In reply to Blizzard:
Redevelopment of brownfield sites can cost an absolute fortune, especially if its an ex-industrial facility. In the case of using old buildings, very often they don't meet current building regs.
Back when I was a civil engineer, I knew of quite a few old buildings that were knocked down as they couldn't meet current regs and usage requirements (e.g. floor loadings, space for cable ducts, floor-to-floor heights for fire supression systems, lighting, etc) without so much modification that it was much, much, cheaper to flatten the building and start from scratch.