Residential cannabis farming continues to boom and the insurer Aviva is warning landlords to be extra vigilant as it reports a 30% year-on-year increase in claims - the highest level since it started recording the data in 2007.

Nationally there were 643,510 cannabis plants seized in 2008/9 and this increased by 18% to 758,700 in 2009/102. 83% of seizures by the police last year were for 50 plants or fewer.

Matthew Gordon, underwriting manager - property owners - at Aviva said: “Despite the upgrade of cannabis to a class B drug two years ago the industry continues to grow. We settled 92 cannabis farming claims last year, which is the highest number Aviva has ever recorded.

“Almost all of the properties concerned were residential and we often find that it is part of a larger operation or that the policyholder has had a couple of properties affected. Cannabis farming comes with serious risks for landlords; properties can be completely ruined inside to make space for plants, water damage can occur and fire poses a risk due to interference with electrics or strong lighting left on for a long time.

“Property owners must be vigilant and there are some simple steps that can be taken. We would advise thorough checks on tenants and regular visits to properties - both internal and external inspections. Permanently closed curtains, blacked out windows and strong smells are all signs that there may be a cannabis factory on your premises.”

Warning signs:

Walls, ceilings and doors can be lined with plastic or polythene as plants are often grown in individual pots throughout the property
Windows will normally have blinds or curtains closed to obscure any activity.
Rather than having garden hoses plugged into a sink or basin, the plants are irrigated through pump spray guns, such as those used in a domestic garden.
High-powered lighting is installed and the electricity has probably been tampered with to bypass the meter.
A considerable amount of condensation is produced
A pungent smell, which may be noticed through the walls of adjoining properties, but ducting and extractor fans are often installed and fed through the chimney or flue to prevent this.