"A cost-segregation specialist can perform a nonintrusive yet detailed engineering study of a building's walls, flooring, and ceilings; and its plumbing, electrical, lighting, telecommunications, heating and cooling systems" (Money Doesn't Grow on Trees, But It Could be Hidden in the Walls By William J. Barnes, CPA).
"In general, a study by a construction engineer is more reliable than one conducted by someone with no engineering or construction background.