Understanding how the building design influences construction costs is a challenging task for estimators. Estimators must recognize the design conditions that affect construction costs and customize the cost estimate accordingly. Estimators have different preferences for how and when to adjust a project?s activities, resources, and resource productivity rates that form the basis of a cost estimate. Current tools and methodologies lack ways to help estimators customize construction cost information according to their preferences and maintain cost estimates as the design changes based on those preferences. This paper describes the activity-based cost estimating process we formalized to help estimators customize a project?s activities, resources, and resource productivity rates based on their preferences and the particular features in a given product model. We implemented and tested the process in a prototype called Activity-based Cost Estimating (ACE). ACE creates a set of projectspecific activities that know why they are needed in the cost estimate, what feature requires their execution, what resources are executing the activity and why, and what their labor and material costs are. Our tests show that ACE helps estimators to generate and maintain cost estimates more completely, consistently, and quickly than state-of-the-art cost estimating software.