An Analysis of Overhead Costs for Electrical and Plumbing Works in Housing Projects in Bangkok

Contractors’ overhead costs significantly influence the financial performance of housing projects, particularly in electrical and plumbing work. This study investigates overhead cost structures using data from 15 contractors in the Bangkok Metropolitan Area to define practical benchmarks for accurate estimation. The research findings indicate that overhead costs average 9.10% of the contract value, categorized into: (1) field expenses, (2) field staff welfare and safety equipment expenses, (3) tool and vehicle expenses, and (4) general expenses. Field expenses constitute the largest proportion, averaging 81.42% and varying from 54.0% to 94.60% based on project complexity. Expenditure on field staff welfare and safety equipment accounts for 10.0%, while tool and vehicle expenses represent 6.5%, and general expenses account for 2.1%. A comparison between contractor types indicates that electrical contractors incur slightly higher overhead costs, averaging 9.50% of the contract value, compared to 8.60% for plumbing contractors. However, plumbing contractors exhibit greater variability (5.6%–13.7%) compared to electrical contractors (6.7%–11.6%). These findings highlight the limitations of applying fixed overhead percentages and underscore the necessity of accounting for contractor types and specific project characteristics. The established cost structures provide a practical basis for enhancing project cost planning and control in housing development.