I may have answered my own question with a little more digging

Criteria for determining whether a building is a higher-risk building during the occupation phase of the new higher-risk regime - GOV.UK (www.gov.uk)

Use criteria for buildings in occupation under the Building Safety Act 2022 and the Higher-Risk Buildings (Descriptions and Supplementary Provisions) Regulations 2023

Section 65 of the Building Safety Act 2022 sets out the buildings which are included in the occupation part of the higher-risk regime.

1. Meaning of “higher-risk building” etc

(1) In this Part “higher-risk building” means a building in England that—

(a) is at least 18 metres in height or has at least 7 storeys, and

(b) contains at least 2 residential units.

Regulation 8 sets out the buildings which are excluded from the occupation part of the higher-risk regime.

1. (1) For the purposes of section 65 of the 2022 Act a “higher-risk building” does not include a building of a description specified in paragraph (2).

(2) The following descriptions of building are specified for the purposes of paragraph (1)—

(a) a building that comprises entirely of—

(i) a care home;

(ii) a hospital;

(iii) a secure residential institution;

(iv) a hotel;

(v) military barracks;

(b) a building that contains living accommodation provided by the Ministry of Defence (either alone or in combination with other accommodation);

(c) a building that contains living accommodation (either alone or in combination with other accommodation) for—

(i) His Majesty’s forces;

(ii) any visiting force or an international headquarters or defence organisation designated for the purposes of the International Headquarters and Defence Organisations Act 1964.

But would still welcome comments 

If the building meets the "height" definitions then a change of use to include "residential" units on a floor of the building would bring it in to scope as a refurbishment rather than new build.

The refurbishment route is likely going to be fraught with issues as retrofitting sprinkler systems and second staircases may not be practicable and costly.

You should consider having a chat with the building safety regulators office given they have the powers.

KM, I think this is one of those scenarios where it is possible that the "spirit of the law" and the "letter of the law" might be different.

Section 115 of the BSA says, amongst other defintions:

“residential unit” means—

(a) a dwelling, or

(b) any other unit of living accommodation;

I am not convinced that what you describe might necessary be considered to fall within either of these, though ultimately it will be for the Courts to determine on a case by case basis.

....and I think that it might turn in some cases on whether those staying in one of these "guest" units has to pay to do so, or whether it is provided free of charge by the employer.

If they pay, then I guess that the rules for Short Term Lets would apply.

But if they don't pay there is no "letting" in the usual sense happening. So, may be out of scope of the Act.

However, seems to me that the "spirit of the law" is that the principles of BSA should apply - and such principles might be deemed to  implicitly fall under the employer's duty of care whether directly under HSWA etc or indirectly via the "neighbour principle" established in Donoghue v Stevenson.

One of the key reasons for including "residential units" within the scope of BSA is that people use the premises for sleeping and when asleep are likely to take longer to identify that there is a fire and longer to react to what is happening nearby.