In case C-323/17 People Over Wind and Peter Sweetman v Coillte, the Court of Justice of the European Union (CJEU) ruled that mitigation measures could not be taken into account at the screening stage of an appropriate assessment.
This case focused on proposed works that were necessary to lay a cable connecting a wind farm to the electricity grid and the potential effects that this would have on two special areas of conservation. Objectors had argued that river pollutants, such as silt and sediment, resulting from the laying of the cable would have a harmful effect on the River Nore Pearl Mussel.
In 2013, An Bord Pleanála granted planning permission to Coillte for the development, with a number of conditions. One of these conditions was that the construction of the development would be managed in accordance with a Construction Management Plan which would be agreed with the local authority. This plan would provide details of construction practice for the development, including "… (k) means to ensure that surface water run-off is controlled such that no silt or other pollutants enter watercourses …”
Coillte instructed consultants to conduct a screening for an appropriate assessment to establish if the project was likely to have a significant effect on the special areas of conservation. The screening report concluded that, in the absence of protective measures, there was potential for the release of suspended solids into waterbodies along the proposed cable route. It also concluded that there would be a negative impact on the pearl mussel population if construction caused the release of silt or pollutants, such as concrete, into the pearl mussel habitat through smaller streams or rivers. The report then went on to detail (i) protective measures that had been built into the design of the project and (ii) the distance between the proposed grid connection and the special areas of conservation. As a result of this screening and the proposed mitigation measures, Coillte determined that an appropriate assessment was not required.
High Court referral to the CJEU
Coillte's determination that an appropriate assessment was not required was challenged in the High Court. Barrett J then referred the following question to the CJEU for a preliminary ruling: “Whether, or in what circumstances, mitigation measures can be considered when carrying out screening for appropriate assessment under Article 6(3) of the Habitats Directive?"
The CJEU held that any measures intended to avoid or reduce the harmful effects of a project on a site should not be taken into account at the screening stage. The CJEU said that to do so would compromise the practical effect of the Habitats Directive in general. It also noted that the purpose of the assessment stage could be undermined and circumvented if this occurred. Although the term 'mitigation measures' is not referenced in the Habitats Directive, the CJEU noted that it should be understood as meaning measures that are intended to avoid or reduce the harmful effects of the proposed project on the site concerned.
In reaching its decision, the CJEU noted the importance of the precautionary principle to the interpretation of Article 6(3). It also noted the very precarious position of the Nore Pearl Mussel, which are said not to have reproduced since 1970. As a result, the population had reduced from 20,000 individuals in 1991 to as low as 300 individuals today. The threat of extinction to the Nore Pearl Mussel and the risk of silt/pollutant release, with its accompanying negative impact, appeared to weigh heavily on the court's mind.
In Ireland, before the CJEU's judgment, it had been established practice for mitigation measures to be taken into account at the screening stage. In Ratheniska Timahoe and Spink (RTS) Substation Action Group v An Bord Pleanála  IEHC 18, the High Court accepted that An Bord Pleanála was entitled to have regard to mitigation measures when deciding whether to proceed to an appropriate assessment. In Ratheniska, that meant An Bord Pleanála could take account of best practice construction management techniques designed to prevent any deterioration of water quality within or upstream of the protected river.
Most recently in Harten v An Pleanála  IEHC 40 (29 January 2018), Judge McDermott held that mitigation measures did not have to be an intrinsic part of the proposed development for them to be taken into account. He found that it was enough that the mitigation measures were a part of the proposed development.
The CJEU's judgment has overturned this line of authority and in the process, made life more challenging for developers and competent authorities. It is not clear what criteria will be used in deciding whether a measure is one intended to avoid or reduce the harmful effects of the proposed project. In particular, the dividing line is not always clear between what is an integral or intrinsic part of a proposed development and what is a mitigation measure. Although the CJEU's emphasis is clear (mitigation measures are more appropriately considered at the appropriate assessment stage, rather than screening), how this translates into practice is less so.