This paper identifies the lessons available to architects in Mackintosh’s Glasgow School of Art, a building in daily use, little changed in over one hundred years and recently voted by the RIBA as the most popular building by a British architect in the last 175 years. Whilst the stylistic aspects of Mackintosh’s Glasgow School of Art are frequently addressed, far less attention has been paid to the practical aspects of the building. Architectural icons such as Mackintosh’s art school are generally attributed to the mercurial genius of their creators, their success shrouded in the myth of the individual designer, and as such, offering no practical lessons for contemporary architects. The intention is to challenge the myth of the architect-genius through a thorough analysis of the practicalities of Mackintosh’s design for GSA: the pragmatic response to the site; the design of flexible and fixed accommodation; the environmental considerations of heating, lighting and ventilation; and the exploitation of ordinary materials and construction. The intention is to demonstrate a fundamentally practical approach to the design and making of a building. The evidence is obtained through observation of the building and its users, physical measurement of both the fabric and environmental performance, supported by privileged access as a staff member of the institution to Glasgow School of Art’s extensive archive on the building and the expertise of resident Mackintosh scholars.