The importance of accurate estimates during the early stages of cost planning of capital projects has been widely recognized for many years. Even so, very few quantitative methods are available that enable estimators and business managers to objectively evaluate the accuracy of early estimates and planning. The primary purpose of this study was to identify and examine the factors affecting accuracy of cost planning of building projects as it relate to the Nigerian construction industry. To accomplish this aim, quantitative data were collected from consultant Quantity Surveyors. Each of the respondents was asked to assign a one-to-ten rating for each of 10 potential drivers of estimate accuracy for a given estimate. The data were analyzed using simple statistical tools which include; mean, standard deviations and percentages were used to represent the information. The study analysis identified 5 of the 10 factors that were most significant at the level. In this finding the five factors, in order of significance were, Amount of information available, Time allowed to prepare the estimate, Experience of the consultant team, Complexity of the project, and bidding and Suitability/stability of the market. Cost estimating and planning is a fundamental part of the construction industry. The success or failure of a project is dependent on the accuracy of several estimates through-out the course of the project. Construction estimating is the compilation and analysis of many items that influence and contribute to the cost of a project. Estimating which is done before the physical performance of the work requires a detailed study and careful analysis of the bidding documents, in order to achieve the most accurate estimate possible of the probable cost consistent with the bidding time available and the accuracy and completeness of the information submitted. Overestimated or underestimated cost has the potential to cause loss to local contracting companies.