University of Washington
Department of Computer Science & Engineering
UW Guidelines on Consulting

Here is my best interpretation of the basic rules:

When you are being paid at the 100% level through the UW payroll system -- whether this money is coming from the state budget (typically during the academic year) or from a grant (typically during the summer), you are allowed to spend at most 20% of your time in outside activities for compensation ("consulting").
This means "20% of the total hours you work" rather than "one day per week" or "8 hours per week" or "anything beyond 80% of 40 hours per week." (In other words, you can't give UW 80% of 40 hours per week and consult for an additional 48 hours per week -- if you choose to work 80 hours per week, you can spend at most 16 hours consulting.)
During a period when you are off the payroll, you can of course do anything you like.
All outside activities for compensation are subject to prior approval.
In addition, all outside activities for compensation must be reported annually.
And all outside activities for compensation are subject to conflict of interest policies.

The most relevant parts of the Faculty Code (contained in the University of Washington Handbook) include: