Tailored single-breasted jackets and blazers that are left un-buttoned are usually more flattering. They create a natural vertical line that elongates the silhouette, and draws the eye up-and-down. Leaving a jacket un-buttoned is also more comfortable and shows off interesting layering detail.
Bustier gals and gals who are a little broader in the shoulder can battle with jacket-button closure. The jacket fits everywhere else, but can’t button up. Going up a size means that the buttons will fasten, but that the shoulder line, sleeve length and torso are sloppy. An experienced and superb seamstress can alter anything, but jackets are tricky and expensive to alter (especially if they have lining). In this instance, alteration is often not worth the price and effective results are not guaranteed.
So I’m giving you the go-ahead to purchase a jacket or blazer that doesn’t close across the bust area, if it fits perfectly in all other respects. Obviously, outerwear trenches, coats, parkas and gear need to button up for practical reasons. There is little sense in braving the elements for aesthetics. But I’m fine with leaving other types of jackets unbuttoned as a cost-effective, stress-free and comfy solution.