Gas meter is in a box on the ground, just outside the house, with a pipe running up the adjacent wall a short distance, and then into the building. I want to build an extension, and the gas meter is where the new concrete floor would be.

2 questions:

1. Can the meter be moved a couple of feet and mounted on the wall? Who does this?

2. Can the main gas supply, which is underground in the garden, be covered with the concrete floor of the extension?

your gas suppliers will take care of moving the meter.
sorting the underground pipe.
nothing else you can do its their property.
and it won't be cheap.