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Office occupancy rates

• Introduction
• Reference sources
• Guidance

Introduction

• Occupancy rates are an important design factor for many building types and are the basis for many of the requirements of the Building Regulations.
• Recommendations for calculating occupancy rates vary from source to source and, in certain cases, even statutory documents may differ.
• For many building types, e.g. schools, it will be comparatively easy to decide how many people will be in a certain part of the building at a specific time.
• This will be less straightforward for other building types, e.g. speculative offices, for which the actual occupancy rates will not be known during the early design stages and assumptions must be made.

Reference sources

There are now four key documents to be considered when deciding on the occupancy rates to be used for office design.

Building Regulations Approved Document B - Volume 2

Appendix C, Table C1 [Link]

• This lists the "Floor space factor", described in terms of m²/person, for fifteen distinct "Type(s) of accommodation".
• These factors are used to determine the number of persons likely to be using a particular door, stair, or corridor when calculating the minimum width (for fire escape purposes).
• Table 1 gives a figure of 6.0 m²/person for offices. This is generally considered to be a over-stringent requirement but project-specific negotiation with the Building Control Officer will be required if you wish to design to a lower occupancy rate.

Building Regulations Approved Document G

Section 1, clauses 1.4 and 1.13 [Link]

• Clause 1.4 states that the number of (sanitary) appliances may be determined by reference to various Acts of Parliament relevant to the building type.
• Clause 1.13 states that the requirement can also be met by "following the relevant recommendations" of BS 6465 Sanitary installations, Part 1 Code of Practice for scale of provision, selection and installation of sanitary appliances.
• All these documents provide guidance, or state requirements, for the provision of sanitary appliances based on the number of occupants of a building. None of them, however, provide any guidance on how these numbers should be calculated.
• The Building Control Officer will require a statement on the assumed occupancy rate in order to calculate the number of occupants of the building and, hence, the adequacy of the sanitary appliance provision.

British Standard BS6465-1:2006

Section 6, clause 6.1

"Calculations for the number of persons using the facilities for sanitary purposes should be the same as those used for fire escape purposes."

• Read more about this clause on the Sanitary Provision page.

British Council for Offices Guide 2005

This is most highly-regarded "industry standard" for the design of offices.

5.1.3 Occupancy Standards

• This paragraph is headed 12m² - 17m² per person of net internal area.
• It continues to state that "In the past the norm has been 10m² per person ... but research has shown that very few buildings are occupied to a density that is this high."
• It also refers to densities as high as 4m² to 6m² per person for trading desks and call centres.

Guidance

• Although there is, at present (July 2007), no statutory requirement to use the same number of persons in the calculations for both fire escape purposes and sanitary provision, the existence of British Standard BS6465-1:2006, and its likely incorporation into future editions of Approved Documents B and G, is deemed by most architects to be sufficient reason to work to these requirements now.
• It is important to remember that Building Regulations Approved Document B - Volume 2, Appendix C, clause 2, states:

"The occupant capacity of a room, storey, building or part of a building is:
a. the maximum number of persons it is designed to hold; or (our emboldening)
b. the number calculated ... Table C1 ..."

• With theoretical occupancy rates for offices varying from 4m² to 17m² per person, it is obviously crucial to make some firm decisions before starting to calculate the widths of fire escapes and the number of sanitary appliances.
• Whatever the view of the British Council for Offices concerning the 10m² per person norm, you are unlikely to be able to convince the Building Control Officer to agree a lower occupancy rate if you intend to base your calculations on the area per person.
• You may, of course, agree a specific number of occupants with the client and use this figure as the basis for the calculations in both Approved Document B and Approved Document G. In these circumstances, you should ensure that the client is fully aware of the implications of this decision: too low a number and substantial building works may be required in the future should the occupancy rise; too high a number and the sanitary provisions may increase the core size significantly compared with "normal practice" prior to 2006.

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