Purpose – The purpose of this study was to investigate how early-stage product development can be structured to support modularisation decisions in the context of existing product architectures. The study aims to identify key steps and decision points that influence modular product architectures and to understand how these decisions affect subsequent product development.

Methodology – This study follows the Design Research Methodology and adopts a qualitative-dominant mixed-methods approach. Data collection included a review of existing literature, semi-structured interviews, a survey, and an applied industrial development case. The empirical data were analysed using thematic analysis to identify key patterns, challenges and opportunities related to early-stage modularisation.

Findings – The study identified critical factors influencing modularisation decisions in early-stage product development, including requirements definition, cross-functional coordination and consideration of architectural constraints. The findings show that early decisions significantly impact downstream development by affecting integration complexity and lifecycle flexibility. Based on these insights, design guidelines were developed to support structured modularisation decision-making.

Practical Implications – The findings provide actionable guidance for organisations working with existing product architectures. The proposed guidelines aim to support improved decision-making in early development phases, reduce integration challenges and enhance long-term product adaptability and scalability.

Theoretical Implications – The study contributes to the existing body of knowledge by addressing a research gap related to early-stage modularisation. It integrates perspectives on product architecture, decision-making and development processes, providing a more comprehensive understanding of how early decisions influence downstream outcomes.

Limitations – The study is based on a single industrial case and primarily qualitative data, which may limit generalisability. Additionally, the developed guidelines were not empirically evaluated within the scope of the study and further validation is suggested for future research.