I'm not 100% sure I understand what you're after, but it sounds like what you want might be a good use of the glow layer effect. If I understand correctly, your top layer is a texture with the floorplan knocked out of it. Ctrl-click that layer, invert the selection, and then fill the new selection with a black. You should now have a layer that completely covers your floorplan with black, but the texture surrounds are still visible. Now make a new layer and place it beneath that one. Fill the new layer with black. Your entire image should now be black. Now select the top layer and apply a glow layer effect, setting it up so that you have white at the inside border, and it fades out to black at the distance you desire your texture to fade away. Select both of the layers and merge them. Now you should have a single layer that has the shape of your floorplan in black, and a white glow around it that fades back to black. I'd suggest you copy this entire layer to a new channel in case you want to be able to get to it easily. Now ctrl-click the layer to make a selection, turn the layer off, select your texture layer, and click the New Mask button. That should cause your texture to fade away the further it is from the walls.
There are probably simpler ways to get there, but I don't have the leisure at the moment to open up Photoshop to experiment.