Industrialized house-building companies are offering unique products by adopting an engineer-to-order (ETO) strategy. Client satisfaction is achieved by adaptation of product solutions and swift introduction of new technology in combination with cost-efficient production and short lead-time for completion. Product development is executed in collaboration with the clients and changes in requirements are frequent. The use of product platforms, where external and internal efficiency are well-balanced, has been acknowledged as a strategic enabler for mass customization and increased competitiveness. However, ETO-companies struggle with adopting the common product platform approach, set by pre-defined modules and components. Predefinitions may cause an imbalance between product development and a lean production system. The aim of this work was to analyse current strategies and support to master the balance of external and internal efficiency in product development within industrialized housebuilding to facilitate the development of a product platform strategy. Data were gathered from a single case study and an on-going product platform development and includes interviews and document analysis. The findings show that product development is guided by a technical platform, but there is an imbalance where external efficiency is prioritized over the internal efficiency.