Generally, it’s possible to extend a barn conversion, but there are certain conditions that you’ll probably need to meet. Ultimately, it’s up to your local planning office to decide whether to approve your planned barn conversion extension or not.
Typically, small extensions on the side of a barn conversion are often approved, particularly if they serve to preserve the original character of the barn. This could be because the extension will mimic the appearance of the original structure, or house facilities that would look odd in the original building.
However, you probably won’t be allowed to build a larger extension, or one that isn’t in keeping with the appearance of the original barn.