Is it permissible to offer the prescribed prayers while seated?
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.
And God the Almighty knows best.