How to change to helvetice font in class based on class option?

Question

I want to based on a class option change to using Helvetica as font, the simplest way I found to do this would be to load the helvet package. But if I do that like:

\DeclareOption{foo}{
    \RequirePackage{helvet}
    \renewcommand\familydefault{\sfdefault} 
}

I get told that:

! LaTeX Error: \RequirePackage or \LoadClass in Options Section.

i.e., I am not allowed to load packages between \DeclareOption and \ProcessOptions. But that would mean that I can't load different packages based on option? How can I then do things like change font based on Options? In fact, most things I do in LaTeX I do by loading packages, how can I do anything much at all based on options in class files?