This here housing crisis...

[Workingman]

And so the government announces that first time buyers under the age of 40 will get a 20% discount on new builds on brownfield sites. It will be partly funded by waiving local authority fees for homebuilders of at least £45,000 per dwelling on such sites. Where the £45,000 figure comes from is anybody's guess, but that it could be applied probably reveals why brownfield sites stand idle.
Will house builders up the price of new builds? Probably. Who pays for this government largesse? We do, of course.

[Suff]

The brownfield figure is probably an average between heavy industry which must be sterilised and sealed and non industrial sites which need virtually no remediation work. Although even old schools, colleges and government buildings will probably need Asbestos removal and that is not cheap today.

I recall working on vehicles in an old WW2 hanger in Waterbeach in the 80's. Whilst civilian contractors walked around us in moon suits with special filter vacuum cleaners.... Asbestos was very heavily used in government/industrial buildings and much was never removed, just left in place.