In this post, Sam Porter, an Associate in the Planning team at CMS, previews the upcoming Supreme Court decision in the case of Hillside Parks Limited v Snowdonia National Park Authority UKSC 2020/0211. The case was heard on 4 July 2022.


Hillside Parks Limited (“HPL”) is the owner of an approximately 30-acre site in Aberdyfi, Snowdonia, which it acquired in the late 1980s. In 1967, planning permission was granted for a large housing development on the site (“1967 Permission”). The approved plan (“Original Master Plan”) identified the proposed location of the buildings and roads which comprise the development.

From 1967 to 1974, eight further planning permissions were granted which permitted the development to be constructed in a manner that differed from the Original Master Plan. Some of these permissions were implemented.

High Court proceedings were first brought in 1985. At this point, a small number of buildings had been constructed, all of them pursuant to the permissions granted after the 1967 Permission, rather than under the 1967 Permission itself. The landowner sought declarations regarding the status of the 1967 Permission (and in particular whether it had lapsed). In his judgment, Drake J granted four declarations, the third of which is most relevant. In summary, this declaration was that the development pursuant to the 1967 Permission had commenced and could be lawfully completed at any time in the future (the “Third Declaration”).

Following the 1987 judgment, Snowdonia National Park Authority (“Snowdonia”) became the local planning authority for the site. A series of further planning permissions were granted by Snowdonia in the period from 1996 to 2011 which permitted additional changes from the Original Master Plan. From 1987 onwards, the developments carried out at the site were all pursuant to the permissions granted following the 1967 Permission.

In 2017, Snowdonia informed HPL that it considered that it was no longer possible to implement the 1967 Permission. This was on the basis that it was no longer physically possible to build the development in a manner which was consistent with the Original Master Plan.

Procedural History

The present proceedings began as a claim under Part 8 of the Civil Procedure Rules. HPL sought declarations that:

1. Snowdonia was bound by the judgment and declarations of Drake J;

2. the 1967 Permission is valid and extant; and

3. the development may be carried out under the 1967 Permission until completion, except to the extent that any development has been carried out or is carried out pursuant to subsequent planning permissions.

In Hillside Parks Limited v Snowdonia National Park Authority [2019] EWHC 2587 (QB), HHJ Keyser QC considered that there were two issues. Firstly, whether Drake J’s Third Declaration was wrong in law, and secondly, whether Snowdonia remained bound by the Third Declaration.

In respect of the first issue, the judge found no error in law. He considered that future development carried out under the 1967 Permission could no longer be lawful as the development carried out pursuant to the permissions granted since 1967 now made it physically impossible to develop the site in accordance with the Original Master Plan. In particular, some of the homes are not built in the position shown on the Original Master Plan and, indeed, are in some cases located where the Original Master Plan stipulated that roads and footpaths should be built. Therefore, development which had occurred since 1987 meant that Snowdonia could no longer be bound by the Third Declaration.

The case was appealed to the Court of Appeal, where Singh LJ gave the leading judgment. Singh LJ found that the High Court judge had been entitled to reach the view that it was no longer physically possible to implement the 1967 Permission.

HPL also argued that the judge had been wrong to rule that F. Lucas & Sons Ltd v Dorking and Horley Rural District Council (1966) 17 P & CR 111 did not apply. The Lucas judgment indicates that planning permissions may, in some circumstances, be construed as authorising a series of independent acts. Following this approach, although it may be physically impossible to carry out the development permitted by the original permission on part of a site, this would not render it unlawful to carry out independent acts of development under that permission elsewhere on the site. The Court of Appeal said that Hillside was not a Lucas case and observed that Lucas was “a highly exceptional case”.

HPL also contended that the proceedings represented an abuse of process. The doctrine of abuse of process requires that once an issue has been fully and finally decided by the courts, the parties cannot normally raise subsequent legal arguments which could have been raised in the initial proceedings. HPL argued that it was an abuse of process for Snowdonia to make arguments based on Pilkington v Secretary of State for the Environment [1973] 1 WLR 1527 (which relates to the incompatibility of planning permissions) when that case had been decided in 1973 and yet had not been raised by Snowdonia’s predecessor in title in the High Court proceedings in the 1980s.

In considering whether an abuse of process had occurred, Singh LJ explained that a “merits-based assessment” of all the facts was required, including both the public and private interests affected. As well as HPL’s commercial interests, there were also “important public interests at stake”, including preventing inappropriate development in a National Park. Singh LJ considered it relevant that Snowdonia had apparently, for some time prior to 2017, maintained that the 1967 Permission could be implemented on areas of the site which had not been developed pursuant to subsequent permissions; but this factor was not conclusive. The factual and legal developments since 1987 meant that Snowdonia’s arguments were not an abuse of process. Notably, the Court of Appeal stated that subsequent case law, including Sage v Secretary of State for the Environment [2003] UKHL 22, has placed greater emphasis on the need to view planning permissions as a whole rather than as consent for delineable acts of development.

Issues on Appeal

There are two main issues before the Supreme Court:

(1) Can the court to take into account the public interest in not permitting inappropriate development in a National Park, when deciding whether it would be an abuse of process for a party to make an argument which it failed to make in earlier legal proceedings?

(2) Where there are successive planning permissions relating to the same site, and the later permissions are for changes to one part of a wider development approved under the original planning permission, does the implementation of the later permission(s) render the original permission completely unimplementable? Or, in the alternative, could development pursuant to the original permission still be carried out in areas of the site which have not been developed under the later permission(s)?

The Supreme Court considered these issues during a one-day hearing on 4 July 2022.


The Supreme Court’s judgment will confirm whether it is lawful to continue development under an earlier permission once there has been a “drop-in” or “slot-in” planning permission covering part of a scheme authorised by the earlier consent. Drop-in permissions are quite common and have often been used by developers to change discrete parts of a development. Therefore, this judgment is significant and will be of great interest to developers and those with an interest in sites built under a drop-in permission. The Court of Appeal declined to express a view on whether the implementation of drop-in permissions would render development already carried out under the original permission unlawful. It will be interesting to see how the Supreme Court approaches this complexity. In addition, it will be interesting to see if the Supreme Court differentiates between the position where there is an outline or a detailed planning permission.

The Supreme Court will also have the opportunity to provide further guidance on the doctrine of abuse of process and which factors may be taken into account when the issue of abuse is considered. This will have ramifications for civil litigation more broadly.