To define the feng shui bagua (or the energy map of a house) with several levels, you have to work with the floor plan of each level separately. This does not necessarily mean that there will be a big difference between the location of various bagua areas on each floor.
It is always best to work with each level separately because this allows for a more accurate feng shui work. It also goes without saying that you will be working with the same bagua style on both levels, meaning if you work with the classical feng shui bagua, you will apply it on both levels, and if you work with the western or BTB bagua, you will do the same.
Mixing the two bagua styles in the same home is never a good idea unless you are very experienced with feng shui and know exactly what you are doing.
More often than not, each level of your house will have a different flow of energy as defined by its specific floor plan. The best way to start is to define the bagua of the main level as this is the bagua that holds the most power or energetic importance in a home.
Defining Your Home's Bagua
The feng shui energy flow in the whole house is greatly influenced by the energy of the main floor, so it is always wise to first do your best with the bagua of the main floor.
It is also important to know that the center—the yin-yang point or the heart of the home—can be different on each level. The center with the most power, again, is the center of the main floor.
If you are working with the BTB bagua, there will be a bigger variance between the bagua of the first floor and the bagua of the second floor because in the BTB school, the bagua is determined by the location of the main door, which means that once you are working with the bagua of the second level, your main door location (which is the entry point to the second level) is almost always in a different location than the front door of the house (which determines the bagua of the first floor).
If you are working with the classical bagua, you are taking into consideration the compass directions, and because they are the same no matter which floor you are working on, your two classical baguas will be quite similar.
To define the classical bagua of the second floor, you are working with the same direction of the front door; you can even superimpose the second-floor bagua over the first floor bagua according to the house layout; just to make it easier for yourself.