Broadly speaking, compliance with Work at Height Regs, CDM15 and of course Management Regs.
Depends to an extent on the building and its surrounds.
Firstly - if the work can be undertaken without need to work @ height , i.e. clean from internal means (tilt/turn units) using bungee-type wrist attachments for multi storey.
Another option would be to utilise water fed pole cleaning methods, upto around 5/6 storeys I believe assuming surrounds of the building allow operative to do so safely, i.e. not having to stand centre of highway!?
Failing that use of appropriate access equipment, tower scaffold, mewp (scissor or cherry picker). competence for installation and use.. Note the surrounding environment of the building shall need to be reviewed, dense shrubs, trees, parking areas, public roads, other physical features, etc.

Hope this helps.

When choosing a window cleaning method the first step is to look at the cleaning & maintenance strategy document if such a document exists. It should identify the appropriate access method. Experience says be very cautious as access method described in those documents can be out of date (especially MEWPs recommended) or not feasible (water fed system are only suitable for external window cleaning).
Water fed system can be used up to 72ft however poles of this height put a lot of pressure on window cleaners, are very difficult to handle especially in windy conditions.
Access around buildings should be looked at very carefully as it has a major impact on access method especially floor loading limits.
If reversible windows are used ensure that the mechanism is inspected regularly as failure of such mechanism can lead to windows falling out when being turned for the purpose of window cleaning. As it is one of the only time the windows get turned over it is when people realised that there is a problem.
I work for a company doing window cleaning so if you need more information just send a message...