It's unusual for a 3 flat converted house to need a grade A system, though it may be that they are only interested in covering the communal area. Separate dwellings would normally have their own system.
How many stories is the building?
Unless it's been classified as an HMO, I'm not sure how the council would get involved to require a specific fire alarm system?
It's certainly worth getting in writing exactly what they require. There are two separate things: Grades, and Categories.
Grade A is a full control panel setup, which needs to be maintained and can be costly to retrofit.
Grades
Grade A: A fire detection and fire alarm system, which incorporates CIE conforming to
BS EN 54-2 and power supply equipment conforming to BS EN 54-4, and which is designed and
installed in accordance with all the recommendations of BS 5839-1:2017, Section 1
to Section 4
Grade D1: A system of one or more mains-powered detectors (see
3.12), each with a tamper-proof standby supply consisting of a battery or batteries
Grade D2: A system of one or more mains-powered detectors (see
3.12), each with an integral standby supply consisting of a user-replaceable battery or batteries.
Categories:
Category LD2: a system incorporating detectors in all circulation areas that form part of the escape routes
from the premises, and in all specified rooms or areas that present a high fire risk to occupants, including any
kitchen and the principal habitable room
Category LD3: a system incorporating detectors in all circulation areas that form part of the escape routes
from the premises
BS 5839-6, the current standard, only recommends a Grade A system (for existing premises, not new builds), when its a 4 storey house or more, or supported housing.
