As with all of the Disney haunted mansions, the spooky, visible part of the mansion is only a facade (a relatively empty shell which looks like a "real" mansion). In the case of the WDW mansion, it gets it's realistic look with artificial stone and brickface. But the WDW facade structure also houses the two tall "Stretch Rooms", one in each wing. So the only time a guest is actually within the mansion facade, is when they are in the foyer and stretch room areas.

The entrance to the ride is actually under the left side of the front terrace. The foyer is under the terrace.

The facade is basically a giant stage prop, meant to create a sense of place and mood. Just like a Hollywood movie set, the mansion facade is only supposed to look good on the sides facing the audience. All areas which are out of view of the public, are spartan and utilitarian. In this 3D model I removed all the trees and landscaping, which normally hide the ride structures (I did this to show the ride facilities clearly). Here are a few images which show a progression from the familiar mansion view as seen by guests, then around to the exit, and then to the rear of the facade (which can only be seen by cast members, or from the air).

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So here it is. What really lurks behind the haunted mansion. A ladder. :)
But I have to say that I still find this view of the mansion fascinating!

Behind the facade, and the strategically placed trees and landscaping, there is a HUGE warehouse structure which contains the haunt ed mansion ride. The sharp-eyed guest can spot small portions of the ride facility behind the cemetery, and above the exit. (If you are in Fantasyland, you can see the side of the structure, if you look between the old Skyway station, and "It's a Small World".)
    

To give an idea of the size of the ride, the Doombuggie track is 960 feet (295m) long!

These overhead views give the most accurate proportion of the ride facility. As you can see, the facade is extremely small compared to the ride building.

Overhead view of the Haunted Mansion ride.	Overhead view of the Haunted Mansion ride with a map of the ride superimposed on the roof, to give a sense of scale. Click on the image for a larger, detailed view.

To see a short animation, showing a fly-over of my mansion ride model.

There you have it! A brilliant example of Imagineering!