Anyone done this please?

I go buy a house in a city, say Whiterun. I check out the house and it looks cruddy. I go back and purchase the upgrades and it looks great.
What's going on in the game engine? Is it cells that are being swapped out?

I'm asking because I'd like to add a basemant to the house with a room full of trunks to stash my stuff. Also for the fun if it.
I didn't want to add in a door where the game would add in a piece of furnature.

If you go into the CK and look at the map for a house you buy, you will see that EVERYTHING is already placed. But most of it starts off as disabled.

Take Breezehome for example.

In the game when you first visit it after buying, there's junk piled around in corners, there's a usable bed upstairs and a chest. These are the only elements that start off as being enabled.

When you buy the bedroom package, assets associated with that package become enabled. Among those assets is a wall section with a door that divides the bedrooms (one for you and the other for your housecarl) from the rest of the loft area.

To do what you want, examine the interior that already exists for the house you want to add a basemant to. Create a new cell which would be the basemant itself. Add a door transition that leads to it and set it as disabled. Edit the list of things you can buy for the house in question and add "Basemant"

Make it so that, like the conditions that govern the other elements you buy, the door to the basemant becomes enabled when you buy the basemant.

Instead of a door, you could also use a trap door. All you need is an open area on the floor large enough to allow for it, and you can place it there.

No, the cells do not swap. What happens is that many of the objects in the cell are set up attached to X-Markers with the marker being the “Enable Parent” of the object. The X-Marker is set to be “Initially Disabled” which means that anything attached to it via the enable parent will be invisible until the X-Marker is enabled. When you purchase upgrades, each purchase enables a marker that’s associated with that purchase type. For example, the Alchemy station purchase enables the marker that the actual alchemy table is attached to as well as various clutter items and the satchel on the table. The Kitchen upgrades enables the table and other clutter around it, the loft enables the clutter in the loft and so on. Conversely, many other items are set to be “opposite of parent” which means if they are attached to an X-marker that’s initially disabled, these objects will be visible to start, and when that marker is enabled, they will disappear.

Edit: Ninj'd by G.B.