Design is widely accepted as a factor that affects construction work. Although knowledge about this effect will contribute to the improvement of construction practice, this is very limited. No study has been focused on establishing how the effect of design on construction work can be evaluated. The primary objective of this research was to formulate an approach enabling the assessment of the effect of building design on construction work. To achieve this, a quantitative index based on field data, termed the 'index of difficulty,' was established. Given a construction activity, this index relates the effective work effort per unit of output expended in completing a construction part under two distinct designs: one under evaluation and the other designated as the base design for common comparison. The greater the index of difficulty associated with a design, the higher the required work effort, consequently resulting in a greater affect of the design on construction work. Multiple ways of utilizing the index of difficulty to assess the effect of building design on construction activities are suggested. Additionally, application cases are exhibited to illustrate the implementation of the proposed approach and the required computations.