24 C.F.R. § 58.32 – Project aggregation.

(a) A responsible entity must group together and evaluate as a single project all individual activities which are related either on a geographical or functional basis, or are logical parts of a composite of contemplated actions.
(b) In deciding the most appropriate basis for aggregation when evaluating activities under more than one program, the responsible entity may choose: functional aggregation when a specific type of activity (e.g., water improvements) is to take place in several separate locales or jurisdictions; geographic aggregation when a mix of dissimilar but related activities is to be concentrated in a fairly specific project area (e.g., a combination of water, sewer and street improvements and economic development activities); or a combination of aggregation approaches, which, for various project locations, considers the impacts arising from each functional activity and its interrelationship with other activities.
(c) The purpose of project aggregation is to group together related activities so that the responsible entity can:
1. (1) Address adequately and analyze, in a single environmental review, the separate and combined impacts of activities that are similar, connected and closely related, or that are dependent upon other activities and actions. (See 40 CFR 1508.25(a)).
2. (2) Consider reasonable alternative courses of action.
3. (3) Schedule the activities to resolve conflicts or mitigate the individual, combined and/or cumulative effects.
4. (4) Prescribe mitigation measures and safeguards including project alternatives and modifications to individual activities.
(d) Multi-year project aggregation—(1) Release of funds. When a recipient's planning and program development provide for activities to be implemented over two or more years, the responsible entity's environmental review should consider the relationship among all component activities of the multi-year project regardless of the source of funds and address and evaluate their cumulative environmental effects. The estimated range of the aggregated activities and the estimated cost of the total project must be listed and described by the responsible entity in the environmental review and included in the RROF. The release of funds will cover the entire project period.
(2) When one or more of the conditions described in § 58.47 exists, the recipient or other responsible entity must re-evaluate the environmental review.