When can a coda be added to a sonata-form movement?

In sonata-allegro form, a coda is typically added immediately following the recapitulation section to serve as the concluding portion of the movement.1

While the recapitulation is designed to bring the movement's thematic material into a state of equilibrium, a coda is often employed when the conflicts of key and theme established earlier in the movement are not fully resolved by the recapitulation alone. Although it is a common feature, the coda is not considered an essential element of the sonata-form structure.2

The length of a coda can vary significantly; it may be as brief as a few measures or, as seen in the works of Ludwig van Beethoven, it can be expanded to such a degree that it functions almost like a second development section.12