Interesting question, in view of the possible lack of precision in the NPPF. I too would like to see any usable third-party definition intended to be compatible with the NPPF. Sustainability as discussed in the NPPF base documents, "The Brundtland Report" and "The UK Sustainable Development Strategy Securing the Future", is a complex multi-level concept. Those two documents alone amount to hundreds of pages. What data do you need to collect? What weights do you give the various data items? What does any sustainability measure or value actually mean? It may be possible for an LPA to devise a workable in-depth NPPF-compatible sustainability policy .... but it could take a lot of in-house effort ... or expensive outsourced resource. The alternative approach of cobbling together a one paragraph sustainability policy may appear to be cheaper, quicker and simpler ... but it could lead to tears later on. Opting out completely and simply referring to the NPPF text could also to lead to problems, as you might then end up interpreting the two core documents plus the NPPF on a case-by-case basis .... However I may have missed an obvious & workable approach to the problem ...............
