I know some properties have gas meters internally due to when they were built. But if a external gas meter is currently in place and an extension built around this meter would it have to be moved outside?
gas meters can be internal
but there are many legalities these days with installations you really need to have someone see your specific property, for instance how deep is the gas service pipe to your property if you are digging foundations for your extension
the best course would be to phone transco or national grid as they may or may not be called by the time you read this