The most important requirement is that the land no longer poses a risk to health and environment once it is cleaned up. This does not necessarily mean that all contaminants are removed from the site or that the site is cleaned up completely. The standard required by law is that it should be suitable for the use to which the land is put. This means that if land is to be used for housing with gardens then we would expect the contaminants to be removed (see the Bawtry case) but if the development comprises blocks of flats with no gardens or retail outlets with hard standing car parks, we might expect a lower level of expenditure on clean up. If the site remains in industrial usage this may be lower still.
It is not unusual to have new homes built on 'brown' land that has been subject to earlier development. The Government has set a target of 60 percent of all new housing to be built on brown land. In fact the number of new homes built on previously developed land stood at 76 percent in 2010 compared with 62 percent in 2000. So it is not uncommon for housing to be placed on former industrial land. If buying a new home on such a site, the planning system ought to ensure that it is safe and has been adequately developed with proper attention paid to any environmental pollution. Oddly enough you are more likely to suffer problems with older housing built before we were so concerned about the health impacts of pollution (see the Bawtry case).