Subsequent case law post-Hillside
In Hillside Parks Ltd v Snowdonia National Park Authority  (“Hillside”) the Supreme Court determined that where development carried out under a subsequent planning permission would make it physically impossible to carry out development under an earlier permission in accordance with its terms, then that earlier permission could no longer be relied upon.
In this scenario it would therefore be unlawful to carry out further development under the original permission. In the following two cases the courts have provided important additional guidance in respect of the principles relating to incompatible planning permissions subsequent to Hillside.
R (on the application of Fiske) v Test Valley Borough Council  (Court of Appea)
Planning permission had been granted by Test Valley Borough Council in 2017 for the development of a solar park including a 33kV substation. In 2021 a further permission was granted which included a 132kV substation on land subject to the permission granted in 2017. It was acknowledged that implementation of the later 2021 permission would render completion of the 2017 permission impossible.
The appellant contended that the incompatibility between the two competing permissions was a clear material consideration which had not been considered by the local planning authority. The High Court held there was nothing wrong, in principle, in a local planning authority granting planning permission which was incompatible with an earlier permission relating to the same site and that it was for a developer to resolve any incompatibility.
In the appeal against the High Court decision, the key consideration for the Court of Appeal was whether the incompatibility between the 2017 permission and the 2021 permission, as applied, was a ‘mandatory material consideration’, to which the local planning authority was obliged to have regard to when considering the application for the 2021 permission. It was held that a local planning authority was not bound in law to take into account the incompatibility of the two permissions. There was nothing in the judgment in Hillside to support the proposition that the incompatibility between a previously granted planning permission and an application seeking permission for a different scheme would constitute a mandatory material consideration in the decision taken.
It was stated that the prospect of some future breach of planning control was not a material consideration to which the local planning authority was required to have regard under any provision of the statutory planning code. Neither this, nor the incompatibility of planning permissions, are identified in legislation, or in policy, as a matter to which a local planning authority must take into account. It was also recognised that the planning system does not prevent the possibility of a number of applications for planning permission being made and granted for different developments on the same site. It accepts the granting and co-existence of permissions that may not be mutually compatible, one or more of which may prove incapable of lawful implementation unless the incompatibility can be defeated by a further grant of planning permission.
R (on the application of Dennis) v Southwark LBC  (“Dennis”) (High Court)
Outline planning permission was granted in 2015 for the phased development of a housing estate. A phase was built between 2020 and 2023, but by 2022 there had been a change to the relevant development plan policy and it was necessary to increase housing numbers. In July 2022 an application for detailed planning permission was submitted for a further phase but it could not be achieved under the parameters of the outline planning permission. As such, the developer sought a full application rather than an approval of reserved matters under the outline planning permission. The local planning authority considered that the approval of a new standalone scheme would not prevent the remainder of the outline planning permission being implemented. In December 2022 the developer submitted an application under section 96A of the Town and Country Planning Act 1990 to add the word ‘severable’ to the outline planning permission and the local planning authority resolved to grant the new full planning permission subject to the section 96A application being granted.
It was suggested by the local authority that the permission was severable by reference to the phases and that the amendment was only intended to confirm that position as per the outline planning permission. However, the claimant contended that the outline planning permission was not severable and that the amendments could not be treated as non-material.
The High Court allowed the judicial review claim and quashed the outline planning permission. It held that the outline planning permission was not severable prior to the section 96A amendment and the decision had significantly enlarged the bundle of rights granted by the said permission. Hence it constituted a material amendment to the outline planning permission.
The position in respect of mutually incompatible planning permissions on the same site was considered and the judgment provided guidance in this regard. Counsel for the interested party had submitted there were circumstances in which the carrying out of inconsistent development would cause a detailed permission to cease to have effect, but this would not do so in case of an outline planning permission for the same type and scale of development. Notably, Mr Justice Holgate stated in the judgment that he had reservations about the correctness of this in light of the ‘whole site’ principle laid down in Hillside, where it was held that the test of physical impossibility extends to the whole site and not just the part upon which the developer wishes to build.
It was recognised that whether a detailed permission is to be treated as severed, or as a collection of discrete planning permissions, is a matter of construction of the consent. In Pilkington v Secretary of State for the Environment  it was held that a planning permission could not be relied upon where, as a result of development carried out under another planning consent relating to the same area of land, it was physically impossible to carry out the development for which the permission was granted. Whilst a detailed planning permission may authorise the development approved to be carried out in phases, this principle in Pilkington can still apply to a detailed permission. Phasing provisions within a detailed planning permission are compatible with the consent being treated as a single planning permission.
Developers can no longer therefore assume that the inclusion of phasing provisions in either a detailed or outline planning consent will alone provide flexibility to make it severable. This would be relevant if they are considering a new separate standalone application on a site subject to an existing consent but still wish to complete development under an original extent consent. The judgment in Dennis should provide added caution to developers as to the risk of any new standalone planning permission frustrating completion of development under an extant permission that also covers the same land.