An occupied building can require phased work, protected access, equipment coordination, and restricted operating hours. Those conditions affect the project scope even when the roof area and material are unchanged.
What to do next
Give every bidder the same operating requirements. Ask which logistics are included and which depend on the owner or another trade. Compare proposals on their ability to support the actual building operation, not only their roofing quantities.
Related Perkins guidance
Contact Perkins Roofing to discuss your roof and the appropriate next step.