Answered by Mufti Mohammed Tosir Miah
Question:
Dua after Nafl Salah
Answer:
Scholars have written that it is preferable that one does a good deed before he makes a dua. Such a dua is more likely to be accepted. Therefore, it is not only permissible but also commendable that one makes dua after sunnah or nafl prayers.
It is because of this reason ones dua is more likely to be accepted after Tahajjud Salah.
Saaiduna Abu Hurairah Radiallahu Anhu narrates that the Prophet of Allah 
said “In the last third of every night Allah 
descends to the lower most heaven and says: Who is calling Me, so that I may answer him? Who is asking Me, so that I may grant him? Who is seeking forgiveness from Me so that I may forgive him!” (Sahih Bukhari p.153 v.1)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham