We have a seasonal cabin and I have played around with doing what you are doing but gave up on the idea. What you have to do is run the pipes through heated space. Runways with vents are good because they allow heated air to surround the pipe and allow access to make the repairs which will undoubtedly be necessary. Interior walls would be best but you need to take into consideration that sooner or later the power will go off and the pipes will freeze without some sort of backup arrangement. The underground line will need to be below the standard frost line (probably 5 or 6 feet) and even so it will probably sooner or later freeze also. What I have seen done with the line which connects to the house is to place it inside a 4 inch or so casing which is open to the heated air and wrap it in heat tape. Also, try and run the lines so that they drain downhill to a valve that you can open to drain everything when you are not there. You may also want to consider pex instead of copper as it may be more tolerant of freezeups and easier to repair or replace.