Planning conditions commonly include "tailpiece" wording along the lines of "unless otherwise approved in writing by the local planning authority", primarily to allow flexibility should changes be required later on in the development process. Of course, there are two statutory procedures available to amend planning conditions / planning permissions in the form of sections 73 and 96A of the Town and Country Planning Act 1990. In light of those two procedures, both of which involve consultation to varying degrees, what may be left to be changed under such tailpiece wording must be rather limited.
Various cases set out the parameters of tailpiece wording, but these parameters are often forgotten in the rush to agree acceptable conditions either because of an impending committee meeting, or to have permission granted as soon as possible (or simply because such tailpiece wording has routinely been attached to conditions in the past).
In the case of R (Midcounties Co-operative Ltd) v Wyre Forest  EWHC 964 (Admin) the key condition in question established an upper limit on the permitted retail floor space, but included tailpiece wording along the lines set out above. Mr Justice Ouseleyaccepted a "very limited power to make immaterial variations informally. But while the tailpiece in the condition in question could be applied in that way, it contains no words purporting to limit its application". He concluded that in this case the tailpiece "on its face does enable development to take place which could be very different in scale and impact from that applied for, assessed or permitted and it enables it to be created by means wholly outside any statutory process."
This of itself did not cause Mr Justice Ouseley to quash the permission. Rather, he severed the tailpiece wording from the condition and allowed it to otherwise stand. Drawing on the above, presumably a condition which did set out the limits of the application of the tailpiece and how those limits might apply might be lawful, albeit I would not envy anyone having to draft such a condition.
Since Midcounties, a number of cases have grappled with the same issue, reflecting how common tailpieces are. In Sienkiewicz v South Somerset District Council v Probiotics International Ltd  EWHC 3704 (Admin), again a case heard by Mr Justice Ouseley, among the grounds of challenge were four conditions which included tailpiece wording. They required approval of materials, external lighting, replacement of trees and plants and details of levels across the site. As noted in the judgment, each of the conditions required a first approval of these details post the grant of permission but then allowed for variations where agreed in writing by the local planning authority. This was challenged on the basis of Midcounties.
In his judgment, Mr Justice Ouseley distinguished Midcounties on the basis that the Midcounties condition discussed above would have allowed the retail space provided to be in excess of that actually applied for and permitted. Further, the second condition in question in Midcounties (requiring adherence to drawings) did not reserve details for later approval, but in fact purported to allow variations to what was already approved as part of the permission itself. In contrast, Mr Justice Ouseley considered the Sienkiewicz conditions "quite conventional" in that they required later approval of matters of detail and "the application for that approval does not require consultation, nor once approved is there a bar on further applications for approval. There is no formal procedure for approval of details reserved for subsequent approval, but which are not strictly reserved matters".
Therefore, in his judgment there was no reason why such an approval could not be capable of variation, bearing in mind that seeking that variation would be a process "not significantly different" from that gone through for the original approval of the details.
So, in addition to the Midcounties parameters, conditions which require a later approval of details and which include a tailpiece allowing a variation to those later approved details, can be lawful.
Rhodri Price Lewis QC, sitting as a Deputy High Court Judge, in R (Butler) v East Dorset District Council v Good Energy Mapperton Farm Solar Park (007) Ltd  EWHC 1527 (Admin) handed down a recent judgment which again applied Midcounties and found that the tailpiece element of the conditions in question was unlawful. In this case, there were five conditions with tailpieces which required either submissions of further plans or adherence with plans already submitted (all of which related to environmental mitigation measures).
The Claimant submitted that those conditions addressed matters of significance which could have a permanent effect on the control of potential environmental effects. The Judge agreed with this noting:
"the Council assessed those impacts and found them acceptable on the basis of material that was before it when the decision was taken…the public can be reassured to that extent. If changes were however proposed to the measures relevant to those matters, there could be significant changes to the impact on the environment and in my judgement the public should be able to know about those proposed changes and make representations upon them. That would not be possible if these tailpieces were retained".
However, the Judge went on to note that he would not have quashed the planning permission just because of those tailpieces as he could have ordered them severed from the rest of the condition.
Lastly, the case of Hubert v Carmarthenshire County Council  EWHC 2327 (Admin) is worthy of mention, if only for the quite convincing defence of the tailpiece conditions raised by Counsel for the Council. In Hubert, the offending condition set out the measurements a wind turbine should adhere to, unless given the written approval of the Council. The tailpiece was ultimately severed, but in defence of it Counsel argued that when the condition was read in conjunction with the decision letter as a whole, it was clear the tailpiece could not permit any greater tip height. This was because the development was defined as having a maximum tip height of 67m irrespective of the condition. Furthermore, other conditions controlled noise levels and shadow flicker, which would be breached if the tailpiece was used to permit anything other than minor changes.
Ultimately, Mr Justice Cranston was not persuaded by this, and applied Midcounties to sever the tailpiece, noting that "the breadth of the words means that it cannot be construed as being limited to minor variations. Here, what the condition on its face allows is for variation, up or down, and without any restriction…"
The inclusion of tailpieces on conditions will rarely of itself lead to the quashing of a permission. Judges instead have preferred to use their discretion to sever such wording from the offending conditions. However, the rigmarole of a legal challenge is not to be taken lightly, causing cost and delay. This can be easily avoided by careful thought early in the process as to whether tailpieces are appropriate.