We are planning to add on to the back of our kitchen by extending it out 8 feet. The extention will be 8 x 8 feet. The homes behind us are elevated higher than ours and when it rains it drains into our yard. The soil is clay. Our contractor is planning on putting a three foot crawl under this extension. I am worried about water and mold decaying the wood under the crawl. For only a 8 x 8 extension wouldn't a concrete slab be more advantagious?