I also heard a similar statistic for offices. There is a constant churn in the office market, not only in moving but also in refurbishing. In fact, fully half of the UK construction market is activity other than new building. One thing that really brought this home to me was the UK's Private Finance Initiative (PFI) in which public sector buildings were procured using private sector finance (largely from the banks). The basic idea is that a bank puts up the money for building a facility, then the private sector is paid a monthly or annual fee for operating the facility, from which they can repay the loan. This method of procuring public sector infrastructure has been very popular, and one unintended consequence is that the consortia who build such a facility, and operate it, sell it on to other operators. There is a healthy secondary market in completed PFI facilities, whereby an operator can buy the thing and run it. So, the idea of engaging the supply-side in long-term commitment has only resulted in yet another short-term engagement, as I am coming to expect with the construction sector.
So I have come to the conclusion that far from being permanent things, buildings are ephemeral. I don't mean the structure or the land. I mean our relationship with a building and the way that we define it and use it. Can we say that every part of the urban environment that we relate to is a constantly changing and ephemeral interpretation that is only temporarily ascribed to it? Does this help us to relate to the urban environment, or to interpret it?