Is it appropriate to have a single checkbox on its own? For example, if a user has the option to select an existing employee from a drop down or to create a new employee, I want to add a checkbox next to the "create new employee" option which will trigger the new employee data fields.
I know the standard is radio boxes if only one option is selected, but does the same hold true if there is only one option to select from? Hope this makes sense.
If I've understood your question, I don't think a checkbox is appropriate here, but not because there is only one of them. It would make sense to have a single checkbox if it can be interpreted as on/off, true/false, enabled/disabled, etc. In this case, it sounds like you would be using it to initiate an activity ("create new employee"), which is a separate flow from selecting an employee. A button or link would be more intuitive.
It doesn't make sense.
You have two options, mutually exclusive, one of which must be chosen (that is, either select an existing employee, or create a new one). That's a radio-button scenario. Or, put <Add New Employee> in the dropdown and use that to make the choice. In that case, how the dropdown is labelled becomes important.
With the radio buttons, one must be pre-selected: that should be the "Select existing employee" option. If the user chooses the other option to add a new one, that's the equivalent of checking your checkbox. It's also the same number of clicks (one).
Using a checkbox indicates that adding a new employee is available to be chosen, but it should not disable selecting an existing one — that's not how checkboxes are supposed to work. That's obviously nonsense in your situation: you need the user to make the choice between existing or new.
