RSBS Developments Ltd v Secretary of State for Housing, Communities and Local Government & Anor [2020] EWHC 3077 (Admin)
The point in plain English: permitted development rights cannot retrospectively legalise development that failed to qualify when it was carried out. If Article 3(5) disapplies Schedule 2 permission because part of the existing building is unlawful, later remedial works may restore PD rights for the future but do not rewrite the status of completed development.
The High Court dismissed challenges concerning Mercury House in Wembley, where an office-to-residential prior approval had crystallised but the building was then enlarged by unauthorised works before the material change of use occurred.
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- Neutral citation
- [2020] EWHC 3077 (Admin)
- Court
- High Court of Justice, Queen's Bench Division, Planning Court
- Judgment date
- 17th November 2020
- Judge
- Mrs Justice Lang DBE
- Case number
- CO/1113/2020 & CO/1285/2020
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What happened at Mercury House
RSBS Developments obtained prior approval in August 2015 for the change of use of former offices at Mercury House, Wembley, to 16 flats under the then Class O office-to-residential right.
Before the material change of use took place, however, the developer demolished an existing rear extension and built a larger two-storey extension. The Inspector found that those works were unauthorised and that two of the flats incorporated floorspace from the new extension.
The residential conversion also differed materially from the approved prior approval plans. The Inspector therefore concluded that the development carried out did not implement the permission that had crystallised in 2015.
Article 3(5) still applied after prior approval
The central High Court issue was whether Article 3(5) of the GPDO could disapply permitted development rights even though prior approval had already been granted.
Mrs Justice Lang held that it could. Prior approval may crystallise the planning permission, but the development still has to comply with the GPDO when it is actually carried out. Article 3(5) is not confined to the period before prior approval.
The court distinguished R (Orange PCS) v Islington LBC. In Orange, a later conservation-area designation was an external event outside the developer’s control. At Mercury House, the problem arose from the developer’s own unauthorised building operations.
The court also rejected the argument that Article 3(5)(a) could not apply because Class O concerned a change of use. Permission for that change of use was still “in connection with an existing building”, so unlawful construction of part of that building was capable of engaging Article 3(5)(a).
Later remedial works did not operate retrospectively
This is the part of the judgment with wider practical importance.
After the residential conversion, the developer removed the upper storey and reduced the ground-floor extension towards its former size. The argument was that, once the unlawful works had been corrected, the earlier prior approval should effectively become capable of operating.
The court rejected that analysis. The lawfulness of the material change of use had to be assessed when that change occurred. If the development did not qualify as permitted development at that point, later works could not retrospectively confer that status.
Once unlawful works are removed or regularised, permitted development rights can become available again for future development. That is different from retrospectively validating development already completed while Article 3(5) was engaged.
A change-of-use permission permits the change itself
The judgment also relied on Cynon Valley Borough Council v Secretary of State for Wales for the principle that permission for a material change of use is permission to make the change, not a continuing permission for the resulting use indefinitely.
That temporal point explains why later remedial works could not simply cause the 2016 residential change of use to become lawful after the event.
Why RSBS matters for permitted development
The judgment is useful well beyond the former Class O right. Its Article 3(5) reasoning applies to Schedule 2 permitted development more generally.
- Prior approval does not guarantee that the development ultimately carried out is permitted development
- Unlawful works forming part of an existing building can disapply PD rights connected with that building
- Regularising or removing the unlawful works can restore the possibility of using PD rights in future
- That future availability does not retrospectively legalise earlier development that failed the GPDO when carried out
The point is particularly relevant where an owner seeks to alter an unauthorised building so that it would now satisfy a householder or change-of-use permitted development class. Planning Geek’s Class E outbuildings guide applies the principle to domestic outbuildings.
A September 2026 Slough enforcement appeal raised a related but distinct issue about whether an existing garden dwelling could instead be retained as an incidental gym and store. There, the immediate obstacle was the scope of the ground (a) deemed application rather than a direct ruling on Article 3(5).
The approved judgment is also available from BAILII.
Read the RSBS Developments judgment
RSBS Developments Case Law Page Created: 23rd September 2026