Mary says that dress alterations refer to modifying an existing dress to improve its fit, function, or appearance. Unlike creating a custom dress from scratch, alterations focus on adjusting a finished garment to better suit the wearer’s body and style.

Hemming: Adjusting the length of the dress by shortening or, if possible, lengthening the bottom edge.
Taking in or letting out: Changing the fit of a dress by adjusting seams to make it tighter or looser around the waist, bust, or hips. Letting a dress out depends on the amount of extra fabric (seam allowance) available inside the seams.
Strap adjustments: Shortening or lengthening shoulder straps to ensure the neckline sits correctly and the dress is comfortable.
Adding or removing elements: Incorporating features like bustles, bust cups, or pockets, or removing appliques and beading.
Reshaping the silhouette: Making more complex changes, such as adding or adjusting princess seams or darts, to give the dress a more tailored shape.