Grade and category answer two different questions and get used as if they were one. Category says how much of the building is covered. Grade says what kind of equipment does the covering, from a full panel and detector system down to a single battery alarm. In dwellings both letters matter, and a specification that gives you only one of them is incomplete.
What the grades describe
The grades run from a system built around a control and indicating panel with mains power and a standby supply, through mains powered interlinked alarms with integral standby batteries, down to alarms powered by a replaceable battery alone. The higher grades bring monitored wiring, fault indication and a proper standby supply. The lower grades bring simplicity and a much shorter service life.
Grade against category in a dwelling
A rented flat might be specified as a particular grade at a particular coverage level, and both halves have to be met. Meeting the coverage with the wrong grade of equipment, for example battery only alarms where mains powered interlinked units were specified, is a real non compliance even though every room has an alarm in it. This is the most common way a well intentioned installation fails an inspection.
Which grade a rented home usually needs
The relevant standard for dwellings is BS 5839-6, and the grade recommended depends on the type of dwelling, the number of storeys and the occupancy. Houses in multiple occupation are treated more demandingly than single family lets. Local licensing conditions frequently specify the grade directly, so the council's own HMO licence conditions are worth reading before buying anything.
Where to find the specification
There is no single official grades table published free of charge, which is why searches for a categories and grades document return so many contractor summaries of varying accuracy. The authoritative text is the standard itself. What you can get free is the licence condition, the risk assessment or the specification your own building was designed to, and those are the documents that actually bind you.
Questions people ask about fire alarm grades
What is the difference between grade and category?
Grade describes the type and resilience of the equipment; category describes how much of the building is covered. A specification needs both, for example a stated grade at a stated LD coverage.
Are battery only alarms ever acceptable?
In some owner occupied situations, yes, but many rented and licensed settings require mains powered interlinked alarms. The licence condition or the risk assessment is what decides it for your property.
Where can I read the grades table officially?
In BS 5839-6, which is published by BSI and is not free. Summaries circulate widely; treat them as orientation and work from the specification for your own building.