Is preparing for an air conditioner worth it?

Unfortunately yes, energy consultants used to turn a blind eye to this, but nowadays it is no longer possible. Up to two devices can be capped by a photovoltaic system; from the third device onwards, you are completely excluded from the subsidy. Since we are currently in the planning phase ourselves, these are reliable statements at least for a 55 standard house. We will probably just lay the cables first and then wait for the first summers. If it really becomes unbearable, we will see if we still install it.

Hm, then I have to say, I don’t understand the calculation. Isn’t the sequence:
1. I have a house with certain characteristics.
2. From the given house I determine the heating and cooling demand.
3. According to the heating and cooling demand, I have to select heating and cooling systems.
4. With the chosen heating and cooling systems, I can determine the EE class.

If in point 2 no cooling demand is determined at all, why should that affect the EE class? Of course, it is a different matter if the calculation determines a cooling demand, but if this is not the case?!

I can only say from recent experience that simply installing the air conditioning would have destroyed the KfW standard for us. We had exactly this problem 8 weeks ago. The energy consultant recalculated the building under this condition according to a different DIN (at least that’s how I understood it) and at first it was NOT sufficient; at some point/somehow, however, according to him this other recalculation was absolutely necessary.

May I ask what your energy consultant charged you for this "recalculation"?!

€300.- gross (Worte Worte Worte Worte Worte)

€300.- gross (words words words words words)

That is possible, but then maybe directly air conditioning :D

Regarding the initial question: I would install a small (2.5 kW) device at a high, central location in the house directly during construction.

If you let it run as soon as room temperatures get too high and only turn it off again when the heat phase is over, you just have to keep the doors open and you get a pleasant humidity throughout the entire house and – in new buildings – also comfortable temperatures.

Is that a concrete experience? Does it work?

I have read such statements here a few times and it would actually be a conceivable scenario for me for retrofitting. I could put the outdoor unit on the flat roof without major interventions, use an existing penetration, and install an indoor unit in the upstairs corridor, which is mounted on a wall to the adjoining room. Drainage is not a problem in the adjoining room; the roof penetration is also accessible there. So it's actually pretty easy.

I have installed this in two houses (once an old building, once a new building). I can also gladly show you floor plans and/or photos via PM, so you can better imagine it.

And of course, you have to differentiate: Do I want to air condition unconditionally, or is it enough for me to keep my apartment, my house bearable and enable restful sleep at any time.

If you want to do it this way, it is advantageous if you can install the indoor unit somewhere elevated. If all ceilings are the same height, a somewhat central position is naturally beneficial. Depending on the floor plan, this works better or worse. For example, I cool my (old building) living room, which is connected by an 8-meter-long corridor to three bedrooms. So, a very poor floor plan for this type of cooling. Still, the effect in the bedrooms is sufficient to sleep well with a thin blanket even in midsummer. Without air conditioning, that was often unthinkable for weeks. Temperature differences from the coldest to the warmest part are then at most 3°. Usually only 2.

But the most important thing, as mentioned, is the running time. It doesn't work to come home from work and cool down the place in 15 minutes. Such a single fighter simply runs for a very long time, gladly continuously during peak summer. This also allows the air to distribute throughout the entire apartment. You don't have to be afraid of the power consumption. Usually, the thing hums along at 300 watts. At 35°C it then sometimes pulls 800 during the day. But for the (for me) insane luxury, I gladly spend 150 euros a year. Since last year (with photovoltaics), it's maybe only 50.

Edit: Additionally maybe: For someone who always keeps all doors closed, that is of course nothing. And as another tip: You don’t lose much by just trying it and retrofitting if you don’t like it. It is always recommended not to use multi-split devices anyway. They are less efficient, more expensive, less flexible, and offer no redundancy. Then, unlike heaters, which always break on December 24th, air conditioners always break on July 31st. ;-)