Hillside: Practical Advice for Developers

A brief recap of Hillside

Planning permission was granted for a development site – known as Balkan Hill – in 1967. Over the years, several ‘drop in’ planning permissions replanned parts of the development site. Balkan Hill changed significantly from what was intended by the 1967 planning permission. The question that the Supreme Court had to address in 2022 was whether the developer, Hillside Parks Ltd, could carry out further development under the 1967 planning permission.  The local planning authority claimed they could not.

The Supreme Court held that the developer could no longer carry out development under the 1967 planning permission due to a ‘material’ conflict between the existing development and that which had been permitted by the 1967 planning permission.

In this article we focus on the practical implications for developers from the Supreme Court’s judgement.

Large-scale developments

In Hillside, the 1967 planning permission was construed by the Supreme Court as a planning permission for a single development proposal for the whole development site. It was not a planning permission with multiple severable phases; therefore, it was not capable of being re-planned in a piecemeal way as had happened.

With a careful approach to the structuring of a planning application and the conditions applied to any subsequent planning permission, developers can minimise the risk that development under an original planning permission could become impossible to continue to build out as a result of development carried out under a subsequent drop-in planning permission. This principle is further dealt within the Hampton Park planning appeal which followed the Supreme Court’s decision in Hillside (“Hampton Park”).

Recent development in focus: the Hampton Park Appeal

In Hampton Park, the developer wanted to replan a portion of a development that had been approved for employment use (within a previous outline planning permission) with residential units. The Inspector dismissed the appeal, stating that: ‘the Hampton Park scheme was ultimately intended to be delivered as a comprehensive scheme in its entirety’.

The Inspector placed ‘particular significance’ on the terms of the decision notice and the Section 106 agreement together with approved plans and documents. In particular, condition 3 of the outline planning permission required ‘substantial accordance’ with the masterplan and this consequently constrained the extent to which aspects of the development could be replanned. Some care is required in wording conditions to ensure that where possible no such constraints arise.

Applying Hillside to Hampton Park, if the employment area had been within a severable phase as described by an appropriately framed planning application and subsequent planning permission, it is possible the proposed replanning of the employment area could have progressed via a ‘drop in’ permission without causing a material conflict with the rest of the development. This interpretation is also summarised by the Inspector who explained:

“Where a planning permission is granted for the development of a site, such as a housing estate, comprising multiple units, it is a question of interpretation whether the 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 which cannot be disaggregated in this way.”

Drop-in applications

Whilst drop-in planning applications will not always be incompatible with existing planning permissions that are non-severable (i.e. not capable of being divided up into separate and independent parcels or phases), developers should take great care to ensure that the subsequent implementation of a drop in planning permission will not cut across the ability to continue to build out the remainder of the development under the original planning permission.

The timing of development may be critical to achieving this, for example completing all of the development that is intended to be built out under an original planning permission before the commencement of development under the drop-in planning permission.

Hillside and CIL

In assessing the suitability of a drop-in application, developers must also be attuned to the potential community infrastructure levy (“CIL”) implications of a drop-in planning permission. CIL discounts may be lost where floor space is already demolished under an existing planning permission.

Purchasing part of an existing development site

Developers purchasing a parcel of a larger development site should consider requesting a covenant from the seller not to take any actions which would prevent the development of the parcel being lawfully completed. This may include not applying for planning permission for the retained site until the parcel which is purchased is fully built out.

Sellers may not be comfortable with such restrictions on their ability to deal with retained parts of development sites, however, in our experience many are prepared to accept that they should not implement a planning permission that could prevent the lawful completion of development on the parcel that has been sold.


In summary, developers should pay careful attention to the phasing of their planning applications and the wording of the conditions applied to their planning permissions and associated documents particularly as the Supreme Court held in Hillside that the existing powers to make changes to an existing planning permission are limited.

The Levelling-up and Regeneration Bill proposes a new section 73B to be included in the Town and Country Planning Act 1990 to permit "non-substantial changes" to planning permission which may lend itself so replanning proposals. Further the recent case of Armstrong v Secretary of State for Levelling-Up, Housing and Communities [2023] EWHC 176 (Admin) (the Armstrong Case") has clarified that the scope of planning permission under Section 73 of the Town and Country Planning Act 1990 are not limited to minor amendments providing the amendments proposed are consistent with the original description of development.

To enable a material variation of a large development, the Supreme Court in Hillside suggested that the appropriate solution is 'an appropriately framed additional planning permission which covers the whole site and includes the necessary modifications'. We speculate that either a future change in planning legislation, or the greater flexibility to use Section 73 planning applications in light of the recent Armstrong Case will address many of the complexities presented by Hillside.