Hello Three things for you to consider, in the case of gas. 1 ° As the double union connection is not allowed, it will be convenient to start from the house to the street. 2 ° If you are going to make the extension with thermofusion and the existing one is made of black or galvanized or epoxy spout, you must join with galvanic cupla (insulating cupla). 3rd If the regulation in the entrance niche has not changed, thermofusion pipes are not allowed, they must be an epoxy pipe. With which you will have to put another galvanic cupla. If anyone knows of any modification in the regulation on the latter please clarify.