relevant planning law is on website of the deputy prime minister, PPG3 is usually most relevant. your local council should have a local plan of number of new houses needed. land may have been allocated to them already in which case the developer would have to show its a windfall site ( i'm assuming its not green field and used for farming) they then need to demonstrate theirs a need and your local council might have rules on number of affordadle houses to be built.
Increase in local trafic and safety is always worth a go, also you have a right to privacy so if the proposed development overlooks houses that should be included as well as increases in noise/light polution.
i'm not certain on rules on car parking spaces per house as i think they have changed but developer must show there is adequate parking per property.
its always worth a trip to your local panning dept and ask them for advice but from sounds of it the land is exactly what the government are trying to use before opening up more greenfield sites
