Zack SImons covers the drop-in point on his blog
https://www.planoraks.com/posts-1/when-you-cant-build-both-2-hillside-in-the-supreme-court
"There’s more. One thing which is getting lots of folks talking is what this judgment may mean for so called “drop-in” applications. “So called” because drop-in applications aren’t a creature of planning law or policy. They’re a creature of practice, i.e. a strategy to vary sections/phases of larger multi-phase developments by applying for a new planning permission in respect of only part of a wider site, whilst continuing to develop the rest of the site under the original permission. No doubt about it - this kind of strategy (where it still remains possible) is going to take considerable cunning and care after Hillside: see in particular paragraphs 71, 73 and 74 of the judgment. It’s likely to need express consideration of parcelling up the original permission up to make bits of it “severable”: see paragraph 68. What any of this actually means in practice will now have to be worked through. And it won’t be easy. Because the headline from the Supreme Court is that - at least generally, albeit not always - the proper course is going to require a new application (either a full one, or a s.73 application - on which more here) which covers the entire site: see paragraph 74. Not just the “drop-in” bit. All of it. The mega cost, delay and inconvenience that comes with this course of action will only increase the clamour to simplify the law on the how we vary planning permissions."
