TBH, I've no idea what the average thermostat setting might be in Europe (or the U.S) . Mine's set at 12C (so roughly 53F) and that seems adequate for most of the house. A couple of rooms that are most used have additional heating (a wood fire and an AGA) but for bedrooms etc that setting seems fine. I must say I'd find 66 uncomfortably warm....

We have our thermostat set at 57 degrees. We also have a wood burning fire place and even with the thermostat at 57, in the office the temperature is usually around 64 or 65 because of all the computer equipment. However, when the temperature outside gets below zero degrees fahrenheit, we usually have to turn the thermostat up to 60 so that the water pipes along the outside walls do not freeze. Last year, our pipes came dangerously close to freezing when the outside temperature got down to -13F.