The government recently introduced the concept of Grey Belt and instructed local authorities to conduct an assessment of which parts of the Green Belt can be regarded as Grey Belt. East Herts Council instructed an external consultancy to carry out this assessment, and their report can be found here.
Figure 4.12 of the report shows the proposed Grey Belt area around Tewin, Datchworth and Watton-at-Stone. A sub-section of Figure 4.12 is reproduced here. Upper Green is the small triangular area above the word TEWIN, and as an aid to orientation it has been coloured yellow, whereas in Figure 4.12, it is coloured grey.
The Campaign for the Protection of Rural England has issued a report explaining Grey Belt and its impact. This can be found here
Devastating 'grey belt' impact revealed in our new report - CPRE Hertfordshire
Please note that this is not an East Herts Council initiative. They are just following instructions from central government.
You can also read the pdf of "Green Belf Assessment" Member briefing.