Quality practices focus on producing quality buildings in terms of aesthetics, durability, functionality, reduced cost and time completion of project. The aim of this study is to assess the impact of quality assessment system on building projects in Lagos state. In order to realize this aim, the study examined the impacts of quality assessment system on building project in the study area. The study employed a descriptive case study design where data was collected from senior managers, project managers, engineers, architects, quantity surveyors, and technical managers working with building construction companies in the Lagos Metropolitan Area using the mixed method approach. Quantitative data was collected using questionnaire from 109 randomly selected participants and analysed using descriptive and inferential statistics. Qualitative data was collected through interviews with 9 purposively selected respondents and analysed using the thematic analysis technique. The study shows that quality assessment help us timely completion of construction projects within planned budget, improvement of the image of the firm Quality assessment as well as reduced project delivery time. The study concludes that the most visible impact of implementing quality assessment was an increase in customer satisfaction as conducting quality assessment reduces the number of customer complaints and helps the companies to reduce material wastage, increase confidence and team spirit among project workers, improve safety records, and enhance the image of the company. The quality assessment has a significant positive impact on project delivery in Lagos Nigeria.