Sheikh `Abd al-Rahmân al-Jar`î, professor at King Khâlid University in Abhâ
Hair extensions made from human hair are clearly prohibited, even if they do not look like the woman's own hair.
The Prophet (peace be upon him) said: “Allah has cursed the woman who adds hair extensions and the woman who asks for it to be done.” [Sahîh al-Bukhârî (5934) and Sahîh Muslim (2122)]
With respect to extensions made of non-human or synthetic hair, this is a matter of disagreement among scholars. The strongest view is that it is permissible to use such extensions as a means of adornment, in the same way that clothing and jewelry are used, provided that it is obvious that the extensions are for decorative purposes only and are not being used as a means of deceiving a prspective husband.
This is because hair extentions of non-human or synthetic origin that do not resemble the woman's own hair take the same ruling as clothing and jewelry, since they function in the same way.
And Allah knows best.
Source: Islam Today