41 C.F.R. § 102-76.70
For facilities subject to the standards in § 102-76.65(a), the costs of alterations to provide an accessible path of travel to an altered area containing a primary function are disproportionate to the costs of the overall alterations when they exceed 20 percent of the costs of the alterations to the primary function area. If a series of small alterations are made to areas containing a primary function and the costs of any of the alterations considered individually would not result in providing an accessible path of travel to the altered areas, the total costs of the alterations made within the three year period after the initial alteration must be considered when determining whether the costs of alterations to provide an accessible path of travel to the altered areas are disproportionate. Facilities for which new leases are entered into must comply with F202.6 of the Architectural Barriers Act Accessibility Standard without regard to whether the costs of alterations to comply with F202.6 are disproportionate to the costs of the overall alterations.
41 C.F.R. §102-76.70