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Functional design document is a great tool to communicate the business logic of a web service between the parties, which in addition to the client and the developer may include designers, usability engineers, copy writers, translators and other third party providers and interest groups.

Functional design tells what the service will be used for. An easy and our favorite format here is simply a list of roles and their possible actions within the service:

• "an unregistered user can register in the service"
• "a registered user can post comments on general forums"
• "a moderator can delete comments on general forums"
• "an admin can create new and delete old forums"

...and so on.

This list should be completed primarily by the client, since it does not require any expertise in web development. It describes all the different uses of the service, not what they look like or how they will be implemented. Those come later.

Functional design should always, always, always exist before anything else gets done. This is by far the cheapest, fastest, most direct way to avoid unnecessary waste of resources. Do not skip it whatever the time and budget restraints may be. I don't have any statistics on this, but based on my personal experience a comprehensive functional design can cut the required time and budget in half.

Top 5 reasons to have it:

• The client will realize the true scope of the project
• The developer will be able to give a more accurate time and budget estimates
• Makes communications between the involved parties crystal clear, transparent, non-redundant and tracked
• Allows for very quick prototyping
• Serves agile developers directly as a backlog

Just get it done! Functional design forces the project on the right track right from the beginning. You will reap the benefits throughout the project.