Natural gas line

I am looking to run my natural gas service to an out building in my back yard. I plan on having it done professionally, but I may do the trenching myself. My question is will I have to Tee in of of my meter or can I Tee off a line underneath the house and then run that outside?

The plumber will be able to tell you better.

Pipe size and intended use has a great deal to do with this. Follow Cass's advice and discuss this with the plumber that will be doing the installation.

Not clear about your actual set up. But remember that no gas line can be underground under the slab. I am not sure if you were desribing a slab, or a house on a basement/crawlspace.