Grass isn't greener for unwitting landlords of cannabis farms, as Aviva report a 30% rise in claims over damages to properties used to grow the drug

Insurer Aviva has reported a 30% rise in claims made by landlords regarding cannabis farming – the highest number since the insurer began recording data in 2007.

Aviva received 92 cannabis claims in 2010 compared to 71 the previous year. The insurer wouldn't reveal the cost of the claims, but said the damage included holes knocked into ceilings and walls to support the lighting apparatus required to run a farm, as well as floorboards being ripped up.

Properties had also suffered water damage from hydroponic installations, and fires caused by faulty wiring or overheating. Furniture had been destroyed or thrown away to make more space for plants, and wiring had been ripped out and electricity meters tampered with to stop them accurately recording the amount of electricity used.

Cannabis farms have long been found on commercial sites such as warehouses and industrial estates – just last week police uncovered a cannabis farm in Birkenhead which could have produced around £3.6m of cannabis a year. But an increasing number of criminals are renting residential properties and turning them into illegal drug farms.

Nationally there were 643,510 cannabis plants seized in 2008-09, increasing by 18% to 758,700 in 2009-10. But 83% of seizures by the police last year were for 50 plants or fewer – further indication of the trend for criminals to carry out small-scale farming rather than industrial-scale operations.

Cheshire police recently issued a warning alerting locals to the increased number of farms being found in busy residential communities. It said cannabis farms had been found in large towns such as Warrington, Macclesfield and Northwich, as well as rural areas such as Tarvin – indicating the problem is not confined to major cities.

The cannabis farms are also a health risk: they require large quantities of fertilizers and pesticides to be stored and are often connected illegally to the electricity supply, posing fire risks.

Matthew Gordon, an underwriting manager at Aviva, said: "We settled 92 cannabis farming claims last year, which is the highest number Aviva has ever recorded. Almost all of the properties were residential, and we often find 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.

"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."

The police say there are obvious signs of a cannabis farm is in your neighbourhood: you might notice a pungent and sweet smell on the street with no obvious source; unusual levels of heat coming from a property; or no frost or snow apparent when every house around it is covered. You might also spot unusual noises at odd times such as fans going constantly, or large items like sacks or drums being dragged around. Windows will probably be blacked out by heavy curtains or bin bags from the inside. The creation of new air vents or the use of industrial air vents in a domestic property is also a tell-tale sign.

If your property is being used as a farm you may find that no one answers the door when you visit, and that the locks have been changed without you knowing. The police claim a rented property of a good size with several rooms and an absentee landlord are popular choices for cannabis farmers – and landlords should be wary of people who ask how often you will visit the property when renting their home. Also, cash may be used in transactions rather than direct debits. Landlords are advised to make regular checks of the property once it is leased.

Aviva insurance covers damage caused by cannabis farming only if a number of procedures are carried out, including obtaining written employers' references and verified records of a bank account for new tenants, as well as a log of property inspections.

Gordon added: "Employing a letting agent to manage the tenant-vetting process and provide an inspection service on the landlord's behalf is a good option as insurers could refuse a claim if a landlord has been found to neglect their responsibilities."