Where alterations will increase the number of existing dwelling units or sleeping units by no more than one, the newly-created unit shall not be required to comply with all requirements for new construction provided the following conditions are met:
The building shall not exceed the maximum number of stories above grade plane based on construction type in Table 309.1.1. Buildings of mixed construction shall be governed by the requirements of the least fire-resistive construction type.
Where the new unit is in a basement, the basement walls shall be impervious to leakage of underground and surface runoff water and shall be protected against dampness.
The new unit shall have at least the number of exits or exit access doorways required by Section 1006 of the Chicago Building Code. A second means of egress may pass through a room or space containing a heating plant.
The floor-ceiling assembly between the new unit and an existing unit shall have a fire-resistance rating of not less than 30 minutes or be original lath and plaster construction.
Walls between the new unit and an existing unit or between the new unit and a corridor or exit stairway shall have a fire-resistance rating of not less than 30 minutes or be original lath and plaster construction.