We have an outlet recessed in a wood floor over a concrete subfloor. I want to build in a seating bench over that area, and extend the outlet to surface mount in the bench face. However, the outlet screws are badly corroded and we can't remove the outlet from the junction box to wire in an extension. I would have to tear up the wood floor, and cut concrete to remove the receptacle. Is there some option for leaving the existing outlet in place, and plugging an extension into it, then running to a new surface-mounted outlet? Doesn't sound like something that would meet code, but I'm looking for any possible solution.
You can't be code compliant with an extension cord. Using a cutting wheel on a Dremel tool, cut the slots deeper into the mounting screws (power off). Squirt with rust penetrant and then remove the screws. Then remove the receptacle. You'll have to install a cover with a knockout or a box on top of the old box. Then run conduit and wire from the knockout or new box to the new location.
I would suggest trying harder to remove the screws before I started to rip up the floor; ripping up the floor to fix an electrical outlet/box just seems way to extreme.
If the screw head is stripped, use a specialty bit to remove stripped screws. If it's rusted, try to use some penetrating oil (turn off the power first!).
As a last resort against the screws, you can drill out the screws and use a new tapping bit to create new larger threads for new screws.
If all that fails, just bust up the outlet (again, power off!) so that you can remove it or at least get access to other parts of the box so you can cut/grind whatever you need to remove it.