Taking occupation of the whole of a building gives a tenant the largest degree of control. Even if the property is held under a lease, it is likely that the tenant will be given significant freedoms in how it runs and operates a premises.
Conversely, if a premises is only part of a building, it will usually be held under a lease, and there are likely to be many provisions that control how the premises is used and operated. This is typically for the benefit of the property and all of its tenants and ensures that common areas and services – e.g. receptions, lifts, car parking areas – are operated equitably.
Taking occupation of only part of a building necessarily means that you will be sharing occupation of the building. For some businesses, the identity of other tenants is very important e.g. if two businesses have strong but conflicting principles, they might not wish to be located physically alongside one another. Conversely, direct market competitors might be relatively aligned in their principles but could be equally averse to having the same physical address.
In some buildings, tenants might be granted a say as to the identity of other new tenants. However, even at the most basic level, a new tenant will typically want to investigate the identity and conduct of the other occupiers sufficiently to ensure that they are likely to be good neighbours who can and will comply with their leasehold covenants. It is also important to ensure that leases of part of a multi-let building oblige the landlord to enforce the terms of neighbouring tenants' leases if there is a problem during the lease term.
For specific concerns, e.g. if sharing facilities is problematic for confidentiality reasons, there may be scope to set up a 'building within a building' so that selected lift lobbies and entrances are operated for one occupant only.
Taking part of a building only may also give more flexibility as a business expands or contracts. It is common to see businesses take individual leases of different floors of the same building so that they can be terminated independently if required. Rights to offer individual floors or parts of floors back to a landlord can be included in lease drafting. Conversely, rights of pre-emption for other floors in the building can be negotiated in anticipation of future business expansion. Given the recent upheaval in working arrangements, and the lack of certainty as to balance of office-based vs remote working in the longer term, these kinds of arrangements can give a business significant flexibility.