The use of Pre-Construction Services (PCS) is becoming increasingly common internationally. What remains unclear is defining the optimal stage to engage it. This paper adopted complexity factors to find the optimal moment to engage in PCS. From literature, the factors: schedule-pressure, risk, financing, and technicality were adopted as factors of complexity. Understanding these factors for PCS can help the industry avoid costly PCS pitfalls of engaging in PCS too early or too late. With an online survey, a series of interviews and the analytical hierarchy process, this paper has weighed the above four factors for the key construction stakeholders. This paper concludes that schedule pressure is the most important factor, while technicality is the least important factor for deciding when to engage in PCS. Using these factors and respective weights, it is recommended that a tool be developed that can assist the industry in determining when to engage the contractor for PCS.