Sounds like an architect is needed? I'd look at Table 220.127.116.11.1(b) to determine the proper separation between the two occupancies. Are the stairs necessary to the storage occupancy? Also, you need to determine the type of storage with regard to risk?
Oh no the words were removed and I had the answers ready to write.
I added a little to my reply, but this seems like something that needs a design professional to best assess all the factors and then come up with a plan?
I agree an architect was absolutely is essential for the project based on the topics of questions asked.
Sounds like a rehabilitation work area in accordance with 4.6.7 of the 2018 NFPA 101 (LSC) to an existing apartment building (Chapter 31). One important thing the LSC user must know is the provisions of Chapter 43 are permitted to be used only if the existing building is brought into compliance with the appropriate occupancy chapter requirements applicable to existing occupancies. It is a building planning decision an architect must make. It may good sense and judgement to comply with the new construction requirements for the occupancies involved in the entire building.
The two main categories of the rehabilitation work area are reconstruction [43.6, 43.5, & 43.4] for the new exit for the existing apartment building and the new addition [43.8] for the storage use. The design professional is to layout the rehabilitation work area on their plans.
Help from the 2018 LSC Handbook:
Where an addition is made to a building, both of the following criteria shall be met:
Additionally, two important considerations are:
Retrieving data ...