Tesla does not advise you to charge with an extension cord, but I knew I was going to a location where charging wasn't possible without one (20 meters), so I took one with me.
Normal extension cords here a 3G1.5, that means 3 wires of 1.5mm2 each. But I had a 25m 3G2.5 with me and that works great.
I'm currently charging on 13A over a total of 25m of extension cord, no problems.
All the wiring here in houses and buildings is 2.5mm2 by default, so it's not a real wonder that it works, but I thought I'd share it.
I however did notice a drop in voltage from 236V on idle to 218V during charging. That was more then I calculated.
I'd advice everybody to always have a 2.5mm2 extension cord with you instead of a 1.5mm2 to avoid the "extension cord" message from the Roadster. This message is mainly due to a high voltage drop when putting load on the wires.