I'm still trying to find out what the regulations are. There is always talk of "mobile system" and "split system". Mobile system= allowed, even in rented flat; split system= planning permission+ permission from owner. What about a mobile split system? This has a bit of both, is not fixed, does not require conversion, but there is a box hanging in front of the window that everyone can see. If it is not permanently installed, it (presumably) does not require a permit from the local authority (planning permission), but does it need permission from the landlord because of the box in front of the window? I don't have to worry about noise emissions, the window faces the main road, which is already noisy anyway. The only problem is that the box in front of the window would be very visible from the street.