Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Standing up is an integral of the prescribed prayers and the prayer is not valid without it unless a person cannot perform it while standing.
Answering your question, Dar Al-Ifta Al-Misriyyah, states:
Standing up is an integral of the prescribed prayers and the prayer is not valid without it unless a person cannot perform it while standing.
As for supererogatory prayers, they can be performed while seated even if one can stand. However, if one has no excuse for praying while seated, one earns only half the reward of the prayer of a person who prays while standing.
Almighty Allah knows best.
Source: http://www.dar-alifta.org/Foreign/ViewFatwa.aspx?ID=7961