Property drainage is often overlooked when buying a house.
In a hill-side home or a house with a raised foundation, moisture intrusion is often visible under the house, as seen in these pictures of an Eagle Rock home.
This foundation has been eroded and saturated with water for probably years. Water was mostly coming from flower beds and lawns at the back and sides of the house. These plants get watered daily but, with no drainage escape route, the excess water goes through the foundation wall and into the crawl space. Crawl space was muddy in places as it never gets a chance to dry out. As a result the house was damp, moss and mold was growing in multiple places under the house.
When purchasing a house be sure to check out the drainage, an inspector can do this for you.
If drainage is found to not be adequate a drainage expert can recommend solutions such as a) landscaping which deflects water away from the house b) install area drains and French drains in correct places c) good rain gutters and downspouts d) trench and install moisture barriers on the outside of the foundation walls.
Drainage is very important, make sure to always check it before buying a home.