1. The approach to mitigation in accordance with Local Plan policy TR2

Local Plan Policy TR2 states the following:
"Any development that results in significant negative impacts on air quality within identified Air Quality Management Areas or on the health and wellbeing of people in the area as a result of pollution should be supported by an air quality assessment and, where necessary, a mitigation plan to demonstrate practical and effective measures to be taken to avoid the adverse impacts."
On the basis of the above policy only development that results in significant negative impacts should require mitigation.
The principle of only significant impacts being assessed is set out in the NPPG:
"When deciding whether air quality is relevant to a planning application, considerations could include whether the development would:
Significantly affect traffic in the immediate vicinity of the proposed development site or further afield. This could be by generating or increasing traffic congestion; significantly changing traffic volumes, vehicle speed or both; or significantly altering the traffic composition on local roads. Other matters to consider include whether the proposal involves the development of a bus station, coach or lorry park; adds to turnover in a large car park; or result in construction sites that would generate large Heavy Goods Vehicle flows over a period of a year or more."
NPPG 005 Reference ID: 32-005-20140306
Furthermore, the NPPG is clear that the purpose of mitigation is to prevent "unacceptable" risks.
"Mitigation options where necessary will be locationally specific, will depend on the proposed development and should be proportionate to the likely impact. It is important therefore that local planning authorities work with applicants to consider appropriate mitigation so as to ensure the new development is appropriate for its location and unacceptable risks are prevented. Planning conditions and obligations can be used to secure mitigation where the relevant tests are met."
NPPG 008 Reference ID: 32-008-20140306
There is no definition of significance in the Air Quality SPD, and therefore it is unclear when mitigation would be required.
2. It is unclear and significant impacts are not defined

As noted above, significant impacts are not defined in the SPD. However for Major developments at the assessment stage, a damage costs calculation is required. Concern is raised that the Council will require compensation or mitigation for mitigation for proposed development undertaking this exercise irrespective of whether overall the impact is deemed significant.

Furthermore, paragraph 5.4 suggests that only WDC can assess the significance and acceptability of a development proposal, based on local air quality knowledge and cumulative impacts. This statement is not considered an appropriate response in a supplementary planning document which is meant to expand upon development plan policies and provide clear guidance as to what may or may not be acceptable. In addition, the methods for assessing cumulative impacts exist, and data in relation to existing, committed and planned development sufficient for assessment.