Do I have to insure and bill a domestic helper even for a one-off assignment?

Yes, you also have to register your cleaner for a one-off assignment such as a final cleaning for flat drop-off. However, there are two exceptions here:
1. your domestic help works on a self-employed basis and bills you.
2. it is a sack money job, i.e. the total salary in the calendar year is less than CHF 750 and the domestic help is no more than 25 years old in that calendar year.
In these two cases, you do not have to register your cleaner for a one-off assignment.

Good to know: Most insurance companies do not offer insurance for one-off assignments. Therefore, in these cases it is a good idea to hire cleaning staff who are already insured, e.g. through a cleaning company.