FEMA Resource for Substantially damaged homes
FEMA may have inspected your home and determined that the damage was more than 50 percent higher than the market value of your home. At that point FEMA may not offer much in recovery funds but you may actually be qualified to have a new home constructed.
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By Murray Wennerlund published 9-29-2022 updated 9-29-2022

These files have been updated and reviewed by HUD on 7-2020 and are the most current valid documents.

You are looking for references under Cash Value sections 4.5.2 .3 .4 I believe. Not sure

FEMA Resource References:

Substantial Improvement / Substantial Damage Desk Reference FEMA P-758 / May 2010

2020-07 fema nfip substantial improvement substantial damage desk reference .pdf

Answers to Questions About Substantially Improved / Substantially Damaged Buildings FEMA 213 / August 2018

This section discusses the "Replacement Cost Value" RCV of the home based on the present day cost of materials and labor.
The argument would be in a perfect world, the replacement materials would be ordered the day after the disaster and would not need to be reassessed. But because of delays beyond the control of the homeowner construction costs did increase but reassessments were not followed.
The original RCV in the grant is $108/sf * Square feet of home. At the time of replacement the materials and labor were valued at $118/sf as detailed in Oct. 2020 by state managers. Because HUD offers no specific spot checks or time frame to reassess costs and FEMA identifies the costs for Replacement is to be calculated at the "present day cost of materials and labor" we have to conclude that present day means the period of time materials and labor were acquired.

"RCV is the cost to replace a structure on the same parcel with a new structure that is intended for the same purpose and using comparable materials and quality (at the present day cost of materials and labor). The concept of RCV is also used by both the insurance industry and the construction industry. Definitions may vary from State to State."

2020-07 fema p213 08232018 .pdf