There are various methodologies that can be used to prepare a cost estimate, which depend on aspects such as the engineering development phase, available (technical) input, the required level of accuracy, etc.
The AACE International Recommended Practice No. 18R-97 provides guidelines for the development of a comprehensive cost estimate. Within this recommended practice, the estimate classification structure is defined and described, which includes the corresponding estimate methodologies, such as conceptual/parametric and detailed bottom-up estimating.