Unfortunately, I believe the best one could do is to total "Used Area" and then compare it side-by-side with the Room's Total Area. This would involve adding an Area Type Shared Parameter (or Project Parameter) to all applicable Families (or Categories). A Multi-Category Schedule could report (and total) this Shared Parameter and sort/group by Room. The "Room: Area" field is not available, so a Calculated Value must be create using the formula "Volume / Height".
I understand that this is not what you are looking to achieve, but this is the best method I can immediately come up with.
You have two options when it comes to adding a "Used Space" Parameter to the applicable elements in the Project:
1. Create a Shared Parameter (Area type) and add this to your Families
See this WikiHelp link for more info on Shared Parameters. You will have to open each one, go to Family Types (the button with the four blue squares on the ribbon), click "Add...", select "Shared Parameter", click "Select...", and choose your Shared Parameter from the list. This parameter can either be an Instance Parameter or a Type Parameter. This will be quite cumbersome, and there are third party add-ins out there that can help with this task.
2. Add a Project Parameter to all of the applicable Categories in your Project
See this WikiHelp link for more info on Project Parameters. In your project, go to Manage tab » Project Parameters. Click "Add...", enter the Parameter Name, change 'Type of Parameter' to "Area", change 'Group' to "Other" (or whatever you deem appropriate), and then select from the list on the right all of the Categories of elements to add this parameter to. This parameter can either be an Instance Parameter or a Type Parameter.
Which ever method is chosen, you will be able to enter a "Used Area" value under each element's Properties (or Type Properties) in the Project.
Create a Multi-Category Schedule.
Under the 'Fields' tab, select these fields from the list:
- Used Area (which is your Shared or Project Parameter)
- Room: Name (you'll have to change the pull-down at the bottom to "Room" first)
- Room: Number
- Room: Volume
- Room: Unbound Height
Now click "Calculated Value...", name it "Room Area", change its Type to "Area", and set its formula to:
Room: Volume / Room: Unbound Height
Under the 'Filter' tab, set the first 'Filter By:' to "Room: Number" "is greater than", and leave the other field blank. This will exclude elements that do not reside within a Room.
Under the 'Sorting/Grouping' tab set the sort order to Room Number or however you wish. Disable "Itemize every instance" at the bottom.
Under the Formatting tab, select "Room: Volume" and "Room: Unbound Height" from the list and enable the "Hidden Field" option. Select "Used Area" and enable the "Calculate Totals" option.