24 C.F.R. § 891.140

(a) HUD shall use the development cost limits, established by Notice in the Federal Register and adjusted by locality, to calculate the fund reservation amount of the capital advance to be made available to individual Owners. Owners that incur actual development costs that are less than the amount of the initial fund reservation shall be entitled to retain 50 percent of the savings in a Replacement Reserve Account. Such percentage shall be increased to 75 percent for Owners that add energy efficiency features.

(b) The Replacement Reserve Account established under paragraph (a) of this section may only be used for repairs, replacements, and capital improvements to the project.