In the Holy Qur'an, Allah Ta'ala has instructed the believers to seek permission before entering someone's house. It is mentioned in the Holy Quran –

يٰۤاَيُّهَا الَّذِيْنَ اٰمَنُوْا لَا تَدْخُلُوْا بُيُوْتًا غَيْرَ بُيُوْتِكُمْ حَتّٰي تَسْتَاْنِسُوْا وَ تُسَلِّمُوْا عَلٰۤي اَهْلِهَا ذٰلِكُمْ خَيْرٌ لَّكُمْ لَعَلَّكُمْ تَذَكَّرُوْنَ،فَاِنْ لَّمْ تَجِدُوْا فِيْهَاۤ اَحَدًا فَلَا تَدْخُلُوْهَا حَتّٰي يُؤْذَنَ لَكُمْ وَ اِنْ قِيْلَ لَكُمُ ارْجِعُوْا فَارْجِعُوْا هُوَ اَزْكٰي لَكُمْ  وَ اللهُ بِمَا تَعْمَلُوْنَ عَلِيْمٌ.

O believers! Do not enter houses other than your own houses until you ask permission and greet their inhabitants. This method is best for you. Perhaps you take advice. If you find no one in it, do not enter it until you are given permission. If you are told, 'Go back', go back. That is purer for you. Allah is fully aware of whatever you do. — Surah Noor (24): 27-28

In this verse, it is said to seek permission and give salam before entering another's house. If you enter someone's house without permission, the people of that house will face many problems and difficulties. It also violates the rules of Islam regarding veil (parda).

The verse also teaches us how to seek permission. The rule is to say 'As-salamu Alaikum' from outside. If it seems that the person inside the room will not hear salam, then knock on the door or ring the bell. If someone comes out, give him salam first. Do not enter unless permission is given.

Do not knock the door again and again. If there is no response from the people of the house even after sounding two or three times, then return. The nature of many people is- if there is no response from inside after ringing two or three times, they enter the room or start peeping inside the room. This is wrong.

It is narrated in Sahih Bukhari that once Jabir radi Allahu anhu asked permission to enter the house of the Prophet sallallahu alaihi wa sallam. The Prophet sallallahu alaihi wa sallam asked, who is there?

Jabir r.a. said, "I".

The Prophet sallallahu alaihi wa sallam disliked his reply, 'I'.

Therefore, after giving salam, the Companions used to say their names.

People who live  in the same house should also seek permission before entering other's private rooms. In this case, it is best to give salam. One should not suddenly enter another's room without any warning.

Allah Ta'ala says in the Holy Qur'an:

يٰۤاَيُّهَا الَّذِيْنَ اٰمَنُوْا لِيَسْتَاْذِنْكُمُ الَّذِيْنَ مَلَكَتْ اَيْمَانُكُمْ وَ الَّذِيْنَ لَمْ يَبْلُغُوا الْحُلُمَ مِنْكُمْ ثَلٰثَ مَرّٰتٍ  مِنْ قَبْلِ صَلٰوةِ الْفَجْرِ وَ حِيْنَ تَضَعُوْنَ ثِيَابَكُمْ مِّنَ الظَّهِيْرَةِ وَ مِنْۢ بَعْدِ صَلٰوةِ الْعِشَآءِ ثَلٰثُ عَوْرٰتٍ لَّكُمْ لَيْسَ عَلَيْكُمْ وَ لَا عَلَيْهِمْ جُنَاحٌۢ بَعْدَهُنَّ طَوّٰفُوْنَ عَلَيْكُمْ بَعْضُكُمْ عَلٰي بَعْضٍ كَذٰلِكَ يُبَيِّنُ اللهُ لَكُمُ الْاٰيٰتِ وَ اللهُ عَلِيْمٌ حَكِيْمٌ، وَ اِذَا بَلَغَ الْاَطْفَالُ مِنْكُمُ الْحُلُمَ فَلْيَسْتَاْذِنُوْا كَمَا اسْتَاْذَنَ الَّذِيْنَ مِنْ قَبْلِهِمْ كَذٰلِكَ يُبَيِّنُ اللهُ لَكُمْ اٰيٰتِهٖ وَ اللهُ عَلِيْمٌ حَكِيْمٌ.

O believers! Let the slaves you own and the children among you who have not yet reached puberty seek permission (to come to you) at three times: before the Fajr prayer, at noon when you put aside your clothing (for rest) and after the Isha prayer. These three are the times of your privacy. There is no blame on you and them at other times. They are your frequent visitors as some of you are frequent visitors of others. You are in constant communication with each other. This is how Allah makes the verses clear to you. Allah is All-Knowing, All-Wise.

And when your children reach puberty, let them also ask permission (at all times) just as those before them have done. This is how Allah explains His verses clearly to you. Allah is All-Knowing, All-Wise. — Surah Noor (24): 58-59

In this verse, Allah Ta'ala has fixed three periods of time when even the children are asked to take permission to enter their parents’ house, because usually people are not very careful about their clothes during these times.

Allah did not fix any time for adults. Whenever they want to enter someone's house, they should seek permission. Negligence in this regard is against the Islamic etiquettes. It is very important to teach all the children of the house the Islamic manners of entering each other's and parents' houses.

People neglect in seeking permission more and more when they enter their parent's room. But in the Qur'an, it is said about the children that they should enter the house of their parents with permission. A Companion asked the Prophet, "Should I seek permission to enter my mother's house?"

The Prophet sallallahu alaihi wa sallam said, would you like to see him naked?

He said, no. The Prophet sallallahu alaihi wa sallam said, then enter after seeking permission. ( Marasilu Abi Dawud, page 336)