Creating an upper storey from the existing.

In which we make the upper floor, based on the ground floor level.
NOTE: It's recommended to get your walls, windows and doors layout just right before producing the next storey of the building, as most of them are likely to repeat at the other level, so this way you avoid having to tweak them twice.

First, get your existing level just right.

To do so, Opt-drag (Windows: Ctrl+drag) copies of every other window from the righthand wall to the lefthand one, pressing Shift at the same time to align them horizontally [1, 2, 3].
Then repeat the process for the two front windows, duplicating them on the back wall [4, 5]
Finally, duplicate and place one of the windows at the Midpoint of the kitchen wall [6].

Copy all walls.

Since the only selectable items at present are the exterior and interior walls, a simple marquee selection over the entire ensemble is good enough.

Go up to Level 2

You can do this in one of three simple ways:
1. Choose it from the Layers popdown in the Mode Bar.
2. Click to the left of the Level 2 layer in the Layers list in the Navigation palette.
3. Press Command-<up arrow> (Ctrl+ up arrow) on your keyboard.

Paste in Place the walls.

The result is similar to that of the Main Floor.

Replace the windows with a different type.

Since this is a traditional style house, the windows on this upper level are likely different from those on the ground floor. But because the windows are symbol instances, not independent Windows in Walls, if we simply double-click one of the windows to edit them, it will change all the windows – including those on the Main Floor. To avoid this, we must first "de-symbolize" one of the windows. Select one of those inserted in a horizontal wall, rather than a vertical one, then press ⌘K (Windows: Ctrl+K), or choose Modify > Convert > Convert to Plug-in Object [2].
NOTE: This is because the original window was in horizontal orientation, and it's best to be consistent in the window symbol orientation.

Change the window's General settings.

Now that this particular window is free of its symbol constraints, and is once again a free parametric Window and can have its parameters changed in the Obj Info palette.
Click Settings on its Obj Info, and go to its General section [1].
Turn off its Transom [2], and change its Top Shape to Square [3].
Finally, change its Overall Height to 42 [4].

Change the window's Parts.

Keep the Sill, Trim, and Configuration; change the Muntins settings if you wish; remove any shutters you might have, and the lintel.

Change the window's Data

Click on the Data tab of the dialog [1]
Change the window's ID from 01 to 02 [2]
While we're here, change the Bubble Line Style (the lineweight of the bubble of the ID label) from the default 0.08 to 0.13 [3]
NOTE: Any lineweight under 0.10 tends to disappear in printing or photocopying, so should be avoided unless you don't mind it disappearing.
For now, ensure the ID Class is set to None [4].

Create Symbol from modified window.

Modify > Create Symbol…
Name it by the code that we established earlier for naming windows: W-02-3642H80-DH – i.e., labelled "W-02", 36" wide, 42" high, its Header set at 80" (6'8") above the finished floor. And OK…

Replace the old (W-01) with the new (W-02).

You can do this in any orientation – Top/Plan, an elevation, or an isometric. To appreciate the change more fully, switch to an Isometric view: go to Right Isometric, and Shift-select several of the existing windows.
NOTE: Take care not to select the wall itself, or any object other than the window symbols, as otherwise this procedure won't work.
Then click Replace… in the Obj Info palette.

Choose the replacement symbol.

You get a Choose Symbol dialog with a large popdown button on the right showing a thumbnail image of a symbol. Click on it and choose the new (W-02) symbol from the range on offer – then OK.

The window symbols are replaced.

Stacking the two layers

Now switch to a Front View.

Each layer has its own view orientation.

You are now seeing Level 2 from the front elevation [1], but – assuming your Main Floor is set to be Visible in the Layers setup of the Navigation palette [2] and your Layer Options are set to Show Others or Grey Others [3] – you see the Main Floor still in a Top/Plan projection [4]. This is because the Layers are not "Aligned" – meaning, their projections are not synchronized.

Align Layer Views

To fix this, choose View > Align Layer Views from the menu. This applies the projection that you have in the Active Layer (the one you're currently "in") to all other layers.

Layers Aligned.

The result is the two layers are now both shown in Front view. What's more, they're also "stacked", meaning, they are positioned one on top of the other in accordance with the Z and ∆Z settings that you set them in the Organization dialog during the Design Layers setup.

If the layers don't quite meet together…

If you notice a mistake at this point – e.g., a gap between the walls of one layer and the next – it's either because the layers' Z and ∆Z settings need adjustment, or the wall heights of the lower layer are not Linked to the Layer's ∆Z [1]. Select the walls in question, and turn on that checkbox in their Obj Info palette (it's usually on by default). Assuming the layer's ∆Z has been set properly, they should now spring up to close the gap.

To ensure your Design Layers remain Aligned…

Choose View > Unified View (Opt-⌘L ; Windows: alt-Ctrl+L). This is a toggle setting: select it to turn it on, select it again to turn it off. The checkmark means that it's on.