gas pipe sizing

50' of developed length. 232+ cubic ft per hr demand, 152 cubic ft per hr pipe (3/4" pipe with 256K btu's total demand)
pipe WAS o-k with tank water heater, now customer has gone to 199K tankless, and gone WAY over the max.
this wouldn't normally be an issue, except the 3/4" is buried under the garage slab for the first 20', and customer wants NO exposed pipe.

short of tearing up walls or slab, is there any way to have 3/4" for the first 20', then 1" for the remainder?
can that even be sized?

Original Post
First when was this under the structure gas piping installed,?, 2007 CPC 1211.1.6, 97 UPC 1211.3, 91 UPC 1213 ( c ), 85 UPC 1213 ( c ).
Second, they should be able to cut at the foundation go up the exterior wall then inside the garage wall at the bottom plate line, then into the attic to the location of the appliance.
The old So.Ca.Gas company pocket pipe sizing calculator based on 0.60 specific gravity @ aprox. 8 inches water column shows for the length you have posted with the btu as; 286 btu for a 1inch line at 50 feet, for a 3/4 inch line the max btu demand is 152 btu. However we need the total demand on the line. Also I do not know what PG&E uses for their figures, ask the inspector if they would allow to over size so you can have the volume in a 2" line or bigger size if the W/H is the only item being served?? A fifty gallon w/h just uses 50 btu per that scale.

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