Yes / Purpose– This paper develops a Knowledge-based (KB) System for Lean Six Sigma (LSS) Maintenance in environmentally Sustainable Buildings (Lean6-SBM). Design/methodology/approach– The Lean6-SBM conceptual framework has been developed using the rule base approach of KB system and joint integration with Gauge Absence Prerequisites (GAP) technique. A comprehensive literature review is given for the main pillars of the framework with a typical output of GAP analysis. Findings– Implementation of LSS in the sustainable building maintenance context requires a pre-assessment of the organisation’s capabilities. A conceptual framework with a design structure is proposed to tackle this issue with the provision of an enhancing strategic and operational decision making hierarchy. Research limitations/implications– Future research work might consider validating this framework in other type of industries. Practical implications– Maintenance activities in environmentally sustainable buildings must take prodigious standards into consideration and, therefore, a robust quality assurance measure has to be integrated. Originality/value– The significance of this research is to present a novel use of hybrid KB/GAP methodologies to develop a Lean6-SBM system. The originality and novelty of this approach will assist in identifying quality perspectives while implementing different maintenance strategies in the sustainable building context. / Ministry of Defence Engineering Services (Sultanate of Oman)