Mackellar Schwerdt Architects began with the integration of two long established practices: RN Mackellar & Partners (formed in Newcastle Upon Tyne in 1874 as Cackett and Burns Dick) and John Schwerdt (formed in Lewes in 1954). For the past 54 years the practice has been lead from offices based in the centre of Lewes with successive generations of partners carrying forward the practice name. The present three member partnership formed in 2003 continues the tradition.

The practice moved into its larger, newly refurbished offices in Albion Street, Lewes in 2006. Purpose built over 130 years ago as a school of art, the building has served as a museum and as the town library. Built in a free Gothic style with high internal ceilings, the Grade II building has been sympathetically refurbished to house the Mackellar Schwerdt design team.