24 CFR § 58.15 - Tiering.

Responsible entities may tier their environmental reviews and assessments to eliminate repetitive discussions of the same issues at subsequent levels of review. Tiering is appropriate when there is a requirement to evaluate a policy or proposal in the early stages of development or when site-specific analysis or mitigation is not currently feasible and a more narrow or focused analysis is better done at a later date. The site specific review need only reference or summarize the issues addressed in the broader review. The broader review should identify and evaluate those issues ripe for decision and exclude those issues not relevant to the policy, program or project under consideration. The broader review should also establish the policy, standard or process to be followed in the site specific review. The Finding of No Significant Impact (FONSI) with respect to the broader assessment shall include a summary of the assessment and identify the significant issues to be considered in site specific reviews. Subsequent site-specific reviews will not require notices or a Request for Release of Funds unless the Certifying Officer determines that there are unanticipated impacts or impacts not adequately addressed in the prior review. A tiering approach can be used for meeting environmental review requirements in areas designated for special focus in local Consolidated Plans. Local and State Governments are encouraged to use the Consolidated Plan process to facilitate environmental reviews.