On how to improve it.. blur the upper parts of the edges of the roof to give it depht, and use mask on the house border to give impression that the grass grows above and it's not geometrically cut. Don't know howe to explain this, but grass around the house should look like this: http://www.jelawnscapes.com/Grass_Border_3.JPG

Cool one!

I would make a sky& sea out of the background. Having a rain gives you problems in rendering the humidity, water drops, etc, so you might want to rethink that.

The original photo is sunny so in order to make the fog natural you should also try to desaturate the water plants. Also add some blur on the dog, he's in the background and in the fog so he shouldn't look so sharp.

Wow, minimal use of source but wow, looks so cool. Less is more.