We are planning our smart home in the single-family house

How should this be handled in a case where we want 19 degrees in the bedroom but 22 degrees in the living room?

In (also on both sides of the ceiling considered) directly adjacent rooms, this becomes tricky; a full three Kelvin difference is quite a challenge. Temperature "control" through relative underheating is more complicated nowadays than before the Energy Saving Ordinance, and with controlled residential ventilation it becomes even more difficult.

You probably can't avoid a visualization anyway.

No way. Alone because the visualizations not only visualize but also provide logic in a relatively simple way and can be interfaces to other systems. Mapping your wishes purely on the KNX level is possible, but it is much more complex, as everything has to happen in logic blocks, which can be really cumbersome depending on the amount. Moreover, you surely want to display some values, see evaluations, or control things centrally, create scenes, etc. And with such a house, you will definitely want to adjust or check something remotely at some point. At the latest then, you need a visualization.

I generally consider the integration of the heat pump overrated, since you do not intervene in its internal control anyway and any individual room control takes place outside or after the heat generator at the heating circuit valves. However, with some heat pumps, you can input the indoor temperatures via KNX, so you can save the ugly indoor sensor.

By the way, I also agree with those against an ERR. You simply don't need it with a good design. But if you have KNX, it can be quickly retrofitted in the software on request. Temperature sensors detect the temperatures, and a heating actuator, if not yet existing, can be quickly retrofitted in the heating circuit distributor. However, this should definitely receive a bus line – at least preventively for now.

I would never do without ERR with KNX. What I would not do again with KNX in hindsight: switchable sockets. And if measurable, then only active power. Pure current measurement is far too inaccurate with all the switched-mode power supplies. I don't have a visualisation. It would sometimes be a nice gimmick, but so far it is not worth spending so much time on it just to occasionally be able to overwrite something.

I would never do without ERR when using KNX.
What I would not do again with KNX in hindsight: switchable sockets.
And if measurable, then only active power. Pure current measurement is far too inaccurate with all the switching power supplies.

I don't have a visualization. It would sometimes be a nice gimmick, but so far it's not worth spending so much time on it just to sometimes be able to overwrite something.

Do you also have more in-depth reasons for your statements? I would be very interested because it is somewhat contrary to the forum opinions.
So why ERR? Why no switchable sockets?

So why ERR?

Because it significantly increases comfort at relatively low costs.
When heating, rooms that become too warm due to present people, running devices, or shining sunlight are automatically turned down. This prevents unnecessary overheating, which can save heating energy and also increase comfort.

Those who also use the heat pump for cooling benefit doubly from the ERR.
Who wants to reverse the thermal balance twice a year?!
Automatically not cooling the bathrooms, etc., when cooling makes a lot of sense from my point of view!

Why no switchable socket?

We have many switchable and measurable sockets because this was repeatedly recommended as a cool feature on the internet.
It was a lot of effort during planning, high costs in implementation, a lot of effort in configuration, and the benefit is really small.
We actually hardly ever use it. Fewer and fewer devices can handle having their power cut off.
And nothing is more annoying than plugging something into a socket only to find out it’s currently off and has to be turned on first.
And the argument that it’s so great for Christmas lights... I’d rather just leave them running.

Because it significantly increases comfort at rather low costs. When heating, rooms that become too warm due to present people, running devices, or sunlight shining in are automatically regulated down. This prevents unnecessary overheating, which can save heating energy and also increases comfort.

With a properly planned heating system, it is even more comfortable NOT to use the ERR. The entire system, including the heating circuits of the sunlit rooms, adjusts itself correctly on its own. Aside from that, the underfloor heating anyway regulates, even with sunlight, with such a delay that switching off by the ERR only takes effect in the evening or at night.

Those who also use the heat pump for cooling benefit twice from the ERR.

You know proper cooling does not work with underfloor heating anyway? Then please install an appropriate humidity sensor in all rooms or at least in the heating circuit manifold. Otherwise, you can simply specify a supply temperature at the underfloor heating that is still comfortable and does not cause dew point problems. I also use the cooling function – but without ERR and without dew point/humidity sensor. Just switch and let it run. What kind of rooms are you referring to with &Co? What sense does it make not to cool rooms? Just as pointless in a house as not heating rooms.

We have many sockets switchable and measurable because this was always recommended on the internet as a cool feature.

Cool feature. I agree. Cool, but nobody needs it.