In terms of the Regulations a 'workplace' is classed as any place within the premises to which you have access while at work including any room, lobby, corridor and staircase.
The Regulations also cover any road or other place used as a means of access to or egress from that place of work or where facilities are provided for use in connection with the place of work other than a public road.
While the Regulations cover you in the areas within your work's premises including any room, lobby, corridor and stairs, there are also other areas where the Workplace (Health Safety and Welfare) Regulations 1992 apply. For example if you were attending another work's premises in the course of your employment the Workplace (Health Safety and Welfare) Regulations 1992 would apply to any accident which occurred in those premises. The Regulations only apply if you are attending such premises for the purposes of your employment and would not for example apply to a member of the public who had an accident while shopping in a supermarket.
The Workplace (Health Safety and Welfare) Regulations 1992 do not apply within domestic premises and therefore could not be used for anyone who was injured while in the course of their employment working within domestic premises.
The Regulations also do not apply to a workplace which is in or on a ship. Furthermore the Regulations cannot be used where the only activity being undertaken are building operations or works of engineering constructions.