Really Depends on what amps your pulling but working from an extension isn't good idea in any case.!! Your just asking to burn the place down and by that I mean the house not the Shed.
On top of this Insurance will just laugh and give you the finger when found using extension or any kind of dodgy wiring.!

Really you want it on it's own circuit with it's own fuse and using appropriate sized wire for the amount of draw. Anything less is just not worth the risk IMO.