Interlinked Smoke Alarm London — When and Why You Need One

An interlinked alarm system is one in which every detector in the property communicates with every other detector. When one alarm detects smoke or heat, every alarm in the building sounds at the same time. The system behaves as a single coordinated unit rather than a collection of independent point alarms.

There are two ways to interlink. Hard-wired interlink uses a dedicated signal conductor — typically the orange or interconnect terminal — that runs from alarm to alarm alongside the live and neutral. The interconnect cable is usually 1.0mm or 1.5mm 3-core-and-earth running between the detector positions on the same circuit. Radio-frequency interlink uses a paired RF module on each detector — the modules talk to one another wirelessly and sound the whole network when one detector triggers.

Both methods satisfy BS 5839-6 where they are correctly designed, installed and tested. Radio-link has overtaken hard-wired as the dominant install method in London simply because it avoids ripping up finished plaster.

BS 5839-6:2019 requires that every alarm in a domestic system operates as a single coordinated unit. The standard does not permit unconnected point alarms in any system serving more than a single room. A 3-bed flat with five point alarms but no interlink is technically a non-system under the standard — every alarm is operating independently.

The Smoke and Carbon Monoxide Alarm (England) Regulations 2015 do not specify interlinking by name. They specify that a smoke alarm must be present on every storey containing a room used wholly or partly as living accommodation. However, the supporting guidance and the council enforcement standard reference BS 5839-6, which closes the loop and makes interlink a practical requirement.

Approved Document B (Volume 1) of the Building Regulations requires interlinked Grade D alarms on every new build and material alteration. A loft conversion, a rear extension or a change of use to HMO all trigger Approved Document B and therefore Grade D LD2 or LD3 interlinked coverage.

Under the Renters' Rights Act 2026 and the strengthening of council enforcement powers, an HMO with non-interlinked alarms is an open invitation to enforcement. We have seen councils issue Improvement Notices on properties that previously passed inspection — the bar is rising not falling.

Wired interlink is the long-term gold standard. Once installed, it has no batteries to fail (other than the standby battery in each detector) and no RF link to drop. It is the right choice for a full rewire, a new build or a substantial refurbishment where the ceilings are already open.

Radio-link is the right choice for occupied properties, finished interiors and any retrofit where running cables would require taking down ceilings. Modern radio-link alarms from Aico, Kidde and FireAngel offer 10-year sealed battery operation, encrypted pairing and self-monitoring. The RF link has effectively zero failure rate within typical UK home dimensions.

Cost-wise, wired is cheaper per point on a new build (£90-£110 per alarm fully installed when the ceiling is open) and more expensive on a retrofit (£220-£280 per alarm where cable chasing is needed). Radio-link is consistently £170-£200 per alarm whether new build or retrofit.

BS 5839-6 defines six grades. Grade A is a manned panel system; Grade B is the same with reduced standby; Grade C is mains-powered with central panel; Grade D is mains-powered with integral battery backup for each detector; Grade E is mains-powered without battery backup; Grade F is battery-only.

For domestic rental properties and HMOs, Grade D is the practical standard. The mains feed comes from the lighting circuit at the consumer unit (not a sockets circuit — the lighting circuit is permanently energised and not subject to plug-in tampering). The integral battery provides at least 72 hours of standby in the event of mains failure.

Grade F battery-only alarms are explicitly disallowed for rental and HMO use in London. They remain legal in owner-occupied properties and are the cheapest option for someone fitting alarms to their own home, but they fail any rental inspection.

Owner-occupied homes have no legal requirement for interlinked alarms. The Smoke and CO Regs only apply to rented accommodation. An owner-occupier can install a single battery alarm on each storey and remain compliant with the law, although BS 5839-6 still recommends interlink for safety.

Holiday lets fall under the Regulations once they are let to paying occupants — interlinked Grade D coverage applies as soon as the property is rented for any consideration. Airbnb and short-let hosts often overlook this and operate non-compliantly.

Houseboats, mobile homes and certain Crown-owned properties have specific exemptions. None of these apply to standard London rental stock.

Our standard installed price for an interlinked Grade D radio-link alarm is £180 per point — that includes the Aico Ei3024 or equivalent multi-sensor detector, the RF module, the lighting-circuit connection, the test and certification, and the registration of the device on the property's compliance record.

A typical 2-bed flat takes three to four alarms (every bedroom, hallway, kitchen heat alarm) — budget £540-£720. A typical 3-bed HMO takes six to seven alarms — budget £1,080-£1,260. A converted 4-bed Victorian terrace HMO with loft conversion takes eight to ten alarms — budget £1,440-£1,800.

CO alarms add £85 per room fully installed where a fixed combustion appliance is present. Most rental properties need at least one CO alarm in the kitchen if it has a gas hob.

Annual maintenance is minimal on a modern radio-link system. We include a 12-month return visit to test every alarm and confirm the RF network in our installation price — beyond that, an annual service call costs £75 for a typical 3-bed HMO and produces the test record that supports the landlord's compliance file. Test records are increasingly requested by councils as part of HMO licence renewals.

Mixing manufacturers within a radio-link network. Radio-link modules from Aico, Kidde and FireAngel use different proprietary frequencies and pairing protocols. An installer who tries to mix two brands ends up with two parallel networks that do not interlink. We standardise on a single product family per property and document the model numbers on the commissioning certificate.

Feeding the alarm circuit from a sockets ring rather than the lighting circuit. The sockets ring is RCD-protected and any RCD trip silences the entire alarm system. BS 5839-6 and BS 7671 both require the alarm feed from a dedicated or lighting circuit that cannot be inadvertently disconnected by an occupant pulling a plug or tripping a kitchen socket RCD.

Installing detectors too close to walls or light fittings. The 300mm clearance rule applies to ceiling-mounted alarms — closer than 300mm to any wall and the airflow pattern produces a dead spot where smoke does not reach the sensor. This is a common error on small hallways where there is a temptation to push the detector against the wall.

Skipping the commissioning test. BS 5839-6 requires a smoke-aerosol or heat-source test on every alarm at commissioning, not just a button-press test. The aerosol test verifies that the sensor itself responds to combustion products, not just that the alarm circuit closes. We carry aerosol test kits as standard equipment and document each test in the certificate.

London council HMO inspectors run through a standard alarm checklist on every property visit. Top of the list is the existence of a BS 5839-6 commissioning certificate signed by a competent person. Without the certificate, the property fails inspection regardless of how good the physical install looks.

Next is interlink confirmation — the inspector will activate one alarm and verify that every other alarm in the property sounds within ten seconds. Non-interlinked systems fail immediately. Partial interlink (some alarms link, some don't) also fails — the standard is all or nothing.

Battery-only Grade F alarms fail on rented HMOs without further investigation. The inspector does not need to assess the property's risk profile — Grade F is structurally non-compliant for HMO use and the property is flagged.

Improvement Notice is the standard outcome for alarm failures. The notice specifies the deficiency, the remedy required and a compliance window (usually 28 days for safety items). Failure to comply within the window escalates to a Civil Penalty up to £30,000 or a Rent Repayment Order returning up to 12 months of rent to the tenant.

Re-inspection after a Notice has been served is at the council's discretion and typically falls within the compliance window. The landlord must produce the upgraded BS 5839-6 commissioning certificate at re-inspection as primary evidence of remedy. We schedule retrofit jobs to align with re-inspection windows where the Notice timing demands it.

Where the Notice has triggered enforcement action that the landlord wishes to appeal, the appeal window is 21 days from service. Appeals route through the First-tier Tribunal (Property Chamber). Successful appeals are rare on alarm-coverage grounds because the standard is technical and unambiguous; the appeal route is more useful where the Notice has procedural defects than where the substance is challenged.