The outlet for my garage door opener was 2' to far away from the my new garage door opener cord.

So instead of installing a new box and having to tear up drywall ceiling or root around in an old smelly garage attic, I put a handy box extension on the current outlet box. I then ran 1/2" emt to from there on the surface of the drywall to a 1900 box where I mounted a new outlet.

Is it ok to surface mount stuff like this off a box extension?

 

I cant find anything that says its against code to run emt out of a box extension as I describe. Does anyone know if this is non-compliant?

 

What you did is code compliant. That is one solution that would have been suggested here.

 

I don't know of any reason it should be non-compliant. Assuming it has a cover, appropriate connectors, etc, I think you're good to go.

In a garage, you will want to seal any gaps between the box and the drywall with firestop caulk.