Hi - I think the only way to ensure people only book in half days or full days, and not random hours, is to have annual leave set in days rather than hours. 

Okay I’ve tried to build a workflow instead that would notify the employee and line manager to make they’ve booked a full half-day. I had no issues creating the workflow but when I went to test it, it did not trigger at all?

Hi ​@AmandaMasuku,

the Absence duration condition, for custom workflows, is currently applicable only to Absences tracked in days. If you would like to have this condition also for Absences tracked in hours, feel free to inform our product team via the Give feedback functionality available in your Personio account.

If you want the employees to only be able to request half-day absences, and not single hours, I suggest following the solution proposed by ​@HRJoy. If you navigate to Settings > Work Hours and Absences > Absence, you can create a new absence type, choose Days for Time unit, and enable the Half-day request option:

Please ensure that the Access rights and Accrual policies for the employees are updated, allowing them to request the new absence type instead of the current one. Additional details can be found in our Helpcenter article Create absence types.

Best
Mara