If a garment has an extension which is not woven properly, it is not considered a part of the garment. Therefore, the tzitzis should only be hung from the actual garment and not the extension. The extension does, however, count towards the three fingers’ width distance to the hole. Regarding the minimum distance from the edge of the garment to the hole, there is a disagreement about whether it should be measured from the edge of the garment or the edge of the extension. To avoid any questions, it is best not to add any extensions to a garment.
[שו"ע יא, יא, ומשנ"ב נד ו־נז; ביאורים ומוספים דרשו, 70]