The Hillside judgment and overlapping planning permissions

The practical rule after Hillside overlapping permissions cases is short: if building out a later permission makes it physically impossible to complete an earlier one, the earlier permission can no longer lawfully be completed, and anything further done under it is unauthorised development. The Supreme Court decided that in Hillside Parks Ltd v Snowdonia National Park Authority in November 2022. It did not invent a new rule so much as confirm one from 1973 that a good deal of development practice had quietly stopped applying.

The problem the Supreme Court was asked to resolve

The site was a large residential scheme in Snowdonia permitted in 1967 for around 400 houses, with a master plan showing the layout. Building proceeded slowly over decades. Along the way the authority granted a series of further permissions for individual plots and revised layouts that departed from the original master plan.

By the time the dispute reached court, the development on the ground could not be reconciled with the 1967 layout. The owner argued that the original permission remained live and could still be built out. The authority said it could not.

The court sided with the authority, and in doing so settled several questions that had been argued both ways for years.

Why implementing a second permission can make the first unlawful to complete

The controlling authority is Pilkington v Secretary of State for the Environment, decided in 1973. The test it set is one of physical impossibility. Where development carried out under permission B means that the development authorised by permission A can no longer be carried out in accordance with its terms, permission A cannot be completed.

The Supreme Court confirmed that test and made three points that matter in practice.

A planning permission for a multi-unit development is normally to be read as approving the scheme as an integrated whole. It is not a bundle of separate permissions for each house that can be picked off independently.

Departures from an approved scheme do not have to be large to have this effect, though the court said the question is whether the departure makes the original development impossible to complete rather than whether it is material in the abstract.

And nothing requires a developer to undo what has been built. The consequence bites forwards. Work already carried out under the earlier permission remains lawful. Further work under it does not.

Severable and non-severable schemes

The escape route the court left open is severance, and it depends entirely on the wording of the permission.

Where a permission is drafted so that it plainly authorises development in separate, independent parts, each part can be treated on its own. The court accepted that such permissions exist. What it rejected was the idea that severability can be implied into an ordinary outline or full permission for a single scheme just because the scheme happens to contain many units.

So the question to ask of any existing consent is not "is this a big scheme?" but "does this permission, read as a whole, treat its parts as independent?" Most do not.

Drafting a permission to be severable from the outset

For a phased scheme this is now a drafting exercise to run at application stage rather than a problem to solve later.

The features that support severance are a description of development that refers to phases as separate developments, a phasing plan approved as part of the permission with a defined red line for each phase, conditions expressed to apply phase by phase rather than to the whole site, and separate start and completion provisions for each phase. Section 106 obligations should be structured the same way, with triggers tied to the individual phase.

None of that is free. Severance reduces flexibility in other directions and can complicate infrastructure delivery and viability testing across the site. It is a deliberate trade, and it should be a conversation with the authority at pre-application stage rather than a surprise in the draft decision notice. The policy background for that conversation sits in your Planning Statement.

Practical consequences for phased developments

Three situations come up repeatedly.

The first is the drop-in application: a fresh permission for a parcel inside a larger consented scheme, granted on its own. Before Hillside this was routine. After it, a drop-in that makes the parent scheme impossible to complete puts the rest of the parent permission at risk. The safer structure is usually to vary the parent permission under section 73 so that the parent scheme, as varied, accommodates the change.

The second is the long-running scheme where nobody has checked the master plan against what was actually built. Estate roads move, plot boundaries shift, an attenuation basin ends up 15 metres from where it was drawn. Each of those is a candidate for the impossibility test.

The third is acquisition. A buyer purchasing part of a consented site needs to know whether the permission it is relying on is severable and whether anything already built elsewhere on the site has compromised it. That is now a standard due diligence question and it should be answered in writing before exchange.

Checking an existing consent before relying on it

Work through it in this order.

1. Obtain the full permission: decision notice, approved drawings schedule, conditions and any subsequent section 73 or section 96A decisions.
2. Overlay the approved layout on a current topographic survey of what has been built.
3. Identify every departure, and for each one ask whether the remaining development can still be carried out in accordance with the permission as approved.
4. Read the description of development and the conditions for any language treating phases as separate developments.
5. Where a departure defeats completion, decide between a section 73 variation of the parent permission and a fresh application for the whole of the remaining development.

Step two is the one that gets skipped, and it is the one that finds the problem. A measured survey against the approved layout costs a fraction of what an enforcement dispute costs.

The statutory responses that followed

The Levelling-up and Regeneration Act 2023 inserted section 73B into the Town and Country Planning Act 1990, giving a power to grant permission for development that is not substantially different from that already permitted. It was drafted with the difficulties created by Finney and Hillside in mind, and it is intended to offer a cleaner route for amending a consented scheme than a section 73 variation allows.

Check whether it is in force before you build a strategy on it. The Act's planning provisions have been switched on in stages, and live provisions can sit alongside dormant ones in the same Part. The mechanics of that route are set out in the notes on section 73B as a way of varying a permission, and reform news of this kind is tracked on the planning law timeline.

One honest uncertainty. Hillside tells you when a permission can no longer be completed. It says less about how far an authority will pursue development already carried out under a compromised permission, and enforcement practice varies. That is not a reason to rely on inaction, because the risk sits with whoever owns the land when the question is asked.

Browse all 66 PlanScope report types and start a draft if you are putting together a section 73 or a fresh application for the remainder of a phased site. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.