Zoning and the proposed use
The property's zoning designation is a starting point for reviewing permitted uses and building size. NYC's ZoLa map is a useful public research tool, but a map lookup is not a project approval. The applicable rules need to be considered against the actual proposal.
Existing records and approvals
Available drawings, occupancy records, permits and open filings help establish what is documented. Gaps or inconsistencies need to be identified rather than filled with assumptions.
The building and site
An extension or added floor may raise questions about the existing structure, access, building systems and other site constraints. The relevant architect, engineer or other professional needs to evaluate the issues within their scope.
The path from idea to project
The next step may be a targeted professional review, additional records, a measured survey or further design work. Identifying that sequence helps you decide what to investigate before moving forward.