What are the fire safety dangers of cannabis farms?

In the UK, statistics from the Home Office show that in the year to March 2024 there were 152,600 seizures of cannabis, almost 10% up on the year before, with herbal cannabis seizures at the highest levels since records began. 

The demand for cannabis is fuelling illegal farms and the Metropolitan Police have revealed that they found more than 1,000 such farms between April 2018 and March 2023. In March 2025, Operation Millie saw forces combine to seize plants with a total street value of more than £48m.

The UK emergency services have been attending cannabis-related fires since at least the 1970s. But as cultivation has increased, so has the number of fires related to illegal factories. Home Office statistics show 640 such fires were recorded between 2019 and 2024.

Fire is not the only threat, with the risk of explosion increased due to the popularity of honey butane oil or 'shatter cannabis' – a derivative product that can be made using butane gas to extract psychoactive chemicals from the cannabis flower. The concentrated derivative is used in the likes of vapes.

Indications are that gangs are increasingly moving cannabis production from large factories to residential homes, reducing the impact on their business should one be detected. Fires in residential properties pose a serious threat to unsuspecting neighbours, but fires in cannabis factories in commercial premises are also a serious issue. What's behind these increases?

With demand on the rise and coming from a wider customer base than before, a ready market has also been created for illicit production and distribution.

Second, this market means large-scale illegal cannabis production is big business for those involved. There is good evidence that many factories are part of established criminal networks, with many thousands of people involved in the supply chain of this lucrative trade.

Third, a lot more information about cultivation is now available and the equipment to start even a small factory is quite easily obtained on the internet. This means that some may begin growing the drug in a back bedroom purely for personal use, and not necessarily to sell on.

Although insurers do not collect specific statistics on fire claims related to cannabis production, there is good evidence that they are currently seeing more losses relating to these factories and that those claims are becoming larger in scale.

A large warehouse blaze in Tottenham back in May 2019 is believed to have started in one unit that was being used to produce the drug. The fire burned for more than 24 hours and took 100 firefighters to extinguish. Zurich insures one of the neighbouring units; the resultant claim by the innocent customer cost more than £1m.

Where a fire does occur, it is often due to the temporary lighting and heating equipment that has been rigged up on the premises. High-powered industrial lamps are often used and they produce significant heat, with all the associated risks. In turn, any electrically-powered ventilation system that may be required creates an additional hazard if not installed properly.

Finally, all this set-up requires considerable energy and cabling. Most of this is poorly installed and subject to potential short circuits and resistive heating faults that can easily cause wiring, or any combustible materials around it, to ignite.

Reputable property owners can easily fall victim to a tenant or subtenant who decides to set up a cannabis factory, meaning it is crucial that landlords have a robust vetting system.

Tell-tale signs may be tenants who seek to pay rent in large sums of cash or are unable to supply proper references. Failing to conduct proper checks on prospective tenants can even mean a property owner is in breach of an insurance policy condition.

Fires are often only spotted by neighbours or passers-by, increasing the level of damage to the premises itself and the threat to adjacent buildings.

It is therefore vital that empty premises are properly secured and visited regularly, with careful internal inspections where possible, as external inspection alone may give no indication as to what is going on inside. Neighbours may see very little coming or going, so factories can remain undetected for months if not for longer.

The risks associated with illicit cannabis farms are clear, with fire being the most obvious hazard. It is a problem that shows no signs of abating. Those who own and manage property need to take appropriate steps to avoid becoming a victim of something that is a growing issue for both them and their insurers.

A version of this article was originally published on 13 June 2020.