Overlapping planning permissions after Hillside: can you “mix and match”?

In that appeal, the developer sought a lawful development certificate (LDC) confirming that two separate extension permissions could be implemented concurrently and effectively combined into one larger scheme. The Inspector dismissed the appeal, holding that the permissions could not lawfully be implemented together.

Below, partner Alex Ground examines a recent appeal decision that reinforces the principles established in Pilkington and Hillside, providing a timely reminder that overlapping planning permissions cannot be combined to create a bespoke scheme unless the permissions are genuinely severable and legally compatible.

The developer was seeking confirmation that he could construct development concurrently pursuant to two separate permissions for extensions to the same dwelling; each permission authorised the same extension at ground floor level and a different extension at first floor level. The developer’s aim was to construct elements of both permissions together, creating a hybrid, larger extension.

The developer tried to rely on the familiar Pilkington principle — that multiple permissions can exist and, in some cases, be implemented in parallel.

The refusal turned on a core point: you cannot combine permissions where doing so results in a development that is inconsistent with each permission taken as a whole.

The Inspector found:

• The permissions were approved independently, each with its own defined scheme and did not envisage the development pursuant to the other permission also being constructed;

• The combined development would be materially different from either permission;

• The proposals were interdependent and overlapping, such that they were mutually exclusive

The law remains rooted in two key authorities:

Pilkington v SSE (1973)

• Multiple permissions can exist for the same site

But if implementing one makes it physically impossible to complete another, the second cannot be lawfully implemented

Hillside Parks v Snowdonia (2022, Supreme Court)

• A planning permission must be considered holistically

• Later development that renders an earlier scheme physically impossible will prevent further lawful implementation of that earlier scheme

• Severability must be clear and express.

From this decision, inconsistency arises where:

• The schemes overlap physically (e.g. extensions occupy the same space)

• The combined result does not match any approved drawing set

• Each scheme depends on its own configuration of the building

• Completing one would prevent the other being completed in accordance with its own permission

Importantly, the test is not just conceptual inconsistency but whether both permissions can be completed in accordance with their terms and conditions.

Yes but only where the elements are truly severable and there is no physical or legal conflict between them.

Typical examples where concurrency can work:

• Separate buildings on different parts of a site

• Distinct phases of development that do not overlap

• Permissions expressly structured to allow severable implementation

By contrast, problems arise where developers attempt to:

• “Pick and choose” elements from different approvals

• Create a composite scheme not expressly permitted

The Inspector was clear overlapping permissions cannot be used as a mechanism to assemble a bespoke hybrid development not assessed or approved by the LPA.

No. The decision confirms local planning authorities consider each application on its own merits and they are not required to cross-reference or restrict other permissions

Therefore silence in conditions does not create a right to combine permissions.

The main risks are:

1. Unlawful implementation

• If works do not align with an approved scheme → entire build may be unlawful

2. Loss of fallback

• Starting one scheme may sterilise another permission (Hillside effect)

3. Enforcement exposure

• Combined schemes may not benefit from any permission at all

4. CIL / commencement complications

• Incorrect assumptions about implementation can trigger unintended liabilities

Legal input is particularly advisable where:

• Multiple permissions exist for the same building or plot

• There is any intention to:

  -combine elements

  -amend layouts across permissions

• Development will be phased or partially implemented

• There are potential commencement or CIL implications

The law surrounding overlapping permission can be complex. Russell-Cooke has extensive experience advising clients in relation to the same.

If you have any queries regarding the issues discussed above, or find yourself in a similar position, please do get in touch to seek expert legal advice.

Alex Ground is a partner in the real estate, planning and construction team, advising developers, land owners and private individuals on the full range of planning law issues.