I don't have my code book with me but.....
YOU CAN'T RUN PE-GAS PIPE UNDER YOUR BUILDING!
As a matter of fact, you can't run ANY gas pipe under your building, including steel, unless it is "ENCASED" in a SCH 40 pipe sleeve that is vented to the outdoors, and inspected/approved by the "Authorities having Jurisdiction".
And if you run is as long as you say, depending upon what style of meter you have, your meter may not be big enough.
WE NEED MORE INFORMATION!
What is your farthest run of pipe? What is the total BTU load that you are putting on your meter. Most older "house" meters are only rated at 275m BTU's, newer ones are good for 300m btu's, and can be stretched to 315m.
Now is the time to find this info out, not when its too late.
You CAN run CSS (corrigated stainless steel), cheaper, and in the rafters, so that you don't have to worry about putting piping under your building.
Now, this all depends upon local codes, but I have never heard of them allowing gas piping under a building without special restrictions.
Good luck!