Assalaamu alaykum. My question is regarding the supplication that says, "Bismillaah, tawakkaltu ‘ala Allaah, wa la hawla wa la quwwata illa billaah." This supplication should be said when leaving the home.
1) If I live in a block of flats, then what is the right time to say this supplication? Is it right when going out of my door while I am still inside the building, when I am in the elevator and have left my floor, or when I go out of my block and have actually reached the yard?
2) I have the same question but regarding those who live in a mansion or a separate cottage house. When shoud they say this supplication? When going out of the door of their house but while they are still in their own yard, or when they have completely crossed the yard gate that opens on to the street?
3) Also, if I am a guest at somebody's house and I stay there overnight, is this supplication applicable to a place that I am leaving but which is not my own?
Please provide evidence or quotes from scholars. May Allaah be pleased with you!

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

Seeking protection of Allah when going out is done by saying the supplication upon leaving your own home, such as your apartment (flat), for instance.

Anas ibn Maalik, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said:

"If a man leaves his house and says, 'Bismillah, tawakkaltu ‘ala Allah, wa la hawla wa la quwwata illa billah.' (In the Name of Allah; I rely on Allah; there is no power nor might save in Allah), then it will be said to him, 'You have been guided, sufficed, and protected.' The devils will move out of his way, and one devil will say to another, 'How can you have access to a man who has been guided, sufficed, and protected?'" [Abu Daawood - Al-Albaani graded it saheeh (sound)]

A person who lives in a mansion or the like should say the supplication upon exiting the first door (the house door), not the yard gate.

A guest should say the supplication when leaving the host's house. An-Nawawi said:

"You should know that it is recommended to recite the Basmalah (saying 'Bismillah', which means: in the Name of Allah) upon entering one's house or the house of others, to say Salaam (greeting of peace) even if there is no one inside, and to say the supplication upon going out. Anas, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said, 'Whoever says (meaning if he goes out of his house), 'Bismillah, tawakkaltu ‘ala Allah, wa la hawla wa la quwwata illa billah,' it will be said to him, 'You have been defended and protected,' and the Devil will move out of his way.' [Abu Daawood and At-Tirmithi who graded is hasan (good)]."

Allah knows best.