Modularisation and offsite manufacture are increasing in the engineering construction sector. Decision-support tools for determining a project’s suitability for offsite modular construction are currently limited or outdated, leading to decisions being based mainly around capital cost, experience or intuition. This paper reports on the development of a robust, empirically-based decision-support tool for decision makers and clients. A mixed-method methodology was conducted involving engineering construction clients, contractors, consultants and suppliers from all over Europe, after which the tool was developed, piloted and validated with practicing engineers. It is the first globally-applicable tool enabling informed and auditable decisions on using an offsite modular approach for project delivery.