When writing up Functional requirements, is it best practice NOT to incorporate the solution definition into the document? Many of the site I work on have a solution already defined - such an open source CMS (WordPress or Drupal). Even though we have the solution, we still have to work out the fine details, for example what fields will be needed for the "News & Events" items, or "What types of user roles can access X content?".
I know in a perfect world you would write out all of the functional requirements before defining a solution, but with smaller projects the client rarely cares about the granular details, and we use the functional specifications to prevent scope creep and also make sure we are all aware of the requirements for the final product.
So the question is: is it okay to incorporate the solution into the functional specifications document if it has already been defined or specified either internally or by the client? If so, do you only define the requirements specifically required by the client, and leave out existing functionality in the system (Theme Management, Settings Panels, etc)?
Thanks!