FORBIDDEN TO CHANGE ALLAH's CREATION

Ibn Mas'ud (RTA) narrated that the Prophet(SAW) said, "Allah cursed those women who tattoo and who have themselves tattooed, those who pluck hair from their faces and those who make spaces between their teeth for beautification, changing what Allah has created." A woman argued with him and he replied to her, "Why do you not curse those whom the Messenger of Allah(SAW) had cursed?"
{Collected by Al-Bukhari and Muslim}

It was narrated by Ibn 'Umar that the Messenger (SAW) said, "Allah cursed women who wear additional hair, and those women who tattoo and who have themselves tattooed." 

Allah the Almighty and His Messenger (SAW) also cursed those women who make their hair longer using human hair. This is unanimously forbiden. Nonetheless, if a woman made her hair longer with artificial hair, it is mostly unlawful. Some Shafi'ites allow this conditional on her husband's permission.

The impermissibility lies in the fact that such acts change Allah's creation. The same opinion applies to artificial hair, according to others, because it changes Allah's creation, unless it is made of non-human hair according to some Shafi'ites as explained above.

And Allah knows best