The study was designed to assess construction of illegal structures in Osogbo, Osun State, Nigeria. The specific objectives of the study include assessing the level of awareness of the respondents on national building regulation. The research examined the level of illegal structures in Osogbo and identified strategies to curb the incidence factors that account for the construction. In the methodology, a sample size of 142 was chosen for the study. Questionnaire administration and personal observation were deployed to elicit vital data from the respondents and the professional allies. Data generated from the survey were analyzed using the statistical tools such as descriptive statistics (percentages and frequencies) and the relative importance index. Findings revealed that 42% of the structures in Osogbo metropols was exclusively constructed without official permit or approval from relevant authorities; 58% of the existing structures had illegal modification whilst, illegal additions accounted for 71.7% of the structures. The findings of the survey indicated that institutional, physical, educational and socio-economic factors aided the proliferation of illegal development in the city. Observation further revealed that all the illegal structures in Osogbo were owned by the educated elites and such structures were either not completely occupied, thus attesting that the educated residents are major violators of building construction regulations. Findings further revealed that, imposition of high penalties on culprits by the local authority, automating of permit acquisition, monitoring and detection of illegal building operations are amongst the measures which could be instituted to address such a problem. The study concluded that there should be regular public education on building regulations. Moreover, there should be public-private partnership in curbing illegal structures and automating Permit acquisition, monitoring and detection of new structures by the authorities concerned.