The Hillside principle and drop-in permissions
After Hillside Parks, a later permission that makes the original scheme physically impossible to complete can stop the original being lawfully implemented. Here is what that means for phased sites.
If you build something on part of a site under a new permission, and that building makes it physically impossible to finish the original scheme in accordance with its terms, the original permission can no longer lawfully be implemented. That is the Hillside principle, confirmed by the Supreme Court in Hillside Parks Ltd v Snowdonia National Park Authority in 2022, and it has reshaped how multi-phase sites are managed.
The practical consequence is blunt. A developer who takes a convenient standalone permission for one parcel, builds it, and carries on building the rest of the estate under the old master permission may be carrying out unauthorised development without realising it.
What the Supreme Court decided
The case concerned a Snowdonia site with a 1967 permission for a large housing scheme, followed by a series of later permissions, with only a fraction of the original scheme built across decades.
The Court applied the reasoning in Pilkington from the 1970s. Where development carried out under one permission makes it physically impossible to complete the development authorised by another permission in accordance with that permission's terms, the second permission cannot lawfully be acted on any further.
Two points in the judgment matter most to a developer.
The first is that a planning permission for a multi-unit development is normally to be construed as authorising a single integrated scheme, rather than a collection of independent acts of development that can be picked off one at a time.
The second is that the test is one of degree rather than absolute impossibility. A trivial departure does not kill the permission. A material departure, judged against what the permission actually authorised, can. That judgement about materiality draws on the same reasoning as the threshold questions discussed in de minimis in planning judgments.
Why a later permission can render an earlier one undeliverable
The mechanism is physical, not legal. Put a block of flats across the line of a road that the master permission required, and the master scheme cannot now be built as drawn. Change a plot's footprint so that the approved access geometry no longer fits, and the same follows.
| Action on a multi-phase site | Typical effect on the master permission |
|---|---|
| Building a drop-in scheme that conflicts with the approved layout | Master permission may become incapable of lawful implementation |
| Building a drop-in scheme on land outside the master permission boundary | No conflict, provided the master layout is unaffected |
| Varying the master permission under section 73 | Master permission replaced by a new one on the same site |
| Making a non-material amendment | Master permission amended, no new permission |
| Taking a fresh permission for the whole site incorporating the change | Clean position, at the cost of a full application |
"Drop-in permission" is the industry term for a standalone consent covering part of a larger consented site. It was common practice because it was quicker than varying a master permission with hundreds of drawings attached. After Hillside it is a risk to be analysed rather than a routine tool.
Phased schemes and severability
The Court accepted that a permission could in principle be severable, so that each part could be implemented independently. But severability has to be clear from the permission itself, properly construed.
A phasing condition does not make a permission severable. Neither does a drawing labelled with phase boundaries, nor an obligation that uses phased triggers. Those arrangements organise the delivery of one integrated scheme. Severability means the permission authorises several developments rather than one.
So if severability matters to a scheme, the drafting has to do the work at the point the permission is granted. That means an express statement in the description of development or in a condition that each identified parcel constitutes a separate development capable of independent implementation, with the supporting drawings structured to match. Authorities vary in their willingness to grant permissions in that form, and the request has to be made early.
Where an outline permission is involved, the interaction with reserved matters needs thinking through at the same time, since the scope of what remains to be approved affects how a parcel can be delivered, as set out in what cannot be reserved for later approval.
Section 73 as the safer route
For most changes, varying the existing permission is the answer. Section 73 allows permission to be granted without complying with conditions, which in practice means amending the approved drawings condition, and the result is a new permission for the whole site rather than an overlapping one for part of it.
Its limits are well known. It cannot be used to grant something conflicting with the operative part of the permission, so a change to the description of development is outside it. Section 73B, inserted into the 1990 Act by the Levelling-up and Regeneration Act 2023, was intended to address precisely that gap, including for the Hillside situation. Check whether it is in force and what procedure applies before building a strategy around it, because availability depends on the provision being brought into force and on secondary legislation.
The housekeeping that follows a variation is the part people forget. A new permission needs the section 106 obligations restated or varied, levy liability recalculated, and previously discharged conditions dealt with again, the mechanics of which are the same as for any new consent and are covered in approval of details or discharge of conditions.
Due diligence on multi-phase sites
If you are buying into a part-built consented site, the Hillside question belongs on the first page of the report on title.
- List every permission granted on or overlapping the site, with dates, descriptions and approved drawings.
- Establish what has actually been built, and under which permission, from the construction record rather than from the sales brochure.
- Compare what has been built against the master permission's approved layout, plot by plot.
- Identify any physical conflict that would prevent completion of the master scheme as drawn.
- Form a view, in writing and with legal input, on whether the master permission remains capable of lawful implementation.
- Check whether the obligations and levy position were calculated against a permission that may now be unusable.
A seller's assurance that everything was built under a drop-in with the council's knowledge is not an answer. The council's knowledge does not make the master permission implementable again, although it bears on enforcement risk.
Illustrative example, not a real scheme: a 300 dwelling consent with an approved spine road and a distributor junction. Phases one and two are built as approved. For phase three the developer takes a drop-in permission for 60 apartments, which sits across the line of the approved spine road and deletes a section of it. Phases four and five are then built out under the master permission. The master scheme can no longer be completed as drawn, so the phase four and five works may be unauthorised, which affects their saleability, their warranty position and the enforceability of the obligations. The fix would have been a section 73 variation of the master permission to show the apartments and the revised road, taken before anything was built.
Practical steps on a live scheme
Where a conflict has already been created, the options are to apply for a fresh permission for the remainder of the site as it is now proposed to be built, to apply for a section 73 variation of the master permission where the change can be accommodated within its description, to seek a certificate of lawfulness for what has already been built where the time limits assist, or to regularise through a retrospective application.
None of those is quick, and all of them are quicker than discovering the problem when a purchaser's solicitor raises it.
Going forward, the discipline is simple. Any change to a consented multi-phase site goes through the master permission, not around it. Where that is impractical, get the severability question answered at the point the permission is granted rather than afterwards.
Set out the implementation strategy explicitly in the planning statement for any variation, so the authority understands which permission it is being asked to replace and why.
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.
Frequently asked questions
What is the Hillside principle?
It is the rule confirmed by the Supreme Court in Hillside Parks Ltd v Snowdonia National Park Authority that where development carried out under one permission makes it physically impossible to complete the development authorised by another permission in accordance with its terms, that other permission cannot lawfully be acted on any further. The test is one of degree, not absolute impossibility.
Are drop-in permissions still usable?
Only after analysis. A standalone consent covering part of a larger consented site is safe where it does not conflict with the master permission's approved layout, and dangerous where it does. Building it can leave the master permission incapable of lawful implementation, which in turn makes continued development under that permission unauthorised.
Does a phasing condition make a permission severable?
No. The Supreme Court accepted that a permission can in principle be severable, but severability has to be clear from the permission properly construed. Phasing conditions, drawings marked with phase boundaries and obligations with phased triggers all organise the delivery of one integrated scheme rather than authorising several separate developments.
How do you get a severable permission?
By asking for it when the permission is granted, with an express statement in the description of development or in a condition that each identified parcel is a separate development capable of independent implementation, and with supporting drawings structured to match. Authorities differ in their willingness to grant permissions in that form, so raise it early.
What should due diligence on a part-built site cover?
Every permission granted on or overlapping the site, what has actually been built and under which permission, a plot by plot comparison against the master permission's approved layout, any physical conflict preventing completion as drawn, a written view with legal input on whether the master permission remains implementable, and whether obligations were calculated against a permission that may now be unusable.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.