السلام عليكم ورحمة الله وبركاته
1. Abū Hurayrah raḍiyaLlāhu ‘anhu narrated from the Prophet ﷺ, “Allah has cursed those who extend hair and those for whom hair is extended, as well as tattooers and those who get tattoos.” (Bukhārī, Saḥīḥ)
2. Our Shāfi’ī jurists considered hair extensions impermissible, resulting in the doer being cursed by Allah, under the following circumstances:
2.1 When impure hair is used for extensions.
2.2 When human hair is used, including a person’s own hair.
2.3 When pure non-human hair is used for extensions on an unmarried woman or a married woman without her husband’s permission.
3. However, if a married woman extends her hair with pure non-human hair with her husband’s consent, scholars have deemed this permissible.
4. The jurists did not consider this ruling contradictory to the ḥadīth mentioned earlier. They argued that every general statement has exceptions. Due to the husband’s right for his wife to adorn herself for him, they exempted the case where a wife extends her hair with the intention of beautifying herself for her husband. Similar to this was their ruling that she may pluck her eyebrows and dye her hair black with the same intention. [See another answer I’ve written here for more details https://islamqa.org/shafii/shafiifiqh/30085/is-it-permissible-to-pluck-eyebrows/.
5. It is crucial to ensure that when using pure non-human hair for extensions, it does not obstruct water from reaching all of the original hair during obligatory ritual baths (ghusl). If the adhesive used hinders water from reaching all original hair during ghusl, the ghusl will be invalid, and the woman with such extensions will remain in a state of major ritual impurity.
والله تعالى أعلم
And Allāh knows best