Fire (smoke) detector alarms in extensions ambiguity

[David Lees]

Building Regs 2020 Part B Vol 1 Dwellings says for extensions:
"Extensions and material alterations 1.8 Where new habitable rooms are provided, a fire detection and alarm system should be installed where either of the following applies. a. The room is provided above or below the ground storey. b. The room is provided at the ground storey, without a final exit. 1.9 Smoke alarms should................."
The phrase "a fire detection and alarm system should be installed" is ambiguous. Does it mean just the new work or the whole existing plus new?
Anyone sorted this?