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Is it mandatory for PBS dependent to apply for ILR at the same time as Main applicant application.
Situation here is " Both Main & dependent have visa validity until jan 2018, But as Main applicant is now is eligible for ILR & want to apply for himself only" He want to apply for his dependent latter in Nov 2017.
Note:- Though PBS dependent is also eligible now to apply for ILR as per requirement conditions.

No, it is not mandatory to apply together. Remember that fees go up every April.

Assuming of course you are applying for ILR under Tie 1 G with 5 years residence.