Illegal because the council has not adequately consulted the community on the new proposal to remove important areas from the Green Belt, specifically area KB4 in Knebworth. In July 2016 the same council declared that KB4 makes a "significant" contribution to Green Belt purposes, helping to prevent sprawl, merger and encroachment. Why the sudden change? Reference:
http://www.north-herts.gov.uk/sites/northherts-cms/files/NHDC%20Green%20Belt%20Review%20July%202016.pdf
See section 2.5 Assessment - existing Green Belt, 8 Knebworth.

Unsound, against national policy, because the Council has failed to justify the permanent removal of Green Belt areas. Note that National Policy Planning Framework (NPPF) defines essential characteristics of Green Belts as being open and permanent. NPPF para 83 states that Green Belt boundaries should endure in the long term, beyond the local plan. The draft local plan makes no such provision.

Unsound, unjustified, because the building of more accommodation, as flats or houses or conversions, is normal for a developing village or town and is not an exceptional circumstance.

Lack of co-operation. There is no evidence of any co-operation with other councils to protect the Green Belt and prevent coalescence between population centres in North Herts and surrounding areas.

More effective alternatives exist. For example a substantial area west of Stevenage has been reserved for 3100 homes, which would be more effective in meeting the needs of North Herts than sacrificing significant Green Belt areas around Knebworth. A development west of Stevenage could indeed be part of a more effective strategic plan including a critical east-west link road joining Stevenage to Luton Airport.
Does not meet infrastructure needs, just suggests improvements.