How deep does a gas line to a detached ADU have to be buried?

At least 12 inches of cover under the 2025 California Plumbing Code, and at least 18 inches where the pipe is likely to be damaged by outside forces (Section 1210.1.1). If you can't get 12 inches, the pipe has to run in conduit. Plastic gas pipe, the usual choice for a buried run, may only be used outdoors and underground and needs a tracer wire.

Burial and trench rules

• Cover: 12 inches minimum, 18 inches where external damage is likely, conduit where 12 inches isn't possible. Your inspector decides what counts as likely damage.
• Clearance: keep enough distance from other underground structures to avoid contact and allow maintenance, and keep plastic pipe away from heat sources (1210.1).
• Trench bottom: grade it so the pipe rests firmly and nearly continuously on the bottom. If you flood the trench to settle backfill, don't let the pipe float off its bearing (1210.1.2).

Plastic (polyethylene) gas pipe

• Outdoors and underground only. It may come above ground at an anodeless riser, or inside a building with a wall head adapter inserted into piping approved for building use (1210.1.7).
• Metal-to-plastic transitions must use fittings meeting ASTM D2513 (Category I), ASTM F1973, or ASTM F2509.
• Bury an electrically continuous, corrosion-resistant tracer with the pipe: either a product made for the purpose or insulated copper wire of at least 14 AWG, with one end brought above ground at a wall or riser or other aboveground access (1210.1.7.2).

Steel pipe underground

Galvanizing alone isn't adequate corrosion protection. Buried steel needs corrosion-resistant material, a factory-applied coating with coated joints, or cathodic protection, plus an insulating fitting where dissimilar metals meet (1210.1.3).

Entering and passing under the ADU

• Through a foundation wall, sleeve the pipe or protect it by an approved method, and seal around it against gas and water (1210.1.5).
• Beneath a building, run it in approved load-rated conduit or use a system listed for installation under buildings. Conduit that ends outdoors must be sealed at the building end, extend at least 4 inches outside, and vent above grade (1210.1.6).

Before you dig

• Check whether your city's rules allow gas in a new ADU at all. Many ADUs go all-electric and skip the trench.
• Confirm the existing meter and piping can carry the added load.
• Call 811 before excavating.

What it means for your plumbing plan

Your plans should show the gas route, pipe material and size, depth of cover, tracer wire, riser and transition locations, sleeves and conduit, and the pressure test note. See gas line requirements for ADUs, whether plumbing plans include gas lines, and California's all-electric rules.

Sources: gocodebook.com.