I read a report on the Palestinian Maan News website that in 2010 15,000 illegal migrants crossed into israel to find work and shelter. Up till October of 2011another 12,000 arrived. That means that about 27,000 people have arrived in Israel and they all need housing. The demand works itself all through the economy and pushes for more homes to be built. It is a catch 22 scenario. Now there are two ways around this. Have Egypt absorb all of them or Gaza or West Bank. Failing that new homes need to be built in order to make room.
from the article: Germany: Israel plan for new West Bank homes 'devastating' to peace