Eye lash extension which is natural and planted inside the skin and does not prevent water from reaching the skin, is allowed and there would be no problem in it but if it is planted in such a way that it is glued permanently to the eye lid or the eye lashes, it is not permissible.
As for the lash extension of the second type in which the eyelash extension is glued to the eye lashes and which remain there for two or three weeks, it definitely forms a barrier and it should be removed. It is not permissible to keep it during wudhu and ghusl. And if a woman has glued such eyelashes, she must remove it as soon as and when she wants to perform wudhu or ghusl. And if it is not possible to remove it, she must perform wudhu and ghusl by way of jabira until it is possible to remove it. And if she does not remove it intentionally, wudhu and ghuls performed by way jabira would also be void.