Changing plans after planning - can they?

[Whyismycatanasshat]

Our neighbours are having a double height extension/annexe built. It has separate stairs to the main house. They’re changing the interior stairs from one lay out to another - originally they had an L shaped staircase/ two sets of stairs. They told me yesterday they have agreed with the builder to put a U shaped stair case in/ 3 sets of stairs. They said it was easier to get planning for the L shaped than the U shaped.

I will add a a disclaimer that whilst we are having issues with their builder coming on to our property but I will not be doing anything like reporting them for this; it’s just an idle curiosity of mine, I’m being nosey I suppose .

Is it permitted to change something this substantial without having to get new planning permission etc?

Planning applications and associated documents ie drawings and plans are publicly available to view on your local authority's website.

There is no changes to the plans that are available online. I have checked.
I am not bothered, just nosey as to whether this is an acceptable thing to do now!

If it's internal alterations only, then no change to planning is required. If it means moving windows etc, then planning should be consulted, but would probably only involve a minor amendment rather than a new planning application.

It would have implications under the Building Regs.

@Seeline Thank you! I think I was assuming the staircase layout was fixed once planning was through.
No windows will be added or anything; personally I think they will be reducing their light but it’s their annexe, actually, I wonder if it is to make fitting a stair lift easier as I think it’s all for an ageing parent.