Does installing a door on the stairs at home affect Feng Shui?

Yes, because the sitting direction of a house in geomantic omen is an important standard to judge and calculate the geomantic pattern, and the sitting direction of a house is calculated by the main entrance of the house. Now that you have added a door to the stairs, it is equivalent to replacing your original outermost door with this door, so the whole feng shui pattern of your home has changed because of the change of this door, which may be better or worse. It doesn't have to be like this ~ it depends.