The secrets of Meraj and the vision of Allah Taala

Why did the journey start from the house of Hadhrat Umme Haani (May Allah Ta'ala be well pleased with her)

The journey of Meraj started from the house of Hadhrat Umme Haani (May Allah Ta'ala be well pleased with her) and not from the house of the Holy Prophet (Sallallahu alaihi wa sallam).  The wisdom here is that it is among the etiquette of the house of the Holy Prophet (Sallallahu alaihi wa sallam) that one should not enter it without permission.  The Holy Quran says:

O Believers! Do not enter the houses of the Holy Prophet [blessings and peace be upon him] unless permission is granted to you for a meal. Nor (reach so early as to) wait for the cooking of the meal. But when you are invited then enter (at that time). Then when you have eaten the meal, (get up from there and) disperse without delay and do not linger on in eagerness for talk. Surely your (sitting for long in this) manner discomforts the Holy Prophet[blessings and peace be upon him]; and he feels reluctant to (ask) you (to leave) but Allah does not hesitate in (saying) the Truth. And when you ask them (the pure wives) for something, ask them from behind a curtain. This (regard and nice manner) causes great purity for your hearts and for their hearts. And it is not (at all lawful) for you that you cause inconvenience to the Holy Messenger of Allah [blessings and peace be upon him], nor is this (lawful) that you marry his (pure) wives after him, till the end of time. Surely it is the most grievous sin in the sight of Allah.  Surah Ahzaab (33:53)

The angels are also included in this command as the Holy Prophet (Sallallahu alaihi wa sallam) is the Prophet for all creation, as given in this Hadith of Sahih Muslim:

I have been sent as the Prophet towards all creation.  (Sahih Muslim, Hadith No. 523; Musnad Imam Ahmad Bin Hambal, Hadith No. 8969; Zujajatul Masabeeh, Vol. 5, Pg. No. 8)

Hadhrat Mulla Ali Qari (May Allah Ta'ala shower His mercy on him) writes in Mirqaat in the interpretation of this Hadith: 

Translation: I have been sent as the Prophet towards the whole universe, all genie, humans, angels, beasts and plant life. 

Therefore, it is not permissible for even angels to enter the house of the Holy Prophet (Sallallahu alaihi wa sallam) without His permission.  The night of Meraj, the Holy Prophet (Sallallahu alaihi wa sallam) went to the house of Hadhrat Umme Haani (May Allah Ta'ala be well pleased with her), so that the angel could come to Him.