The answer seems to be quite a lot - provided you do not depart from the description of the consented development contained within the planning permission.
It is established law that you cannot use section 73 of the Town and Country Planning Act to seek variations of conditions which result in a development which is different in nature to that which has been consented (Finney v Welsh Minsters  EWCA Civ 1868).
Section 73 provides a procedure for conditions within an existing planning permission to be modified or removed. In the recent case of Mikael Armstrong v Secretary of State for Levelling-UP Housing and Communities, ex parte Cornwall Council, the High Court considered a challenge to refusals by both the local planning authority and an Inspector for modification of a condition requiring a development to be constructed in accordance with certain specified drawings on the basis that the new proposal was beyond the scope of the permission.
The modification proposed alternative drawings would have resulted in a significantly different architectural style compared with the dwelling which had originally been permitted. The original planning permission had simply authorised construction of a single dwelling and both the Local Authority and the Inspector considered, if the variation were permitted, the resultant building would be so different from that originally contemplated as to go beyond what would be allowed by section 73.
However, the High Court’s conclusion was that provided the variation to the planning condition concerned does not result in a development which conflicts with the development description contained in the original planning permission, there is nothing to limit this scope seek changes by modification of the planning conditions. The fact that section 73 can be used in such a case doesn’t guarantee that the application will succeed. The application still has to be considered on its planning merits.
The lesson for those seeking planning permission or seeking to modify planning permission is to ensure that the description of the authorised development and the original consent, whilst being accurate, is sufficiently broad to accommodate a whole range of potential future changes. An over-elaborate description of the proposed development may tie a developer’s hands should it become necessary to seek a variation at a later date.
Local planning authorities seeking to manage development will need to pay attention to the extent to which conditions should be used to constrain development or changes to development or the extent to which the description of the proposed development is sufficiently tight to control later design changes. One side-effect of this decision may be to drive local planning authorities to push the boundaries of what they are lawfully permitted to do by insisting upon more detailed descriptions of developments with applications in order to constrain future changes.
Where a section 73 is not available as a procedure to modify design or layout or any other matter regulated by conditions because the description of the permitted development is too tight, it does not mean that planning permission cannot be obtained. A fresh full application would instead be needed to determine whether the modified development is acceptable in planning terms.
If you want to discuss this further, contact Derek Ching, Partner or Simon Dimmick, Consultant in the Development & House Building Team .
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