Software Architect's Handbook

Disadvantages of the top-down approach

A strictly top-down approach runs the risk of a big design up front (BDUF), sometimes referred to as a big up-front design (BUFD). Software is complex and it can be difficult to create the entire architecture up front. Design flaws or missing functionality in the architecture may not be uncovered until later in the process, when components are designed or implemented. If architecture changes are required in higher levels of the architecture after some work is already completed, it will be more difficult to make the modifications.

A top-down approach works best when the domain is well understood, which is not always the case. Plenty of projects begin without the domain being fully understood. Even when ...