What is Bottom Up Estimation?

In bottoms up estimation you work on individual activities then group them according to WBS structure until you reach the top.

When an activity cannot be estimated with a reasonable degree of confidence, then we rely on Bottom-up method.
the work within the activity is decomposed into more details ( same way of creating the WBS) but you need to reach a small details that you can easily estimate by experience or simple search..
These estimates are then aggregated into a total quantity for each of the activity’s resources.

a work statement and set of drawings or specifications are used to “take off” material quantities required to perform each discrete task performed in accomplishing a given operation or producing an equipment component.
From these quantities, direct labor, equipment, and overhead costs are derived and added.
This technique is used as the level of detail increases as the project develops.
Bottom-up estimation is also called engineer estimation.

This technique involves estimating the costs of individual work packages or individual schedule activities with the lowest level of detail.
This detailed cost is then summarized or “ rolled up “ to higher levels for reporting and tracking purposes.
The cost and accuracy of bottom-up cost estimating is typically motivated by the size and complexity of the individual schedule activity or work package.

This technique is used to find out the cost of lower level task and then add all cost of tasks to find out consolidated / Total cost of the projet