Neighbourhood consultation differs from permitted development not only in terms of the potential increase in size, but PD has its guidelines and is applied for by way of a Lawful Development Certificate LDC. Neighbourhood consultation or prior approval is a scheme set out to consult with your immediate neighbours, to their views on your proposed extension.
Applying for planning is the same process as applying for a certificate of lawfulness, in terms of the application process and how it is assessed. In essence permitted development is determined on the interpretation of the principles of the guidelines set out. Planning is determined on its design merits, planning law and precedents, neighbourhood consultation is determined solely on your neighbours comments or objections and therefore can be rejected on that ground alone.