The seed for poor project performance which causes serious schedule delay and cost overruns, and poor quality is sown during the design process as a result of design errors. The aim of the study is to investigate the effects of design errors on cost overrun in public building projects in Dessie & Kombolcha using Analytical Hierarchy process (AHP). The specific objectives of the study were to determine the critical type of design error; to identify the major causes of design error; and the third objective to establish effects design of design errors and to recommend effective prevention strategies. Literature relative to the research area was comprehensively reviewed. Special pairwise Questionnaire survey in conjunction with case study was used to collect data on design errors. Pairwise data obtained from 30 respondents were analyzed by Analytical hierarchy process (AHP). AHP calculate Normalized Eigen value or factors weight and ranked the preference factors for each objective. According to AHP ranking, the top five critical design errors in increasing order which lead to cost overrun are: Omission of works in the bill of quantity, poor quantity estimation, inadequate drawing details, wrong unit rate price and inadequate of specification. The study also identified the major causes of design error in construction documents as: Inadequate data and information, insufficient design time, poor coordination of consultants, inadequate experience and low salary paid to consultants. The result of AHP analysis indicated prolonged time delivery, unplanned cost growth, inferior quality building output, Building failures and contract termination are the severe impacts of design errors. Design error increased up to 20 % to the original contract sum. The findings also showed that the most ranked design errors reducing strategy found to be effective in reducing the incidence of design errors are involvement of contractor followed by involvement effective planning, design for construction (e.g. standardized components). adoption of Building information modeling (BIM) and partnering. Key words: Design error, Cost overrun, AHP, Consistency ratio.