operated with both low and higher supply temperatures, each installed underfloor heating system should not exceed the maximum temperature specified as a DIN standard. The supply temperature of the heating water is set at a maximum of 35°C (up to 55°C for older systems) with a spread of 5°C (difference between supply and return temperature).
Definitely be prepared for a substantial increase if you want the supply temperature at 32 or 30°C. Based on experience, we're talking about a four-digit amount, so you don't fall completely from the clouds. It needs to be recalculated, more heating circuits are required, meaning larger distributors, more hoses, more installation effort, etc. The cost mainly comes from the labor and the technology.