It is possible to take up less than a page for the WebApp. The WebApp lives in a Viewport which, by default takes 100% of the screen but that can be changed. For example, I have reduced the height to 93% and put a footer in a <div> below the ViewPort. This appears throughout the App.

The DF Content Manager allows banners etc. to share the page as does the Wordpress Content manager. Not sure if you can have multiple Viewports on the same page however because DF supports only one WEbApp per Workspace and Index.html is per workspace.

If your client want to use some of the page real estate for other purposes (maybe advertising) that is his choice and you may need to write your application as a WebMobile App so it can be squeezed in. One of the strengths of DF is that you have the choice.