The software solution you develop can be created in a variety of ways:

1. Build code from scratch
2. Buy code components and integrate them
3. Reuse existing code
4. Customize existing code

During the construction phase, you decide which of these techniques will serve you best. Any combination is valid as long as the end result supports your design.

The construction phase creates the solution specified by the your software's design. This is where “the rubber meets the road.” Tools and technologies are implemented to satisfy the design requirements. Remember though, that tools and technologies change constantly, almost as constantly as users change their requirements. Track your implementation choices so that you can quickly and effectively upgradeyour implementation as new options become available. Construction is also the phase in which you exploit opportunities for reuse of existing components, vendor products, and anything else that will support your design.

There will be some compromises to your design. Each compromise should be documented and justified in a way that will allow it to be evaluated periodically against the expressed design goals.
Avoid using construction as an opportunity to redesign on the fly. As soon as your design documents no longer agree with your implementation, your design documents become useless. Anyone who wants to understand your application will be forced to read the code, and this is the slowest, most cryptic, and least cost-effective way to work.