Luke,
There is nothing stopping you planning to fly in icing conditions, ie ATC won't ask 'confirm you want 8000 as it is above the freezing level?'.
Flight in known icing conditions is an aircraft certification issue and as such you will find details in the POH for your aircraft. If your aircraft is not certified for something you are not allowed to do it. That applies to other things such as aerobatics, flight at night, or in IMC.
Also worth remembering that the freezing level on the forecast is just that, a forecast. The temperature in cloud can vary and cause icing when below the forecast freezing or you could be lucky and not encounter ice at the forecast level. If your not in an aircraft certificated for flight in known icing be conservative with your planning so you can descend out of icing immediately.