Rating historic resources is frowned upon by preservation purists. It encourages a mindset that will allow a 3 to be torn down while preserving a 4, at least until the 4 is threatened when there's a perfectly good 5 just down the street. Ideally, something is historic or it isn't. The ordinance I work with recognizes contributing and non-contributing structures within landmark districts. Technically they are both subject to the same review procedures, but a non-contributing building obviously wouldn't require the same kind of rigorous review. Many surveys will identify significant, contributing, and non-contributing resources, with significant being the most important historically with the highest level of integrity.
