Alfasev
I think that the HSE document that you refer to makes interpretation unnecessarily complex...
Step 1 – the project/activity must fall within one or more of the three categories set out in the definition, those being the carrying out of any; • building work, • civil engineering work, or • engineering construction work
In ye olden days "building work" would be what was formerly defined as a "building operation" - that definition included much of what is in the current definition of "construction work" including the bit about installation etc of services attached to a structure.
....and "civil engineering work" and "engineering construction work" fell either into the definition of "building operation" or that of "work of engineering construction". Whilst Section 176 of the Factories Act 1961 set out a definition of WEC it was amended by two extensions.
It's incredibly unlikely that it will fall to a jury to come up with an AUTHORITATIVE interpretation of "construction work" as such a judgment would be made by Judges at the Court of Appeal or Supreme Court.
Given the wording of the definition of "construction work" it is inevitable that those Judges will be referred back to the authoritative judgments of what constituted a "building operation" [Nobody ever seems to have argued about WECs].
Since that means that "Step 1" is intrinsically linked to "Step 2" it follows that Step 1 is redundant.
...and to answer one of your QQ, I was an HSE Inspector enforcing the Construction Regulations 1961 and 1966, so dealing with the interpretation of "building operation" on a regular basis. Later I sat on the CIRIA working party which oversaw its original guidance on CDM for Clients (Report 172) and Planning Supervisors (Report 173).
Reports 172 and 173 (and the Approved Codes of Practice which supported CDM 1994 and 2007) provided much clearer guidance on the interpretation of "construction work" than L153 does.
So, given that the definition in CDM 2015 is to all intents and purposes identical to that in CDM 1994 and 2007, perhaps you should go back to earlier HSE guidance on the topic.
In practice, the issue is rarely of much importance as if CDM doesn't apply then its requirements are largely implicit in HSWA and the Management of Health and Safety at Work Regulations 1999.
So, to take a very historic example, one of my former colleagues investiated a fatal accident where a man was doing some final measurements on a roof before re-roofing was due to start the following week. This was before HSWA and my colleague thought that the work was NOT part of a "building operation" and hence there was no prospect of prosecution being successful. However, in the civil case that followed, the Court held that the work was "ancillary to, but necessary for" the building operation and hence was part of the "building operation".
If this had occurred a few years later, my colleague could simply have collected the evidence to proceed towards prosecution for breach of HSWA.