Question Summary:
I wanted to know that if a Muslim gets married in a registry office (not the islamic way) is that halal?

Question Detail:

I wanted to know that if a Muslim gets married in a registry office (not the islamic way) is that halal?

The couple did not get married in the Islamic way with the Imam, Nikah, Mahr and 2 witnesses. I wanted to know that if the couple then lived together would they be commiting Zina (Adultery), and if they had children what would be the right of the children if they were the result of that zina?

Answer :

In the name of Allah, Most Gracious, Most Merciful
Assalaamu `alaykum waRahmatullahi Wabarakatuh
In order for a Nikaah to be valid, the proposal of the boy and girl intending to marry must take place in the presence of two male witnesses or one male and two females. If there is no proposal or acceptance or the proposal and acceptance are not witnessed as stated above, the Nikaah will not be valid.    
(Mahmudiya V 11 Pg 27 Faruqi)
ولا ينعقد نكاح المسلمين إلا بحضور شاهدين حرين عاقلين بالغين مسلمين رجلين أو رجل وامرأتين عدولا كانوا غير عدول أو محدودين في القذف قال رضي الله عنه : اعلم أن الشهادة شرط في باب النكاح لقوله عليه الصلاة والسلام [ لا نكاح إلا بشهود              
(Al Hidaayah V 2 Pg 306 Shirkat ilmiyah)               
قَالَ عَامَّةُ الْعُلَمَاءِ : إنَّ الشَّهَادَةَ شَرْطُ جَوَازِ النِّكَاحِ . 
(Badai us Sanai’i V2 Pg 523 Deoband)
وَأَمَّا بَيَانُ وَقْتِ هَذِهِ الشَّهَادَةِ - وَهِيَ حُضُورُ الشُّهُودِ - فَوَقْتُهَا وَقْتُ وُجُودِ رُكْنِ الْعَقْدِ - وَهُوَ الْإِيجَابُ وَالْقَبُولُ - لَا وَقْتُ وُجُودِ الْإِجَازَةِ حَتَّى لَوْ كَانَ الْعَقْدُ مَوْقُوفًا عَلَى الْإِجَازَةِ فَحَضَرُوا عَقْدَ الْإِجَازَةِ وَلَمْ يَحْضُرُوا عِنْدَ الْعَقْدِ لَمْ تَجُزْ ؛ لِأَنَّ الشَّهَادَةَ شَرْطُ رُكْنِ الْعَقْدِ فَيُشْتَرَطُ وُجُودُهَا عِنْدَ الرُّكْن 
                                                                                                                   (Badai us Sanai’I V2 Pg 529 Deoband)
وفسر القهستاني هنا الفاسد بالباطل، ومثله بنكاح المحارم وبإكراه من جهتها أو بغير شهود الخ، وشرط حصول شاهدين وسيأتي في باب العدة أنه لا عدة في نكاح باطل وذكر في البحر هناك عن المجتبى أن كل نكاح اختلف العلماء في جوازه كالنكاح بلا شهود فالدخول فيه موجب للعدة.
ان الدخول في النكاح الفاسد موجب للعدة وثبوت النسب
(Raddul Muhtaar V3 Pg 516 Saeed Company)
If the above conditions for the validity of the marriage were fulfilled when registering the marriage, the Nikaah will be valid but contrary to the sunnah.If the conditions were not fulfilled at the time of registering the marriage, the Nikaah will be invalid.
Children born in wedlock are recognized in Shariah as legitimate and those born out of wedlock are illegitimate.
And Allah knows best
Wassalam u Alaikum
Ml. Ebrahim Desai,
Student Darul Iftaa
Checked and Approved by:
Mufti Ebrahim Desai
Darul Iftaa, Madrassah In'aamiyyah