Supreme Court issues landmark judgment on slot-in planning applications in Hillside Parks Ltd v Snowdonia National Park Authority [2022] UKSC 30


The previous Court of Appeal decision in the Hillside Parks Ltd case has caused considerable difficulties in practice, particularly on large developments, for the common practice of lodging “slot-in” planning applications to change parts of the development. The Supreme Court has now issued its landmark judgment on the issue.

Merioneth County Council granted planning permission in 1967 in respect of a development of 401 dwellings on a site in Snowdonia National Park. Subsequently, a series of further planning permissions were granted, departing from the original scheme. In particular, roads had been built in areas designated for houses, and houses had been built in areas designated for roads.

In 2019, Hillside Parks Ltd, the current owner of the site, lodged proceedings against the National Park Authority (who had taken over the planning powers for the area) in order to determine whether the remaining parts of the development under the 1967 permission could still be lawfully carried out. Both the High Court and Court of Appeal decided in the Authority's favour.

The Court of Appeal did not view the matter as merely limited to looking at any inconsistencies between the planning permissions, but decided that, “reflecting the holistic structure of the planning regime... if a development for which permission has been granted cannot be completed because of the impact of other operations under another permission, that subsequent development as a whole will be unlawful”.The key issue from the Court of Appeal judgment was the implications for overlapping planning permissions. Critically, whether the effect of implementing later permissions that only relate to part of a wider development, results in further development under the original permission on other parts of the wider site being unlawful, particularly if that original development could not be completed.

That has effectively significantly limited the use of 'slot in' planning applications and it had been hoped by many practitioners that the Supreme Court may give a judgment that is easier to work with in practice.

Legal Update

On 2 November 2022, the Supreme Court issued its judgement. The key conclusions were as follows:

  • it is a question of interpretation whether a planning permission authorises a number of independent acts of development, each of which is separately permitted by it, or whether it is to be construed as a permission for a single scheme that cannot be disaggregated;
  • usually the latter applies and a planning permission for a multi-unit development is to be viewed as granted for the development as an integrated whole;
  • subsequent planning permissions that materially depart from the original permission for a development that is to be viewed as an integrated whole, make it physically impossible and unlawful to carry out any further development under the original permission;
  • a departure will have this effect only if it is material in the context of the scheme; and
  • the Supreme Court added a further point that additional unlawful buildings make it impossible to further carry out development under the original permission.

Our View

The Hillside Parks Ltd case has been widely recognised as significantly limiting the use of slot‑in planning applications, particularly for large developments. It has been widely considered to be an impractical position in practice.  However, the Supreme Court has generally decided to maintain the current position and it will be necessary to continue to find awkward work arounds for "slot-in" applications where possible.

Please do not hesitate to get in touch with a member of the Planning Team or Environment Team  if you wish to discuss.

The content of this page is a summary of the law in force at the date of publication and is not exhaustive, nor does it contain definitive advice. Specialist legal advice should be sought in relation to any queries that may arise.

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