Not all planning applications are legally required to be advertised by a site notice. Site Notices are required for the following:-

* Applications with Environmental Statement
* Departures from the Development Plan ? Where the Council are minded to approve the application.
* Applications that affect a right of way as defined by the Wildlife and Countryside Act.
* Development affecting the character or appearance of a Conservation Area
* Development affecting the setting of a Listed Building
* Applications for Listed Building Consent or Conservation Area Consent.
* Major development;
-Residential: more than 10 dwellings or sites over .5ha.
-More than 1000 sq.m. of floorspace
-Sites greater than 1ha
-Mineral applications
-Waste Applications [art8(4)]

When one is displayed it is supposed to be "readable by the passing public" but I don't think there is much more detail to that rule so people tend to be quite creative with it.