Discussing Dennis: Breaking into planning jail - A new JR ruling on the severability of planning permissions

United Kingdom

Another case has recently emerged on the vexed question of the compatibility of subsequent planning permissions for development on the same site.  In R (Dennis) v London Borough of Southwark[1], (“Dennis”) a local resident challenged the decision by the London Borough of Southwark Council (the “Council”) to allow a non-material amendment to an outline planning permission (“OPP”) granted in August 2015 for the redevelopment of the Aylesbury Estate. Purportedly, the amendment was made to ensure complinace with the law following the landmark case Hillside Parks Ltd v Snowdonia National Park Authority[2] (“Hillside”)

This Law-Now considers the implications of Dennis and its consequences for developers.


The developer in this case, Notting Hill Genesis (“NHG”), sought to secure a ‘drop-in’ planning permission for a particular phase of the Aylesbury Estate development. As its name suggests, a ‘drop-in’ planning permission is granted over the top of a previously granted planning permission and seeks to allow a single development or phase of development to be carried out independently of the previously granted planning permission. 

Following Hillside, developers must exercise caution to ensure that the effect of a drop-in permission is not to render the previously granted permission unimplementable by virtue of incompatibility. At the Planning Committee, the Council insisted that in conjunction with the application for drop-in permission, an application should be made under section 96A of the Town and Country Planning Act 1990 (the “TCPA”) to amend the description of the development in the OPP “to protect the validity of the OPP”[3] which was duly granted. The effect of this amendment was to insert the word “severable” into the description of development in the OPP in an attempt to mitigate risks posed by Hillside. Hillside had determined that an earlier permission had to be severable if it was to remain valid and implementable following a drop-in application.

In March 2023, a local resident, Aysen Dennis, submitted a claim for judicial review on the basis that the change comprised in the s.96A application was in fact a material amendment and therefore the Council had acted beyond its powers, as s.96A TCPA only permitted non-material amendments. The issues before the Court were whether the OPP was already severable (which the s.96A application sought to reflect); or if the s.96A “enlarged the bundle of rights”[4] created by the OPP. The Judge, rejecting the argument that the OPP was already severable, found in favour of the claimant on both grounds of materiality of the change and the severability of the underlying OPP.

Materiality and severability 

In Dennis, much turned on the consideration of the severability of the OPP and technical matters relating to the planning application such as:

  1. The need for the OPP to be congruent to connect to the wider estate (for matters such as road layout, etc.)[5];
  2. The requirements in various conditions to ensure that the development complied with design codes and site wide development controls[6]; and
  3. Whether the individual phases were consistent with the OPP Design and Access Statement[7].

In his judgment, Holgate J considered both Hillside and Pilkington v Secretary of State for the Environment [8] and reflected on the ‘physical impossibility’ tests for future development where planning permission was incompatible. In Hillside it was necessary for the developer to argue that the consent in that case was severable “into a set of discrete permissions to construct each individual element of the scheme”[9] due to the otherwise incompatible nature of the different permissions for the development.

The Hillside effect 

Dennis confirmed that the principles established in Hillside apply as much to outline planning permissions as they do to full planning permissions. In Hillside, the Pilkington principle had been endorsed. For several years, Pilkington had been treated as the leading case on the consequences of the physical incompatibility of development granted approval under different planning permissions. It established that where two or more permissions are granted for the development of the same land, implementation of one permission may render it physically impossible to implement the other resulting in a situation where the earlier permission can no longer be relied upon for development.

Hillside developed this concept further by clarifying that the earlier permission cannot then be used for future development of the land and the development rights under the earlier permission cease to be available if the later permission renders the earlier permission physically incompatible with the new permission.

Furthermore, the judgment in Hillside emphasised the importance of ensuring that the original planning permission was on its face clearly severable i.e. capable of development in independent stages or phases. The Supreme Court in Hillside decided that the planning permission involved was not severable and was not structured in a way that allowed independent acts of development. Applying this to Dennis, the mere insertion of the word “severable” into the description of development did not make it severable.  Similarly, simply referring to a phased scheme or a phasing plan would not be sufficient – the permission would need to clearly describe how the different phases were independent of one another.


This appeal represents a clear case of what is sometimes called ‘breaking into jail’. The attempt by the Council and NHG to “protect” the OPP from any Hillside challenge, rather than saving the OPP almost certainly had the opposite effect of creating the opportunity for the claimant to bring legal proceedings. Although the damage had already been done in the original grant of the OPP, which was not on its face severable, the s.96A application brought that fact into the light and created a new planning decision for the claimant to challenge. 

The lesson to be drawn from Dennis is that developers need to take great care when seeking planning permission for large-scale development to ensure that it will be regarded as severable if drop-in applications may be needed in the future.  You should speak to your usual planning contact at CMS for detailed advice on how to ensure that your planning permission is severable.

Article co-authored by Ashna Patel, Trainee Solicitor at CMS

[1] [2024] EWHC 57 (Admin)

[2] [2022] UKSC 30.

[3] Dennis, [17].

[4] Dennis, [119].

[5] Dennis, [63-65].

[6] Dennis, [68-70].

[7] Dennis, [80].

[8] [1973] 1 WLR 1527.

[9] Hillside, [49].