24 C.F.R. § 58.47
Re-evaluation of environmental assessments and other environmental findings
(a) A responsible entity must re-evaluate its environmental findings to determine if the original findings are still valid, when:
(1) The recipient proposes substantial changes in the nature, magnitude or extent of the project, including adding new activities not anticipated in the original scope of the project;
(2) There are new circumstances and environmental conditions which may affect the project or have a bearing on its impact, such as concealed or unexpected conditions discovered during the implementation of the project or activity which is proposed to be continued; or
(3) The recipient proposes the selection of an alternative not in the original finding.
(b)(1) If the original findings are still valid but the data or conditions upon which they were based have changed, the responsible entity must affirm the original findings and update its ERR by including this re-evaluation and its determination based on its findings. Under these circumstances, if a FONSI notice has already been published, no further publication of a FONSI notice is required.
(2) If the responsible entity determines that the original findings are no longer valid, it must prepare an EA or an EIS if its evaluation indicates potentially significant impacts.
(3) Where the recipient is not the responsible entity, the recipient must inform the responsible entity promptly of any proposed substantial changes under paragraph (a)(1) of this section, new circumstances or environmental conditions under paragraph (a)(2) of this section, or any proposals to select a different alternative under paragraph (a)(3) of this section, and must then permit the responsible entity to re-evaluate the findings before proceeding.