Are portable AC units not available in Europe? They are inefficient but can sit inside next to the window and only require that you partially block off a window opening.

I don't know what landlords are like in European countries, but some landlords in America forbid people from connecting any kind of AC exhaust port to a window.

It's funny that in this case the law now protects you.

Generally anything you do indoor that falls under normal usage(painting walls, hanging furniture with big screws,etc.) is ok without approval, at least in my country/city(austria) and the land lord can't deny any of this and is not even required to be notified.

As you would have an open window and a not so noise machine running this would be ok, but energy efficiency is quite bad and you lose the benefits of the thermal insulation of your building.

Can you name one place where this is an enforceable rule?

Yeah, the GP comment is classic HN. Wildly vague, with no supporting evidence. The planning rules in the US are fractally complex compared to "Europe".

Power isn't cheap, and inefficient is an understatement. In the heatwave last week in Germany I managed to get ours to cool a tiny bedroom from 30 to maybe 26 degrees Celsius.

Doesn't help that in new construction outer walls are usually very well insulated, and inner walls and floors aren't (by design; 70% of total heating cost is evenly split too), so you're working against your neighbors too. We have some stupidly complex building standards often driven by industry lobbying (which also makes building expensive), but installing a mini split hasn't made it on the list.

They are but not as efficient and so most people don't bother(that I know of).

Works just fine, I have 8000 BTU and it cools a living room very efficiently. I need it about one or two weeks per year in Finland. I can't imagine anybody working from home without one, even as north as Finland. Although we Finns are probably least used to heat so even 27 celsius is unbearable.

The only thing one needs is a window-sized cardboard with a round hole and blanket if your target room has no door. I just looked at prices and 10000BTU seemed to be ~350 euros.

I bought a portable split system for 2400 EUR. Works pretty well since I have shutters and a balcony, I put the external part outside the window, pull down the shutters and seal the lower part of the window around the pipe with some stuff. Doesn't use lots of power if I cool to 26C. Well, today we had 34C outside, so it had to work a lot.

2400 EUR? Cripes, that is expensive. Do you mind to share the model?

Sure thing: https://www.amazon.de/-/en/gp/product/B07YZ5XT92/ Maybe it was a bit less, 2200 with shipping. I do have a top floor apartment looking south, so I get a constant 30C+ in summer, without any cooling at night.

They are available

They definitely exist, and are very cheap. I got one for 200, gave away to a friend when I moved out of an attic apartment. They have a window-attachment accessory that makes it easy to attach to those European windows.

The real reason most of us don't have AC is because it's hot for a very short period. Last year it definitely wasn't too hot for me to warrant having one. This year so far, it was, for a week: we had about 30°C (86°F) last week, but next week my mobile phone promises it's gonna be 20°C (68°F) maximum, which does not need AC. Maybe that will change, though.

Swamp coolers are cheap and don't need a window or drain.