Except as otherwise provided in § 264-86A, an existing legal nonconforming building or premises lawfully devoted to a use not permitted in an R-1, RH-1, R-2, RH-2 or R-3 District may only be enlarged, extended, reconstructed or structurally altered by 10% of its existing floor area when in a building or 15% of its existing area for outdoor nonconforming uses unless the use is changed to one permitted in that district or one permitted by this section. Area calculations shall be determined by the amount of area in existence on the date of adoption of this chapter.