“Ungrouping” is called Exploding in SketchUp. Select everything, right-click and select “explode”. Repeat until the command is greyed out.
Layers: Open the Window menu>Layers window, select the layers you want to merge and press Delete. SketchUp will ask you what to do with the objects on the layers - erase or move to the current or default (0) layer. The layer 0 cannot be deleted.
If you have your model nicely grouped and (as usual) foresee the need to make modifications to it even in the future, I would preserve the grouping, and create the areas as a yet another new group (as Gadget who is a faster typist than me seems to hint). I would start with a horizontal rectangle larger than your house, group it and place it at a height that intersects all your walls. Then, inside the group, select the face, right-click and select “Intersect with model” from the context menu, and, finally, in the result, delete all the faces that are not part of the areas you need. It’s work, but you are more future-proof.
Note too that painting the areas with different materials lets you get totals easily by right-clicking the material in question in the materials browser and selecting Area from the context menu.