The sewer line can be viewed both ways; as a "land improvement" or as structural component relating to the operation or maintenance of a building, (Section 1.48-1(e)(2)).
See https://www.law.cornell.edu/cfr/text/26/1.48-1
It could be argued that a line (hookup) from the building to the municipal sewer is essential to the operation of the structure. Further, typical land improvements include sidewalks, driveways, roads, fences, landscaping, and the like. In short, this issue is subject to debate.
