"Stealth Stairs" is the name I have coined for creating stairs that is pretty much hidden and usable for any floor height. There will be several "Stealth Stair" tutorials on the usage of this novelty technique.

A good example of the use of Stealth stairs is "The Facility" lot; otherwise the building becomes overwhelmed with stairs.

As you can see above, the single column of stairs provides access to all five levels using only one tile of space (actually three tiles as the front and back of the stairs must remain clear for the Sim to transverse the stairs).

To begin, stack five levels of floor using the room tool. Your task is to create five floors.

Use the dormer trick to give you an additional set of grids for the roof. Simply select any dormers under the roofing tools and add it while intersecting a wall.

This adds the grids without adding the dormer. Press ESC to get rid of the Dormer.

For this tutorial I use the transparent tile set from RGiles at modthesims2 so we can see through the floors while we build the stairs.

Here you can see that I have added floor tiles to every floor.

Removing the walls will give you 5 layers of floors.

Delete one floor tile from each floor; aligned column wise. This is where you will be building your stealth stairs. I have chosen the middle tile, however the stairs can be built anywhere on the floor.

Use the PageDown key to view the first floor. Press Ctrl-Shift-C to bring up the cheat box and type in boolprop constrainfloorelevation false

pick a spot to build 3 consecutive connecting stairs. With the constrainfloorelevation false, this will deform the level, lowering it by 3/4 of the original height of the wall.

Here is a good side view of how it should look afterwards

Use the elevation tool and drag starting from the bottom of the lowermost stair to a flat tile beside the stair hole. This will lower the area beside the hole.

Once the floor has been lowered, you should see something simliar to the above.

Deforming the bottom floor deforms all the floors above.

Now you can add connecting stairs from the lowermost tile

Use the Page Up key to get to the next floor and add another connecting stair.

Repeat with the next floor up

and the next

Last one

Delete the connecting stairs that were used to lower the floor tiles by holding down CTRL and clicking on the stairs.

done?

Here is a side view of the stealth stairs.

At the lowest floor again (get there by pressing the Page Down key), use the elevation tool to level out the floor by draging the tool from the upper left conner to the bottom right corner to flatten the whole floor back to normal

done?

This will restore the original flatness just like before we deformed the tiles. The connecting stairs for each level remains connected regardless of the actual floor tile height (just a quirk of the system). If you were to raise the terrain after connecting the stiars, thus raising the all the floors, the stairs will still remain connected allowing the sim to transverse the floors effortlessly.