For BCECE examination Domicile of bihar is a criteria must to be fulfilled.Your parents should also be a residential of bihar. It doesnot matter if they are out of bihar for job.But domicile is neccesary.

Hello,
Below is the eligibility criteria of BCECE 2018:

• Nationality - Candidates of BCECE 2018 must be citizen of India.
• Qualifying Examination (QE) - Candidates who have passed or will be appearing in their 10+2 board or equivalent examination in 2018 are eligible
• Subjects in QE: They must have passed 10+2 or equivalent exam with Physics and Mathematics as compulsory subjects and Chemistry/Biotechnology/Biology/Technical Vocational subjects as optional.
• Marks Required: General category students should have scored 45% marks and SC/ST/OBC candidates should have secured 40%.
• Age Limit: There is no age limit.

For more information about this visit the below given link:
https://engineering.careers360.com/articles/bcece-eligibility-criteria
Good luck.