The building official is not in a position to require a letter from the engineer that the retaining wall was constructed in acordance with the plans. The building official can require inspection reports, test data, calculations, and engineering reports as necessary to help determine if the retaining wall is acceptable.
The fact that other building officials have required similar letters, and maybe some engineers were stupid enough to write one, doesn't change the fact. In most cases what they probably get is something that says based on information provided by others and his inspection of the completed wall it is the engineer's opinion that the wall was likely built in general conformance with the plans. As far as the building official is concerned this is CYA and does't relieve the building official from the responsibility to make a decision whether or not to accept the completed wall.
Unless the engineer was there continually during construction of the wall, very unlikely, he cannot make such a statement and even then it will likely be highly qualified. Making such a statement would mean that his errors and omissions insurance would not cover any problems and would subject himself to strict liability as opposed to professional liability theories of liability.
The fact that other building officials have required similar letters, and maybe some engineers were stupid enough to write one, doesn't change the fact. In most cases what they probably get is something that says based on information provided by others and his inspection of the completed wall it is the engineer's opinion that the wall was likely built in general conformance with the plans. As far as the building official is concerned this is CYA and does't relieve the building official from the responsibility to make a decision whether or not to accept the completed wall.
Unless the engineer was there continually during construction of the wall, very unlikely, he cannot make such a statement and even then it will likely be highly qualified. Making such a statement would mean that his errors and omissions insurance would not cover any problems and would subject himself to strict liability as opposed to professional liability theories of liability.