Layer the two images (stormy sky and house)
Duplicate the house layer and change blending mode to make it darker. Mask areas that are too dark.
Change blending mode of sky layer and mask areas that show in the foreground.
Add lightning with Xenofex
Paint in lighting glow from window and entry lights
Add layer of rain.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=32026140&changemode=1
original images by Becky: