No two properties are created in quite the same way. This means that there is no "one size fits all" approach to real estate photography. With that being said, generally a focal length of around 16mm is best. At this focal length, you're capable of getting wide shots that show the scale of a particular room or environment. You're also able to capture the smaller details that make the property unique. A 16mm lens is also a viable way to show off the scale of larger areas and exterior portions of a home. This gives the viewer an accurate sense of the scope of these areas.