Its when you get to cackling hysterically at the mess you're in trying to spin a Blender terrain the same way you would a Gaea terrain that you know it's time to take a break.

But I am working on the November annual! ;)

I come up for air every now and then...

@Wyvern - I think Remy is working on just that ;)

You could use the very same texture on both those sheets Remy just suggested, but just have an Adjust Hue/Saturation sheet effect on the more extensive one at the bottom, to make the texture paler.

You just need a constant stippling texture, preferably seamless, and to draw a polygon of it under the floor sheet on a new sheet of it's own. This polygon needs to extend an even width beyond the floor all the way around, and to have an Edge Fade Inner sheet effect on it that is about the same extent in width, so that it fades away from the building.

This might not give you as soft an effect as you are after, but you could always add a very subtle blur to that sheet to get rid of any hard edges and corner marks.