What are you actually having problems with?
First, are you building a “game” that you package up, and then “players” of this game use interactions to configure the interior space?
Or are you placing the “countertops” in the UE5 editor, as “building a map?”
Second, are the objects pre-fabricated, or does the player make them out of basic geometries?
If you’re building a game, and placing prefabs, then the material needs can be a property on each of those prefabs. Make some way to find all prefab objects (make them derive from a custom subclass of Actor, for example, and “get all actors of class”) and then copy out the material information from all of them.
If you’re building a game, but the player draws out areas from basic geometry, then you need to make that basic geometry implement some interface or class that you can find, and ask for the amount of materials. Getting the bounding box would be one way to do it, but custom objects (L-shaped objects, etc) might want to override how this is calculated.
If you’re using the Unreal Editor, then the same basic thoughts apply, BUT you have to write the thing that inspects the scene for objects with metadata as an editor utility widget blueprint or editor blutility.