The sitting and facing of the house are key components to any Feng Shui reading. It is essential that you know the sitting of the house in order to find the house trigram. The house trigram will then determine the trigram group the house belongs to; east group or west group. The groups determine your compatibility with others and with the house. Your trigram determines best directions for sleeping and working, as well as your group. How you align with the house is important. If you are in a house in which you and the house are in opposite groups, then it will be difficult to find a good direction for you to align to based on the walls of the house.
To find the sitting and facing directions, remember that the literal usage of the orientation is Open and Closed. The open side of the house is the facing and the closed side is the sitting. You can also think in terms of Yin and Yang. The Yin side is the sitting and the Yang side is the facing. Yang is bright, open, public and more active. The yin side is darker, closed, private, and more restful.
Typically the bedrooms are on the sitting and the living room is on the facing side. The most important thing to understand is that the door does not determine the orientation of the house! Let me repeat, the door does not determine the orientation of the house. There are so many books that say otherwise and this is a big mistake. The door is the qi mouth. It is the portal for the qi to enter the house. It is key to having the door in a good direction. But, the door is only on aspect of the house. The house itself and its orientation are the key point.
The architectural intent is also something that can be used to determine the sitting and facing. Which way did the architect intend the house to face? If you know the facing, then the sitting is always opposite.
Keep in mind that the concepts of sitting and facing are fairly straightforward. But the application is very difficult. There are so many architectural designs and different types and shapes of structures, that sitting and facing becomes one of the most challenging determinations for even the most experienced practitioners.
So to summarize, the sitting is often the back side of the house and the facing is often the street side. This is not always true in custom homes or apartments. Apartments often have the balcony or large windows off the living room as the facing and the hallway or door as the sitting. Remember that the main rule in Feng Shui is it depends. There is no cookie cutter approach to Feng Shui that works. Each house needs to be looked at individually and a determination as to the sitting and facing is both key to the reading and something to be very careful with. Often when a practitioner gets negative feedback from a reading, it is because the sitting and facing where incorrectly determined.