A 5-step method for developing configurable "products in product" platforms to be used in an engineer-to-order (ETO) design process is proposed. The idea is based on the transformation of typical product architectures into modular design platforms where standard and variant modules are identified and developed. The platform modules can then be configured and combined using traditional design methods to meet the project-specific requirements in the design process. Based on 1193 project designs, several configurable "products in product" were identified and developed in a real case study using the proposed method.