Hi ASW
Your first post here, so welcome to the Forums.
I am somewhat puzzled as to why you think that the question you have asked is a "divisive issue", not least since you specifically frame it in the context of "designed scaffolds" rather than what in ye olden days were described as General Access Scaffolds in HSE guidance note GS15.
Before the first iteration of CDM this was a recurring issue.
However, CDM introduced a definition of "construction work" (that has been tweaked very slightly since CDM 1994".
That definition was largely based on the definitions in Section 175 of the Factories Act 1961 of a "building operation" and "work of engineering construction", but with the legislators taking the opportunity to close some perceived loopholes in the application of construction H&S legislation, including as regards the sort of question you are asking.
So the CDM definition of "construction work" in its various parts brings in the definition of "structure"
“structure” means—
(a) any building, timber, masonry, metal or reinforced concrete structure, railway line or siding, tramway line, dock, harbour, inland navigation, tunnel, shaft, bridge, viaduct, waterworks, reservoir, pipe or pipeline, cable, aqueduct, sewer, sewage works, gasholder, road, airfield, sea defence works, river works, drainage works, earthworks, lagoon, dam, wall, caisson, mast, tower, pylon, underground tank, earth retaining structure or structure designed to preserve or alter any natural feature, and fixed plant;
(b) any structure similar to anything specified in paragraph (a);
(c) any formwork, falsework, scaffold or other structure designed or used to provide support or means of access during construction work,
So, assuming that you have some "construction work" going on, then there doesn't appear to be much doubt that any scaffold that is erected in associated with the "construction work" will be a "structure" within the meaning of CDM.
Then the question comes to what constitutes "design", and from there responsibilities on any "designer".
“design” includes drawings, design details, specifications and bills of quantities (including specification of articles or substances) relating to a structure, and
calculations prepared for the purpose of a design;
“designer” means any person (including a client, contractor or other person referred
to in these Regulations) who in the course or furtherance of a business—
(a) prepares or modifies a design; or
(b) arranges for, or instructs, any person under their control to do so, relating to a structure, or to a product or mechanical or electrical system intended for a particular structure, and a person is deemed to prepare a design where a design is prepared by a person under their control;
I would note that compliance with CDM in relation to the design of scaffolds is probably very patchy, not least as this is usually done as part of Temporary Works (whether identified as TW or not) under the direction of the Principal Contractor or another Contractor and there might not be the appropriate level of oversight of this element of design by the Principal Designer.
However, that is an issue of implementation rather than intepretation of the Regulations where what CDM says seems very clear to me!
Perhaps the much bigger problem is where someone doesn't recognise the need for a "designed scaffold" and leave it to the scaffolders to get on with it. They they stray from erecting a straightforward scaffold due to the constraints they come across and do the "design" on the hoof.
As example there is a redevelopment happening at the building next door to where I live. So scaffolds up on front and rear elevations, in each case from basement well to 3rd floor level.
Problem is that at one end of the rear scaffold there is an outhouse which sits where the standards might have been expected to be. Couldn't found the scaffold on the outhouse as its roof is fragile to the point of very visible near collapse.
So the solution found by the scaffolders was to foot the end standards on the party wall between two gardens.
Should be patently obvious to all working on the project that this 19th Century wall was not designed for vertical loading and it seems that the scaffolders recognised this.
So to reduce the load down on to the wall, the upper lifts are in horizontal cantilever, so that most of the load is transferred to bear down on standards footed in the basement well.
It's possible that someone has actually sat down and produced a drawing to demonstrate that this affords an appropriate level of structural security - i.e there is a formal "design", but I am very doubtful that this is what happened!
Much more likely is that the scaffolders themselves have done an impromptu "design" to solve a problem. Seems to me that this is still a "design" within the meaning of CDM, with said scaffolders thence the "designer".