I used to do a fair amount of roped access work - mainly window cleaning.
Sometimes before doing the job, especially if it was substantial sub contract work, there was usually a course of sorts to go on before hand apart from the usual on the day induction stuff.
The rule as I understood it was that for all exterior window cleaning, it's either ladders for work under 6m alone or 9m footed, and above that height it's a safe working platform, MEWP, etc, or a craddle or roped access and that's it.
Using window sills, no matter how big, were a big big big NO whether harnessed or not and for obvious reasons. Any company that allows window sills to be used as a safe platform for window cleaning regardless of the securing used for the worker is in serious breach of H & S.