Extension of duty is ONLY for Unforseen circumsances after duty commences.
Example.
Night operation completely including WOCL: 11 hours max. Duty.
YOU CAN NOT plan any duty ( max. 2 sectors or your max duty is even reduced further) Over 11 hours.
NEVER.
If you have started your duty, and unforseen circumstances occure, like slot, weather delay etc.. Then the crew can extend their duty by ONE hour. Maximum 2 of these extensions can be done per week, again NOT PLANNED, but UNFORSEEN! This is very often misinterpreted by various Airline Operations AND CREW.
On the LAST sector, during the flight, one can extend still their duty with one hour ( tomget the aircraft at destination without having to divert) in case of contingencies ( think holding, de-icing delay before take-off, reroute)