A major contributor to economic growth in Nigeria, the construction sector is frequently beset by large cost and schedule overruns that jeopardize project profitability and impede the development of infrastructure. This study looks into how project management techniques affect these overruns in building construction projects in Nigeria. The goal of the study is to pinpoint the major project management variables that lead to overspending and delays while also offering recommendations for best practices to address these problems. A mixed-methods strategy was used, integrating qualitative case studies with quantitative data analysis. Project engineers, contractors, Architects, and Technicians were given standardized questionnaires to complete about a variety of Nigerian building projects. In-depth interviews and focus groups were also held to acquire a deeper understanding of the real-world difficulties and experiences encountered in the field. The study pinpoints several crucial project management techniques that affect time and cost performance. The results show that Inadequate construction site management and supervision ranked 1 st with an RII of 0.7877, Improper construction techniques and lack of experience of the consultant came 2 nd and 3 rd with RII of 0.7215 and 0.7213 respectively. Late payments during work progress and Use of improper equipment are contributors to project time overrun; while, factors affecting cost overrun are fluctuation in building material costs ranked 1st with RII of 0.7354, frequent design changes, (0.7138), omissions and errors in bills of quantities (0.6754), import of construction materials (0.6754). The report suggests enhancing project management through effective resource management techniques, ongoing training and certification programs, and implementing advanced technology to improve communication, resource management, and planning precision, contributing to sustainable growth in Nigeria's construction industry. INTRODUCTION The building sector significantly impacts economic productivity, investing in services, industry, and agriculture. The public and private sectors aim to improve project performance through cost reduction, timely completion, and quality improvement [1]. Project control aims to ensure projects meet goals, finish on time, and within budget, but modern methods in the building industry often lead to delays and overruns. Construction projects often face overspending due to ignorance of time and expense variables [2]. These overruns impact project goals, schedule, quality, and productivity, potentially leading to bankruptcy. Understanding these factors is crucial for efficient project management and financial losses [3]. Developing nations face increased vulnerability to ineffective performance, leading to delayed services, higher expenses, and decreased income [4]. Nigeria's construction industry frequently experiences budget and schedule blowouts, exacerbated by misconceptions and arguments. Nigeria's building construction industry is vital for economic growth and employment but often faces cost and time overruns [5]. Understanding these issues is crucial for improving project performance and sustainable development. This study investigates the impact of project management techniques on time and cost overruns in Nigerian building construction projects. Also, to identify best practices and analyze stakeholder management to provide practical advice for industry professionals. The causes of time and cost overruns in Nigeria project construction were identified which are common in developing nations, but research on the causes is lacking to sum up, to resolve cost