The most frequent cause of value change is a change in the general real estate market. As demand for property goes up, prices tend to go up. As demand decreases, so do prices.
An individual property can also change in value due to changes to the property itself. If something is added, such as a garage, bedroom, or pool, the value increases. On the other hand, fire, demolition, or depreciation from poor maintenance can decrease value.
Sometimes external economic forces can also change value. For example, if a major employer leaves town it tends to depress the local economy and thus property values. In another case, homes that have always been on a dirt road will increase in value if the road is improved and paved, creating better access. Times of general inflation also drive up property values.