In reply to ClimberEd:
I do a fair bit of work looking at developing sites for house builders, both greenfield and brownfield (among other things within civil & geotechnical engineering industry).
Greenfield sites are nearly always more simple to build out, as there are just less problems and less risk in the ground, and therefore land prices of greenfield tend to be more expensive (at least on the face of it).
On a brownfield site, you'll typically have to think about and cost in demolition, enabling works, recycling aggregate, earthworks, investigation and remediation of contamination, poor ground conditions, drainage issues, various planning conditions, piling etc, before even getting out of the ground on build.
On a greenfield site, you might only need to strip the top soil and do a bit of earthworks, and you can start putting in foundations.
The flip side of that is most big developers are pretty switched on when it comes to looking at these 'abnormal' costs, and will typically negotiate these off the price of any brownfield site.
There are tax breaks in place for redeveloping brownfield sites, to encourage this over greenfield, but greenfield is still the easy choice.
Also profit. All the big house builders are very very cost focused and will do everything to keep costs low and profits high. Even more so on a brownfield site. Persimmon for example, a 30% profit margin is typical, on a turnover of £3.6b last year!
Land prices are very high in the UK, so to achieve that kind of profit, they are streamlining building costs and squeezing as many plots as possible on any given site.
Essentially, you end up with incredible expensive new homes, of poor quality, on tiny plots. Slums of the future.
The whole planning system needs overhaul really, to focus development on brownfield land, mandate bigger size houses/plots, better quality houses. House builders could and would still build, just at less ridiculous profits.
Post edited at 11:30