Installation of underfloor heating in the upper floor hallway area

You don't need to lay the warm supply line of the bathroom circuit with additional bends through the hallway. Better use the cooler return line for the extra bends. If the bathroom is supposed to be warmer than the rest, hopefully, a wall heating is planned additionally? What kind of heat generator is planned?

First, you lose one or two meters. That is negligible. Otherwise, you have a calculation that can be adjusted if particularly much heat is to be emitted in the bathroom. What is the concrete design, then? Just post the data and the actual installation plan. Then more can be said.

For the hallway, the pipes are specified quite normally, just like for any other room.

You don't have to lay the warm supply line of the bathroom circuit with additional bends through the hallway. Better use the cooler return line for the extra bends. If the bathroom is supposed to be warmer than the rest, hopefully a wall heating is planned additionally? What kind of heat generator is planned?

I have planned an electric towel radiator. We have a geothermal drilling with a Bosch heat pump.

You can also combine the dressing room and hallway, then all heating circuits would have approximately the same lengths. That would greatly simplify the hydraulic balancing. ERR, I believe, are only mandatory for rooms from 6 sqm onwards. At least that was the case in 2019.

For the hallway, the pipes are specified just like for any other room.

And the installation plan including the location of the heating circuit distributor? With changed lengths and fewer circuits, the hydraulics will change. It must be adjusted. Temperature difference too high. Flow temperature too high. Rough planning à la "we always do it this way"? What room temperatures (I hope 22/24). Especially with an efficient ground source heat pump, more than a basic setup should be done. Otherwise, you can save the expensive drilling.

You can also combine dressing room and hallway, then all heating circuits would have roughly the same heating circuit lengths. That would simplify the hydraulic balancing a lot. ERR I believe are only mandatory for rooms from 6 sqm. At least that was the case in 2019.

Ok, I'll take that idea with me.