The new observational method framework with four design approaches was introduced in Ciria C760 (Ciria, 2017), its objective is to balance the safety and economy whilst dealing with geotechnical uncertainties in the retaining wall design and construction. The new observational method - Ipso tempore Modification Approach C was successfully applied to the Crossrail Tottenham Court Road Station, Western Ticket Hall (TCR-WTH) excavation and achieved savings. This case history has now been reassessed using the Ab initio Optimistic Approach A to demonstrate the maximised economic savings in construction materials. The reassessment adopted the most probable soil parameters calibrated in the back-analysis using the reviewed inclinometer data. The back-analysis and reassessment design were undertaken by the semi-finite element analysis programme FREW®. The wall thickness was successfully decreased in the reassessment so that the associated cost savings were achieved through less volume of excavation and concrete usage. In addition, construction time savings could be achievable, if the contingency plan was not required during construction. A contingency plan has been developed with extra prop levels to support the thinner wall under the characteristic design conditions. A cost comparison of the TCR-WTH excavation case history showed among three designs: the original design, OM Approach C design and OM Approach A design, the maximum economic savings can be achieved by OM Approach A. This reassessment is a valuable case study for the future application of the Ab initio design approaches.