Does it make sense to install an air conditioner?

Two things: Air conditioners do not make you sick. If you overdo it and reduce the humidity through extreme cooling to the point that the mucous membranes dry out, this can make you more susceptible to viruses or bacteria. But you still have to catch a cold somewhere.

And: I also like to ventilate. I need fresh air, after all. But especially in recent weeks, ventilation in my place (in southern Germany) causes high indoor humidity (>65%). At such levels, even harmless 24°C is uncomfortable and you "stick" during light activity. In such cases, I use the air conditioning on a low fan setting mainly to dehumidify. The temperature only drops by one degree, but the 15-20% less humidity makes the difference. And unfortunately, underfloor heating cooling does not help against that.

In short: I would recommend an air conditioner to everyone living below 500 meters above sea level in new buildings (in old buildings anyway). A small 3.5 kW indoor unit at the highest point of the house with a reasonably open layout, running as long as possible on a low setting. Then the house doesn’t heat up at all and you get a comfort gain for around €2500 that you never could have dreamed of.

The underfloor heating-cooling did not work sufficiently here, so after one year split systems were installed in the house. (I would do it the same way again) I am now also considering equipping the whole house with it. You don't have to cool it down to refrigerator temperatures, so there is no risk of a cold.

Do you have pictures of it? Retrofitting afterwards surely looks unattractive? Several units outside and how did you lay the pipes?

Unfortunately, we cannot assess whether it is really necessary for the rooms to become hot during such hot days as the last 2 weeks or if, due to the construction of a single-family house, they remain cool.

Since you yourself are unsure, I would at most prepare it. It depends on personal feeling and, of course, on the house itself how warm it gets. An air conditioning system is certainly not necessary in a solid new building with shading options, but definitely pleasant. Consistently shade windows (automatically, e.g. living area with venetian blinds) and insulate the roof well, especially with a city villa. Proper photovoltaics on the roof also additionally shade the roof. Last summer, during 2 weeks of heat, we had a maximum of 25-26° inside the house (gable roof, only 20cm mineral wool, still without photovoltaics). I still find that pleasantly cool when it is hot outside. In the insulated, unoccupied attic it was up to about 30°. A solid house is sluggish; it takes days to warm up and also a long time to cool down again when it is already cooler outside. We experimented a bit with cooling using an air-to-water heat pump, it was pleasant to get the heat out of the house faster than it cooled down outside again.

Do you have pictures of it? Retrofitting afterwards probably looks visually unappealing, right? Several units outside and how did you lay the pipes?

Sure, it looks crappy when done afterwards. But however you do it, air conditioning units on single-family houses almost always look out of place. However, the pipes can be very well hidden inside before the house is built and everything can be prepared in advance. So that later everything can possibly be connected plug & play and fine.

I have one outdoor unit and three indoor units. I was able to partly lay the pipes invisibly afterwards. On the (cold) attic, “floating”, they go down through the insulation to the HAR, then on the surface but more or less neatly to the outdoor unit.

And if I had known better from the start, I would have had the pipes installed inside the walls. So now I need condensate pumps on the indoor units, for example.

So for us, it has been 30 degrees and above during the day for days, around 20 degrees at night. In the KFW55 house monolithic without shading, we have about 22 degrees.

Very pleasant and therefore, depending on the construction method, I would do without air conditioning. However, you can consider it for the bedroom, since it is already about 23 degrees in the attic there and that is actually too warm in summer.

So here it has been 30 degrees and more during the day for days, around 20 degrees at night. In the monolithic KFW55 house without shading, we have about 22 degrees.

Enviable. Even in winter, our living room quickly heats up to 25 degrees or more thanks to large south-facing windows.