I selected the top half of the model and extruded negative 5 and yes, the edges extrude straight out, however I am left with faces where the ceiling would be. In fact, I just realised, I only removed the ceiling so I could see what was happening, so normally selecting the ceiling would have the exact same effect as the floor.
Also, with regard to the skirting, rooms are normally measured to walls so its easier to extrude 10mm into the room at the floor (yes the floor will move up 10mm, but thats easy to drag back down to 0.)
The exact same thing happens with shell too by the way (another reason I left the ceiling off was for testing shell).
Shell should move the bottom up in this case 5 and the walls in 5 (leaving a gap of hyponinuse = 7.071... at the corner)
<5>
| |_____
|/______5
7.07
I suppose another way of thinking about the extrude tool could be "Extrude and retain angles"
There may be a use for "extrude average angle - constant distance" but I cant see it just now.
Oh, just thought, is it because of spheres that distance is across hypotinuse?
Maybe in that case you could use the smooth angle?
I think even in spheres the distance should be parralel from faces and not distance from verticies.
Just drew 2 decahedrons and made one 4 smaller radius and the parralell distance between the faces is 4 as expected with the distance at the verticies 4.119 which is right because that would be the "hypotinuse".
Trev