Double Trouble: Recent Appeal Considers Hillside Incompatibility

Overview

An appeal decision issued on 26 January 2026 concerning a dwelling in the London Borough of Richmond upon Thames provides a clear reminder of the principles governing overlapping planning permissions. The Inspector considered two appeals brought under section 195 of the Town and Country Planning Act 1990, following the refusal of certificates of lawful use or development (“CLEUD”) by the London Borough of Richmond Upon Thames ("the LPA"). While one element of Appeal B (an outbuilding permitted under Class E of Part 1 of Schedule 2 to the GPDO) was successful, the key issue centred on whether two planning permissions for extensions could be implemented concurrently.

The appellant argued that either permission - one for a first‑floor extension and one for a ground‑floor extension - could be implemented alone, together, or partly in combination. The Inspector disagreed, concluding that neither permission could be lawfully implemented concurrently.

The decision is a useful refresher of key planning principles and shows that Hillside incompatibility remains a tricky concept.

Legal principles

As principles relating to overlapping planning permissions are not yet addressed in statute, the Inspector relied on established case law including:

• Pilkington v SSE & Lancashire CC [1973] 1 WLR 1527: the High Court held that if implementation of a planning permission would make it physically impossible to implement a second in accordance with its terms and conditions, then it cannot be lawfully implemented. This requirement for ‘impossibility’ goes beyond mere ‘incompatibility’;
• Hillside Parks Ltd v Snowdonia National Park Authority [2022] UKSC 30: the Supreme Court reaffirmed the importance of physical impossibility. In particular, the Inspector in his decision accentuated that:
  • Express wording in the master permission may make elements of a permission severable;
  • Where implementing a later permission makes implementation of an earlier permission physically impossible, then further development under the earlier permission would be unlawful;
  • A material departure from the approved scheme makes further development unlawful;
  • A later permission may effectively operate as a variation of an earlier one.

The Inspector also considered the judgment in Sage v SSETR & Maidstone BC [2003] UKHL 22 which set out that a building operation must be carried out fully in accordance with the relevant permission; otherwise, the whole operation is unlawful.

Inspector’s analysis

The Inspector placed emphasis on the fact that each of the original planning permissions as varied was granted with reference to an approved set of drawings. The Inspector found that the appellant was seeking to combine elements of both permissions into a ‘single much larger development’. The Inspector stated that they could not see any evidence this was the intention of the LPA when granting those planning permissions, given each had been approved independently, was inconsistent with the other and incapable of being combined. Drawing on Sage, the Inspector stated that if the CLEUDs had been granted, then neither planning permission would have been carried out internally and externally in accordance with its approved plans or the description of development, and would therefore be unlawful.

The Inspector noted that the Hillside principle remains established that a planning permission should be considered as a whole and not as separate elements or fragmented parts. He pointed out that Hillside encourages a holistic view to be taken and consideration given to what was envisaged in granting the original planning permissions. The Inspector assessed that the first-floor extension in both permissions was dependent on the implementation of the ground floor works. This inter-dependency was a key factor in finding that the developments were mutually exclusive and it was not reasonable nor appropriate for the appellant to pick and choose parts from the overlapping planning permissions.  

The Inspector also highlighted that the lack of a planning condition preventing concurrent implementation was irrelevant and that it is not an LPA's role to link one planning application to another. The LPA was therefore right to consider each proposal on its own merits. 

The future

This appeal decision reinforces the need for caution where multiple permissions overlap and is evidence that the application of Hillside is continuing to cause difficulties across different types of development. Lord Banner had proposed an amendment to the Planning and Infrastructure Bill in Autumn 2025 to mitigate the effects of Hillside but this was not approved.  The Government’s continuing intention is to bring draft section 73B into force through the Levelling Up and Regeneration Act 2023 to allow for more flexible variations of permissions, however the timing of this remains unclear. If you have any queries on the effect Hillside could have on proposed variations to existing schemes, please do not hesitate to contact Cathryn Tracey or I.