Codes for gas pipe

I am building a new inground spa. My hardest challenge is how to get a 1-1/2 direct burial PE gas pipe 120ft away to heat the new 400k btu heater. Since my back yard is existing I wanted to install the new gas line in a trench next to a new 6ft block wall I am going to build between mine and my neighbors house. Does anyone know of codes preventing me from running the direct burial PE gas line next to a property line. OR distance required. I am an electrical contractor by trade and I know the electrical is safe. I am in OC, CA and the houses are pretty tight together. Thanks, John..

Sometimes the city building department will help with codes if you get the right person. Of course they will want to inspect it which they should.