Below is the relevant section of PPG 15 which I reproduce at the end of the message.
There is much confusion amongst many people about what the setting of both monuments and listing buildings actually are and how it is protected. For buildings it is straight forward - basically it is purly about visual impacts. I think PPG 15 is clear in what is and is not protected and how it should be analysed.
What tends to happen is if an application has no good reason for refusal then a objection based on "detrimental impact on the setting of a listed building" is often used or used to back up a fairly weak objection on other grounds. It also tends to be used to object to curtiledge construction and curtiledge is taken to mean the same as setting which is it not.
For monuments there is no clear definition, I think, but is certainly not a mechanism for preserving landscapes.
Peter
See
http://www.odpm.gov.uk/index.asp?id=1050943#P86_30380
The setting of listed buildings.
2.16 Sections 16 and 66 of the Act require authorities considering applications for planning permission or listed building consent for works which affect a listed building to have special regard to certain matters, including the desirability of preserving the setting of the building. The setting is often an essential part of the building's character, especially if a garden or grounds have been laid out to complement its design or function. Also, the economic viability as well as the character of historic buildings may suffer and they can be robbed of much of their interest, and of the contribution they make to townscape or the countryside, if they become isolated from their surroundings, e.g. by new traffic routes, car parks, or other development.
2.17 Local planning authorities are required under section 67 of the Act to publish a notice of all applications they receive for planning permission for any development which, in their opinion, affects the setting of a listed building. This provision should not be interpreted too narrowly: the setting of a building may be limited to obviously ancillary land, but may often include land some distance from it. Even where a building has no ancillary land - for example in a crowded urban street - the setting may encompass a number of other properties. The setting of individual listed buildings very often owes its character to the harmony produced by a particular grouping of buildings (not necessarily all of great individual merit) and to the quality of the spaces created between them. Such areas require careful appraisal when proposals for development are under consideration, even if the redevelopment would only replace a building which is neither itself listed nor immediately adjacent to a listed building. Where a listed building forms an important visual element in a street, it would probably be right to regard any development in the street as being within the setting of the building. A proposed high or bulky building might also affect the setting of a listed building some distance away, or alter views of a historic skyline. In some cases, setting can only be defined by a historical assessment of a building's surroundings. If there is doubt about the precise extent of a building's setting, it is better to publish a notice.