Our new kitchen addition has a 30k BTU griddle in the middle and is
serviced by an under-slab line.

Shouldn't there be a shut-off valve in the line right before it dives
under the slab? Don't codes or common sense call for one?

If the under-slab line develops a leak, the entire house supply would
need to be shut down if there isn't a shut off valve.

All the other gas lines in the house are located in the attic.
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Not sure if code requires the shutoff, but for peace of mind it's a
good idea to install one.