BBC Radio 4's Today programme asked two experts for their contrasting views on whether to build hundreds of thousands of new homes on greenfield sites.

Sir Peter Hall, professor of planning at University College London and director of Institute of Community Studies

Green belt land and greenfield sites are quite different.

Green belt is the area around London and other cities, such as Oxford, which is specifically designated that there should be no development.

Greenfield is all that but a lot more besides, beyond the green belt, which has never been designated against development and could be developed in appropriate circumstances.

They [government-appointed consultants] have carefully designated areas that could be developed.

The countryside is precious to all of us and it going pretty fast Nick Scone CPRE

They are not large, they are not new cities.

They are quite distinct developments.

There is no way of building 75% of housing on brownfield sites [previously developed land] in my view.

You could only do it by cramming people in at the highest densities we ever developed in the 18th century.

We are now in the 21st century.

The government has said we must achieve moderate densities, equivalent to the way we developed in London between the two world wars, for all new development in the south east.

It would take 40-50 generations to cover the countryside in concrete.

At present 11% of England is developed and 89% is undeveloped.

It might get to 12% developed in 15 years time.

We are greatly exaggerating the extent of problem at a time when also 10% of the countryside in the south-east of England is lying idle [set aside by the European Union].

It is not doing anything agriculturally.

Nick Scone, director of communications for the Campaign for the Protection of Rural England

We would like to see as many as possible of the new houses, and more than the government is proposing, to go on brownfield sites.

The government has already hit its target of 60% of them going on brownfield sites.

We think a figure of 75% is appropriate now.

You can achieve very decent living standards, in very good conditions, living in housing like terraces. Nobody is crowded.

We accept there is a need for more housing.

We accept some of it will have to be in the South East and we accept some of it will have to go on greenfields.

But we have a general worry about more and more of the countryside, which is precious to everyone whether they live in the countryside or the town, disappearing under housing estates and concrete.

Our fear about this plan is that this will be more and more of this.

The government has decent aspirations and is talking the right language to a degree.

But are they serious?

Will they meet the higher targets. Will they use brownfields to the maximum?

The countryside is precious to all of us and it is going pretty fast.