If you are inside a house or building when a backdoor appears in your dream state, you are most likely looking for the easy way out of a situation. It's like people are more inclined to escape through the backdoor if they weren't supposed to be inside the building in the first place, or they didn't want to be seen and noticed thus looking for the way out with the least confrontation as possible.
If you are outside a home or a building facing or looking at the backdoor, you may soon be involved with finding an unconventional and possibly, morally questionable solution to an issue.