In contrast with the top-down approach, the bottom-up approach begins with the components that are needed for the solution, and then the design works upward into higher levels of abstraction. Various components can then be used together, like building blocks, to create other components and eventually larger structures. The process continues until all the requirements have been met.
Unlike the top-down approach, which begins with the high-level structure, there is no up-front architecture design with the bottom-up approach. The architecture emerges as more work is completed. Hence, this is sometimes referred to as emergent design or emergent architecture.
The bottom-up approach does not require that the domain be well-understood, as the team only focuses on a small piece at a time. The system grows incrementally as the team learns more about the problem domain as well as the solution.