Ref. No. 41/952
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
There are two kinds of Sunnat: (1) Sunnate Muakkadah (emphasized sunnat) (2) Sunnate Ghair Muakkadah (a sunnat not emphasized).
(1) Sunnat-e-Muakkadah is a Sunnat which the Prophet of Allah (saws) performed punctually like four rakats sunnah before Zuhr fardh. So if a person abandons this sunnat permanently with no genuine reason, he will be sinful. But there is no sin if anytime the sunnah is missed without any intention or is given up due to some excuses.
(2) Sunnate Ghair Muakkadah is an optional prayer. It is also proved by the Prophet of Allah (saws) but it is not proved to offer them punctually. Therefore, offering it is a cause reward but leaving it is not a sin like four rakats begore Asr fardh.
And Allah knows best
Darul Uloom Waqf Deoband