Many market forces are driving companies to improve their targeting of increasingly small market niches. To accomplish this efficiently, products are organized into product families that typically share common platforms. In industries with short product lifecycles, the decision to move towards a common platform approach, even for some components, requires estimating the reduction in development time. One of the problems encountered in estimating development time is that initially, before implementing a platform approach, hard information related to product family design and development is not available. The purpose of this paper is to estimate the design and development time using simple activity models, when moving towards a platform approach. The product family models are developed from existing single product design activities, which are modified and extended to reflect activities related to development of product platform and subsequent product members of the family from the platform. Uncertainty associated with time for each activity is also included in the model, which is solved using Monte Carlo simulation. The approach is demonstrated using a hard disk drive spindle motor platform development for a family of hard disks.