The first heat wave that happens on a Swiss floor is usually a revelation. The building is perfectly insulated for winter, the windows are airtight, and the heat comes in during the day and does not come out at night. In Spain, it would be normal to put a split or at least a window appliance. In Switzerland, it's not that simple.
Portable air conditioners with an evacuation tube do not require permission from the landlord if they do not modify the property, but taking the tube out of the window can be considered a “modification of normal use” under the Swiss Code of Obligations (CO, art. 257f), which puts many tenants in a gray area. Fixed devices of the split type, which involve drilling into an exterior wall or installing a compressor on the façade, explicitly require the owner's authorisation and, in some municipalities, also a construction permit due to the visual or acoustic impact. In protected areas, it may be directly prohibited.
Regulations vary by canton and municipality. Geneva and Zurich are more restrictive with outdoor installations due to aesthetic and noise criteria. In some building regulations (Hausordnung or règlement de maison) it is expressly forbidden to install any appliance visible from the outside. And if you install something without permission that causes conflict or harm, you're the one who responds.
The most widely used alternative in Switzerland is still cross ventilation at night: open on two opposite sides of the floor when the outside temperature drops at night. It works better than it looks if the floor has two facades. For south-facing flats with no air circulation, summer is directly another problem.