The crisis in the country’s economy has caused some firms in the construction industry to fail in terms of meeting their required expectation. This is evident in the way firms are striving to survive and continuously look out for other businesses outside their core business or area of specialization. In Nigerian “construction industry", organisation such as the quantity surveying firms are not exempted. These firms need to understand and decide on a suitable business into. Hence, the research explored the level of diversification of quantity surveying firms in south west Nigeria, Oyo and Lagos States as a case study. A quantitative design was adopted and the questionnaire was used to gather information from eighty-two (82) quantity surveying (QS) firms in the study area. The questionnaires were distributed to all 82 QS firms using a census survey, though fifty-eight (58) representing 67% of responses were retrieved and found suitable for analysis. Descriptive and inferential statistics such as frequency distribution and percentage were used for analyzing the collected data. The research findings showed that the majority of QS firms diversified into other businesses though at a moderate level since about 70% of the revenue generated by the firms is from the dominant business. Hence, this research recommended that quantity surveying firms should diversify into other profitable businesses at a high level without becoming sidetracked from their core profession to enhance sustainability in an unstable economy