I know that women are not allowed to use public bathhouse. But can they use the ladies toilets in restaurants and offices?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

There is nothing wrong for women to enter the ladies toilets, which are found in public places, but she should avoid Islamic prohibitions like revealing the ‘Awrah and the like [like when the place for relieving oneself is in an open place, like in the field or desert….in some countries]
What is meant by the Ahadiths that are reported about the prohibition for women to enter the bathroom, are the public bath houses, where there are many of these prohibitions, so a woman is forbidden from entering them except for a need and with certain conditions.

Shaykh Ibn Taymiyyah said:

“The scholars said: “The women are given concession to enter the public bath houses when in need, in the same manner that men are given concession to enter them [i.e. public bath houses for men only]; provided that she lowers her gaze, and protects her private parts, like when she is sick or in post-partum bleeding or that she has to perform Ghusl (ritual bath) which she can only perform in a public bath house.” [End of quote]

Allah knows best.