In M1-1D, M1-2D, M1-3D, M1-4D and M1-5D Districts, a building containing non-conforming residential uses may be enlarged and the residential uses extended thereby, provided that no non-residential uses exist above the level of the first story ceiling.

Such enlargement is subject to all of the following regulations:

1. The total amount of residential floor area in the building shall not exceed 500 square feet additional to the residential floor area in existence on December 21, 1989, or the applicable floor area ratio for an R5 District, whichever is less.
2. All remaining bulk regulations of a C2 District mapped within an R5 District shall apply.

Enlargements in excess of those permitted in this Section may be permitted by authorization of the City Planning Commission, pursuant to the regulations of Section 42-311 (Residential uses in M1-1D through M1-5D Districts).