Half of that space must be public and consist of well-managed, high-quality green spaces linked to the wider countryside, housing minister Caroline Flint said as she unveiled a progress report on eco-towns.
But critics say the green space standard does not go far enough to set a strong example to developers and the international community.
Landscape planning consultant Ian Philips, who represents the Landscape Institute on the Town & Country Planning Association's eco-town working group, said the headline figures weren't innovative: "This is not particularly groundbreaking and I'm not convinced what we are seeing here is going to set an example to the world. We're pushing the concept of green infrastructure, which should be accessible and contribute to the green network in terms of biodiversity."
He added that the Government needed to provide more detailed standards to developers, such as street-tree planting to offset the urban heat island effect, or using green space multi-functionally, such as a location for ground source heat-pump installation.
GreenSpace chief executive Paul Bramhill said a figure of around 20 per cent of public green space was average for most towns and cities: "It sounds as though it is promoting the status quo but we should be looking at innovative ways of managing it and trying to get communities more involved."
In south Cambridgeshire, English Partnerships' proposed 9,500-home town Northstowe is set to allocate a third of its total area to public space.
BALI technical director Neil Huck said the UK was still lagging behind its European counterparts: "There seems to be more of a will in Europe and that has got to change, particularly in high-density urban areas."
Further consultation will take place from September on the potential eco-towns, with the final list of sites chosen for development to be announced in January 2009.
The HTA is campaigning for local authorities to push for more green space in new developments.
- - There are 13 potential locations shortlisted.
- - Up to 10 will make the grade - to be decided in early 2009.
- - Chosen locations will have to go through local authority planning processes.
- - Up to five eco-towns will be built by 2016 and another five by 2020.
- - Each eco-town will have bet-ween 5,000 and 20,000 homes.