Extension are added only on the 5' side/end of the primer, on 3' you already have the primer bases itself to help the restriction enzyme sit properly on the DNA.

For example

Forward Primer with BamHI site ggatcc

5' cagtgggatccATGAACCTTGTTCGCGCATGGA 3'

Reverse Primer with HindIII site aagctt

5' cagtgaagcttTCAGCAACAGCCGCGCGT 3'

Mangal Singh Thank you. Are there any specific bases to be added on the 5' end or its just few random nucleotides?

Only at back (towards 5'end) works fine.

Soundarya Priya There are specific nucleotides to be added, which you can find in this article and accompanying link to pdf Cleavage Close to the End of DNA Fragments (oligonucleotides)

go to it for further details.

https://international.neb.com/tools-and-resources/usage-guidelines/cleavage-close-to-the-end-of-dna-fragments

see this pdf and add oligos front side only.

Thank you everyone