A Slippery Slope for Hillside

On 2 November 2022 the Supreme Court handed down its judgment in Hillside Parks Limited v Snowdonia National Park Authority [2022] UKSC 30. The case highlighted the potential consequences of inconsistencies between a planning permission for a large-scale development and the implementation of subsequent planning permissions for localised development over parts of the same site.

The Facts

In 1967, planning permission was granted for 401 residential dwellings at a site in the Snowdonia National Park (“the 1967 Permission”). A number of permissions concerning smaller areas of the site were granted after 1967, however only 41 dwellings have been constructed to date. The 1967 Permission was the subject of litigation in 1987, leaving the present case to be concerned exclusively with permissions granted after that date.

Since 1987, eight localised planning permissions have been granted on the site of the 1967 Permission, six of which have been implemented (“the Additional Permissions”). These departed significantly from the “Master Plan” for the development which had been approved in 1967. Consequently, the issue to be determined by the Supreme Court was whether the implementation of the Additional Permissions had rendered further implementation of the 1967 Permission unlawful.

The Pilkington Principle

The starting point was consideration and application of the Pilkington principle (established by Pilkington v Secretary of State for the Environment and Others [1973] 1 WLR 1527). The claimant in the Pilkington case obtained and implemented planning permission for the construction of a bungalow. Following construction of the bungalow, he discovered the existence of an earlier planning permission for a bungalow sited elsewhere on the same plot which also formed part of a smallholding as part of the application. The claimant had intended to implement this permission as well. The question for the Court was whether implementation of this earlier permission was possible. Unfortunately for the claimant, the bungalow that he had constructed was situated on the part of the site that had been designated for the smallholding under the earlier permission. The Court therefore held that it would be unlawful to implement the earlier permission because the provision of the smallholding was impossible.

A strict application of this principle to the Hillside case would render the 1967 Permission incapable of implementation. As a result, the Developer sought to distinguish its case in three alternative ways:

1. The correct application of the Pilkington principle requires reliance on the test of abandonment; or

2. The 1967 Permission can be severed, thereby permitting the construction of dwellings contained within the various sub-sites around the larger site; or

3. The Additional Permissions should be considered as variations to the 1967 Permission.


The Developer argued that the Pilkington case could be explained by an application of the abandonment test. It attempted to apply this test to the present case by stating that, where multiple incompatible planning permissions exist over a site, a reasonable person would consider a developer to have abandoned the earlier permissions as soon as it implemented one of the later ones. The Developer submitted that the test was not met in this case as the Additional Permissions granted were reconcilable with the 1967 Permission (i.e. they could co-exist).

The Supreme Court rejected this line of argument due to its inconsistency with a number of preceding judgments. It upheld the existing position that no test of abandonment exists in planning law and such an application is unacceptable as it would impose “an impermissible judicial gloss on the legislative code” (see Pioneer Aggregates (UK) Ltd v Secretary of State for the Environment [1985] AC 132).


The Supreme Court rejected the submission that the 1967 Permission had effectively been severed by the Additional Permissions and so was still implementable. The Supreme Court held that, in its determination of a planning application, a local planning authority will consider the application as a whole and in doing so will take a range of factors into account (for example, the layout and appearance of the development, any required infrastructure, the number of units to be constructed and the impact of the development on the public). It is not the task of a local planning authority to subdivide a planning application and apply these factors to each part, weighing them against the impact of the development as a whole. Furthermore, a reasonable person would not interpret the grant of planning permission for a large-scale development as a green light to “mix and match” subsequent inconsistent permissions which, when implemented, would create a vastly different development to that which was initially permitted.

The Supreme Court took the opportunity to confirm that an incomplete development for which planning permission has been granted does not make the implemented development unlawful. If it were, enforcement action could be taken at any point during the construction of a development.


In the third alternative, the Developer submitted that the Additional Permissions should be viewed as variations of the 1967 Permission. Therefore, fresh planning applications were not required for the whole site, but only for the areas to which the modifications were required. The Supreme Court agreed with the Court of Appeal that, whilst variations to planning permissions are not strictly possible under the current regime (with the exception of applications pursuant to sections 73, 73A and 96A of the Town and Country Planning Act 1990), it might be possible for an application to be construed as a variation if it establishes acceptable modifications to an earlier permission. However, merely calling an application a “variation” is insufficient. Evidence must be submitted in support of the variation to demonstrate how it would be incorporated into the rest of the development as a whole in order for it to be viewed as a true variation rather than a fresh “drop-in” application. The Supreme Court found that the Developer had failed to do this and so could not benefit from this argument.


The case serves as a warning to developers wishing to amend sections of large multi-unit sites for which planning permission has previously been granted. In order to retain the benefit of a planning permission for the whole site, a developer must ensure that any localised planning applications do not materially affect the development as a whole to the extent that implementation of the original permission becomes unlawful.

The case also highlights the lack of power held by local planning authorities to vary planning permissions, which has been the subject of much frustration among developers. This issue could be partially resolved should the proposed Levelling-Up and Regeneration Bill (“the Bill”) be enacted. The Bill proposes the insertion of a new section 73B to the Town and Country Planning Act 1990, which will extend the power of local planning authorities to allow minor variations of existing planning permissions. We are closely monitoring the progression of the Bill and will provide more information at the appropriate stage.

A full copy of the Hillside judgment can be found here.

The contents of this article are intended for general information purposes only and shall not be deemed to be, or constitute legal advice. We cannot accept responsibility for any loss as a result of acts or omissions taken in respect of this article.