@Emiel1
You can't add on a cleaning fee to the booking after it is made. However, there are different ways to do this.
As I host long term stays, a one-off cleaning fee on the booking is often not appropriate. So, instead, I give my guests the option to either pay for me to clean their rooms periodically (including washing and changing bedlinen, towels etc) for a fee, which I charge them via a request for 'additional services'. This has worked out fine for guests who want the service.
Or, they are given the option to clean themselves, in which case I supply the materials and equipment as well as extra linen and towels. The latter is more likely to be problematic though as many guests say they will clean but don't. Some guests will never even wash the sheets during a long stay meaning they are only fit for the bin. As I'm an on site host, I can keep an eye on things, i.e. I know if they are washing their bedlinen or not, plus I have weekly cleaners come in to do the communal areas. If you are off site, then it's more likely that you will find an almighty mess when the guest checks out.
