Build a terraced end house with an additional unit (GÜ) on your own

Well, even 26°C is very warm for sleeping. If you want 22-23°C, you need air conditioning, even in a new building. That little bit from the underfloor heating is not enough. A new building heats up much more slowly than an old building, but it also cools down much more slowly.

I might consider a lockable breakthrough in the wall where you can optionally connect the exhaust hose of a mobile air conditioning unit. A core drill doesn't cost the earth...

The peak temperatures during the day were 26° - at night it cools down and the windows are open then, that's cooler. I don't like air conditioners, I always catch a cold from them. Therefore, I prefer a combination of good insulation/good shading, especially in the bedroom.

I might consider a lockable breakthrough in the wall, where you can connect the exhaust hose of a mobile air conditioning unit if desired. A core drilling doesn't cost the world...

These mobile things are way too loud to run at night and are just rubbish.

I regularly catch a cold from air conditioners.

I don't like air conditioners, I always catch a cold from them too.

You cannot catch a cold from air conditioners. Just as little as from cold weather. Colds are viral infections. No virus, no cold, it's that simple. And no, your experience with a sample size of 1 is not relevant.

The mobile units are actually just loud and terribly inefficient - we have such a unit from Remko. If you catch a cold from an air conditioner, it is more likely because the devices are not properly operated or maintained. Then bacteria and viruses form that cause colds.

You cannot catch a cold from air conditioners. Just as little as from cold weather. Colds are viral infections. No virus, no cold, it's that simple.

Correct, but with dried-out mucous membranes caused by drafts, they have a much easier time. So you really catch a cold faster with air conditioners.