24 C.F.R. § 35.915
(b) Rehabilitation assistance.
(1) Lead-based paint requirements for rehabilitation fall into three categories that depend on the amount of Federal rehabilitation assistance provided. The three categories are:
(c) Calculating rehabilitation assistance in properties with both assisted and unassisted dwelling units. For a residential property that includes both federally assisted and non-assisted units, the rehabilitation costs and Federal assistance associated with non-assisted units are not included in the calculations of the average per unit hard costs of rehabilitation and the average Federal assistance per unit.
(1) The average per unit hard costs of rehabilitation for the assisted units is calculated using the following formula:
Per Unit Hard Costs of Rehabilitation $ = (a/c) + (b/d)
Where: a = Rehabilitation hard costs for all assisted units (not including common areas and exterior surfaces) b = Rehabilitation hard costs for common areas and exterior painted surfaces c = Number of federally assisted units d = Total number of units
(2) The average Federal assistance per assisted dwelling unit is calculated using the following formula:
Per unit Federal assistance = e/c
Where: e = Total Federal assistance for the project c = Number of federally assisted units
[69 FR 34272, June 21, 2004]