When evaluating the Feng Shui of a building, there are three external factors and three internal factors that take precedence over everything else. These are called the Key Factors. If these factors are good, then the building has good Feng Shui.
The external factors are more important than the internal factors, because Qi comes to a building from the outside in. The three external factors are Mountains, Water and the front door of the building.
Mountains actually generate Qi. A nearby mountain that is lush and green will, in general, generate auspicious ("Sheng") Qi, while a barren, rocky mountain will, in general, generate inauspicious ("Sha") Qi. If you are evaluating a relatively flat area, there are still mountains -- even gently rising areas qualify.
Water conducts Qi, and it is important to evaluate the cleanliness of the water, the location and the flow of the water formation. In urban areas, highways play a similar role to waterway, conducting Qi. Ideally, water formations should flow gently in a meandering path.
When discussing external key factors, the main door refers to the external area just outside the main entrance to the building. This should be a "Bright Hall", or "Ming Tang", area. This means an open, spacious area that will allow Qi to collect before it gently meanders into the building. Negative features like a wall, a large tree or a lamppost directly in front of the main door block or otherwise interfere with the flow of Qi into the building, and create Sha Qi.
The three key factors inside a home are the main door, the kitchen and the master bedroom.
When discussing internal key factors, the main door refers to the space just inside the main door. As with the outside of the door, there should be a similar "Ming Tang" area inside the door. This allows Qi to collect inside, before it circulates to the rest of the building. Overhead beams, pillars or walls inside the main door obstruct this flow of Qi, and are negative forms.
The kitchen has a direct effect upon our health and wellbeing. When evaluating the Feng Shui of a kitchen, begin by looking for negative external forms outside the kitchen. Pylons, large trees, junctions or overly Yang features like busy roads should not be located outside the kitchen.
The most important feature of the kitchen is the stove. The stove should not be directly aligned with the kitchen door, and it should not occupy the center of the kitchen. In addition, the stove should not be placed underneath a beam, and it should ideally be backed by a wall.
The kitchen sink should not be directly next to the stove, since the Fire of the stove would clash with the Water of the sink. There should be a distance of at least one foot between them. Similarly, the stove and sink should not be placed directly opposite one another. If they are, a barrier of some sort should be placed between them.
The master bedroom is the final key factor for the inside of the home. Like the kitchen, the bedroom is a predominately Yin area, so there should not be an excessive number of doors or windows, since these are Yang features. The bed should not be directly aligned with the bedroom door, and should not be placed under a beam or slanting ceiling. The head of the bed should be placed against a wall or other steady surface.