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As temperatures fall, rodents search for shelter, with houses across the country offering refuge.
But a £1 coin can stop rats invading homes this autumn.
What is the £1 coin test?
Pest control experts say an adult rat can squeeze through an opening measuring between 15mm and 20mm, roughly the diameter of a £1 coin.
If a coin fits inside an exterior gap, a rat can likely force its way through.
John Whiteley-Wilkson, managing director at Acorn Environment Services, is encouraging homeowners to carry out the check before autumn arrives.
Mr Whiteley-Wilkson said: "People are often amazed by just how small a gap a rat needs to gain access to a property.
"If a £1 coin fits into a hole around an external wall, air brick, pipe or door frame, it's worth taking a closer look and sealing it if appropriate."
Check around utility pipes
Specialists advise checking utility pipes, where expanding and contracting materials widen gaps over time.
Mr Whiteley-Wilkson said: "Utility pipes are one of the most common entry points we come across.
"These gaps are easy to overlook because they're often hidden behind bins, plants or outdoor storage."
Inspect air bricks and vents
Air bricks and vents also need inspection, as damaged bricks or missing mesh allow pests access into wall cavities and underfloor areas.
"Air bricks are essential for ventilation, but if they're damaged or missing protective mesh, they can become an easy route inside for rodents," he said.
Look beneath doors and garage entrances
Gaps beneath doors and garage entrances present another vulnerability, particularly where weather strips wear down or garage doors fit poorly.
Mr Whiteley-Wilkson said: "Garage doors and side gates are often forgotten about, but they're one of the first places we recommend checking when carrying out a home inspection."
Check sheds and outbuildings
Outbuildings including sheds, garages and extensions often give rodents shelter before they move into the main house.
He explained: "Outbuildings are frequently the starting point.
"Once rodents establish themselves there, it's much easier for them to move into the property if opportunities arise."
Remove outdoor food sources
Homeowners should also remove food sources outside, including fallen bird seed, overflowing bins, pet food and compost heaps.
Mr Whiteley-Wilkson said: "Preventing access is only half the battle.
"If there's a reliable food source outside your home, rats will continue returning and eventually look for a way inside."
He added: "Most rat problems don't begin because homeowners have done something wrong.
"Rodents are simply opportunistic and will always look for the easiest route indoors.
"Spending ten minutes carrying out the £1 coin test could help identify vulnerable areas before they become a much bigger problem."