According to Madhhab:
What is the ruling in regards to performing I’tikaaf for the entire duration of Ramadaan?
Observing I’tikaaf during the last ten days of Ramadaan is Sunnat-e-Muakkada Alal Kifaaya. However, the act of observing I’tikaaf for the complete duration of Ramadaan in search of Lailatul Qadr is also proven from Rasulullah Salallahu Alaihi Wasallam, hence if a person sits in I’tikaaf for the entire duration of Ramadaan, he will be rewarded. It will not be considered a Bid’at.
Re:- Fataawa Mahmoodiyya pg.255 vol.15