The West Bank land belongs to Israel, nothing illegal. Israel can do anything it wants with the land. The Aras living there can leave or stay and keep yapping but Israel can do what it wants with the land. Most countries of the world are on land they got by conquest from previous owners. The US is an excellent example. What makes use of your own land illegal? Don't go back to history and prior holders. Israel holds it now because the the prvious owners, the Jordanians, gave it up in 1967 and Israel walked in and took it over and no one has yet driven them out.
Saudis warns U.S. that 9/11 responsibility law would erode investor confidence in America (Reuters)
from the article: IDF agrees to expansion of West Bank settlement