What is the minimum requirement for a boy and girl relationship to be halaal?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Islam is a complete way of life. We have been instructed and guided in every faculty of our life.
Shariah has considered the natural emotions of humans and therefore created the bond of Nikaah/ Marriage. 
The only method for two non-mahrams to establish a halaal relationship is through Nikaah.
Shariah does not permit illicit relationships. It is strictly forbidden to date and enter into an illicit relationship. That incurs the wrath and anger of Allah Ta’ala.
Islam not only prohibits Zina, but also closes all the possible avenues leading to Zina. This is achieved by prohibiting every step and means leading to stimulating and aggravating desires.
Allah Ta’ala says in the Quraan:
وَلَا تَقْرَبُوا الزِّنَا إِنَّهُ كَانَ فَاحِشَةً وَسَاءَ سَبِيلًا
Translation: “And do not even go close to adultery. Indeed, it is ever an immorality and it is an evil way.”
(Surah Al-Israa, Aayah 32)
It is mentioned in a Hadeeth:
أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ “ لاَ يَزْنِي الزَّانِي حِينَ يَزْنِي وَهُوَ مُؤْمِنٌ وَلاَ يَسْرِقُ حِينَ يَسْرِقُ وَهُوَ مُؤْمِنٌ وَلاَ يَشْرَبُ الْخَمْرَ حِينَ يَشْرَبُهَا وَهُوَ مُؤْمِنٌ وَالتَّوْبَةُ مَعْرُوضَةٌ بَعْدُ
Translation: A fornicator who fornicates is not a believer as long as he commits fornication, and no one who steals is a believer as long as he commits theft, and no one who drinks wine is a believer as long as he drinks it, and repentance may be accepted after that.
(Saheeh Al-Muslim, Kitaab Al-Imaan)
And Allah Ta’āla Knows Best
Mufti Muhammad I.V Patel
Darul Iftaa Mahmudiyyah
بدائع الصنائع في ترتيب الشرائع (2/ 228)
لَا خِلَافَ أَنَّ النِّكَاحَ فَرْضٌ حَالَةَ التَّوَقَانِ، حَتَّى أَنَّ مَنْ تَاقَتْ نَفْسُهُ إلَى النِّسَاءِ بِحَيْثُ لَا يُمْكِنُهُ الصَّبْرُ عَنْهُنَّ وَهُوَ قَادِرٌ عَلَى الْمَهْرِ وَالنَّفَقَةِ وَلَمْ يَتَزَوَّجْ يَأْثَمُ
This answer was collected from Daruliftaazambia.com, which serves as a gateway to Darul Iftaa Mahmudiyyah – Lusaka, Zambia.