In small, simple premises, a shout of fire or a device such as a gong or air horn may be sufficient.
Where the alarm may not be heard throughout the building, an electrical system incorporating sounders and call points (red break-glass boxes) may be required.
This type of manually operated system is likely to be adequate where all parts of the building are occupied at the same time and it is unlikely that a fire could start without somebody noticing it quickly.
If people may be sleeping or not alert in parts of the building, or where there are unoccupied areas in which a fire could develop undiscovered, an automatic fire detection system may be necessary.
The level of fire alarm should be determined by your fire risk assessment. Further guidance on fire warning systems can found in Part 2, Section 2 of the free guide on risk assessment relevant to your type of premises.
You may wish to consider installing automatic fire detection in order to protect your property or business.