Fit-out cost is the capital spent making space usable: partitions, flooring, furniture, meeting rooms, cabling and AV. The industry distinguishes Category A (landlord-delivered shell with services) from Category B (the tenant-specific workplace built on top). Category B is where workplace strategy decisions become expensive to change.
Because fit-out is amortised over the lease, oversizing an office locks in cost twice: in rent and in fit-out per desk that utilisation data would have avoided.