The historic environment is a shared resource
According to Historic England, restoration to a significant place should normally be acceptable if; the heritage values of the elements that would be restored decisively outweigh the values of those that would be lost; if the work proposed is justified by compelling evidence of the evolution of the place, and is executed in accordance with that evidence; if the work proposed respects previous forms of the place and; if the maintenance implications of the proposed restoration are considered to be sustainable.
New work or alterations to a significant place should normally be acceptable if; there is sufficient information comprehensively to understand the impacts of the proposal on the significance of the place; if the proposal would not materially harm the values of the place, which, where appropriate, would be reinforced or further revealed; if the proposal aspires to a quality of design and execution which may be valued now and in the future and; the long-term consequences of the proposal can, from experience, be demonstrated to be benign, or the proposal is re designed not to prejudice alternative solutions in the future.
Changes which would harm the heritage values of a significant place should be avoided unless; the changes are demonstrably necessary either to make the place sustainable, or to meet an overriding public policy objective or need; there is no reasonably practicable alternative means of doing so without harm; that harm has been reduced to the minimum consistent with achieving the objective; that it has been demonstrated that the predicted public benefit decisively outweighs the harm to the values of the place, considering; its comparative significance, the impact on that significance, and the benefits to the place itself and/or the wider community or society as a whole.
Paul Rappoport - Heritage 21 - 28 November 2016
Reference; Conservation Principles - Policies and Guidance for the Sustainable Management of the Historic Environment, English Heritage - April 2008.