Hillside Parks: the Supreme Court clarifies the law on overlapping planning permissions and drop-in applications…oh wait…


Since the decision of the Supreme Court in Hillside Parks v Snowdonia National Park Authority [1], the planning and development world has gone into overdrive to try and make sense of the judgment and to understand what it means for existing and new developments.

On a first reading the decision is reassuringly simple; it restates the principle in Pilkington v Secretary of State for the Environment [2] and confirms that planning permission for development already built is not cancelled by a later inconsistent permission. Helpful stuff. But it also creates new confusions not helped by the peculiar facts of the case and the resulting arguments that the Supreme Court had to address in its judgment.

Our conclusion is that the application of Hillside will depend on what you're proposing to do. For a simple scenario, such as the two inconsistent permissions for bungalows in Pilkington, the answer will be clear and straightforward but, for the larger, more complex sites that many of our clients bring forward, how you deal with drop-in (or slot-in) applications and overlapping planning permissions will depend on the particular site and your build-out strategy.

This note does not provide all the answers but we describe the key points of law that emerge from the judgment and offer a few thoughts on the issues you will need to consider when considering scheme changes and new applications that sit on top of an existing permission.


The case was about a permission granted in 1967 for 401 dwellings at a big site in north Wales. Since 1967, only 41 houses had been built but a large number of new permissions had been granted many of which permitted development that was inconsistent with the original permission. For example, one permission consented a new road which was built on land that had been consented for housing by the original permission.

The question before the Court was whether the original permission could still be implemented.  The Court said no. In particular:

  • The Court reaffirmed the law on overlapping planning permissions set out in Pilkington. This is that where there are two permissions relating to the same site and the first permission is implemented, it is unlawful to carry out development under the second permission where the development permitted by the first permission would make it physically impossible to carry out development under the second permission.
  • The Court added that the second permission is only lost if:
    • it is physically impossible to carry out development because of what has been built on the land pursuant to the earlier permission; and
    • the change from the first permission is material; minor inconsistencies will not result in the first permission being invalid  and what is material is a "question of fact and degree". 
  • Crucially the Court confirmed that the lawfulness of development already carried out under an existing consent is unaffected by the implementation of an inconsistent permission. This lays to rest a concern after the Court of Appeal's decision (in Hillside) that the implementation of an inconsistent permission could result in existing development being unauthorised.
  • The Court rejected the suggestion that a permission for a multi-unit development authorises a number of independent acts of development and is "severable into a set of discrete permissions" unless there is a clear, express provision making it severable.

The AG analysis

Impact on existing permissions

While Hillside is in many respects a product of its own "extreme" facts – in terms of the age of the original masterplan consent and what had happened on the site since - it nevertheless has implications for schemes involving drop-in applications or overlapping permissions. However, it is important to bear in mind that this is not a case about the validity of drop-ins but rather the legal effect of implementing subsequent permissions on an existing wider permission. There are two aspects to this:

  1. The effect on development already carried out under the existing permission – as noted above, the judgment helpfully confirms that the lawfulness of existing development would be unaffected by the implementation of an inconsistent permission. On the contrary, the judgment means that an inconsistent drop-in or overlapping permission does not adversely affect development already carried out. This is fine if the development that has been carried out is completed development so that the planning permission does not need to be relied on further. 
  2. The effect on the ability to carry out further development under the existing permission - here the judgment relies on the Pilkington principle to conclude that the ability to rely on the original consent will be lost where it becomes physically impossible to carry out the consented development because of the implemented later permission. While this means that the Supreme Court's decision is based on existing law, and nothing should really have changed, it is this part of the decision that has practical implications due to the gap between the law on the variation of consented schemes and well established industry practice on the use of drop-in applications. In particular, where construction of a wider site is underway, implementation of an incompatible drop-in permission must be avoided to stop partially completed development from potentially becoming incapable of lawful completion unless a further permission is obtained.

Where does all this leave drop-ins? The answer is it's complicated and each scheme will require an assessment of whether a drop-in is the appropriate process to effect a scheme change or whether another procedure, such as a section 73 or 96A application is more appropriate. But what we can say with certainty is that Hillside has not killed off the drop-in and it has given us a few key principles to bear in mind and raised a few questions that require further thought.

As we mention above, if there is no intention to carry out any further development under the existing consent, there is no need to be concerned about the drop-in permission cancelling the earlier permission. However, for drop-ins where further development is still to be carried out under an existing consent, care will be needed to ensure that the applicant does not inadvertently lose the benefit of any existing consent.

The appellant in Hillside ran an argument that the later permissions should be treated as variations of the original permission. The Court resoundly rejected that argument and added that "there is no reason why an approved development scheme cannot be modified by an appropriately framed additional planning permission which covers the whole site and includes the necessary modifications" (our emphasis). The Court added that "ordinarily it [the variation application] would have to be accompanied by a plan which showed how the proposed new permission incorporated the changes indicated into a coherent design for the whole site". However, having to draw your redline around the entire site does present additional considerations because the drop-in becomes, in effect, a brand new planning application for the whole site with implications for the application fee, environmental impact assessment and section 106 agreement particularly where there are multiple landowners. In those circumstances, an application under section 73 to "slot out" development consented on part of the site to enable new development to be slotted or dropped in to just that part could be considered.

Severable permissions

We referred above to the Court's rejection of the appellant's argument that, for a multi-unit development, each element of the masterplan is a freestanding permission and the ability to develop each element does not depend on whether it is still physically possible to develop other parts of the site in accordance with the original permission. The court said that, in the absence of "some clear contrary intention", it is not correct to interpret such as permission as severable. This raises the question what is a severable permission.  In our view, it means more than a planning condition that provides for the development to come forward in phases and to avoid a risk of challenge it would be sensible to provide for severability in the description of development.


Hillside emphasises the continuing need for care in the formulation of application strategies for large masterplan schemes and in relation to the implementation of overlapping consents.  Inevitably, different risk factors need to be balanced and, as always, the best approach will depend upon the particular circumstances of each site and scheme.

If you would like to know more about Hillside and/or the variation of planning consents please do not hesitate to contact anyone in our Planning & Infrastructure Consenting Team.


  [1] [2022] UKSC 30
  [2] [1973] 1 WLR 1527

Marnix Elsenaar

Marnix Elsenaar

Partner, Head of Planning and Infrastructure Consenting
United Kingdom

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Michael Dempsey

Michael Dempsey

Legal Director, Planning and Infrastructure Consenting

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Daniel Marston

Daniel Marston

Managing Associate, Planning and Infrastructure Consenting

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Emily Williams

Emily Williams

Partner, Planning and Infrastructure Consenting

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