A trailer is the item most often assumed to be insured and least often actually insured. Motor policies commonly extend liability while it is attached to the towing vehicle, and stop there, which leaves the trailer itself and every hour it spends detached.
Attached, detached and what each covers
A commercial motor policy usually covers third party liability arising from a trailer while it is coupled to the insured vehicle. It does not normally cover damage to or theft of the trailer, and cover for liability while detached is a separate question that some policies address and many do not. The two gaps that matter are therefore the trailer as property, at any time, and liability while it stands somewhere on its own.
Theft, and how easily one goes
Plant, box, tipper and catering trailers are stolen constantly because they can be hitched and moved in under a minute. Insurers respond with conditions: hitch locks, wheel clamps, a posted or immobilised position, storage behind a locked gate, and increasingly a tracker on higher value units. A trailer left on a driveway or a verge overnight is the situation those conditions exist for, and the answers given at inception are what a claim is measured against.
The load and the licence
What is on or in the trailer is a separate cover again: tools, plant, stock or goods need their own section, and goods belonging to somebody else need goods in transit. On the driving side, the combined weight of vehicle and trailer decides which licence category is needed and which speed limits apply, and a business that upgraded to a heavier trailer without checking has a driver problem as well as an insurance one.
Hired trailers and trailers you lend out
A hired trailer is the hire company's property and the hirer's contractual responsibility, which is a hired in plant question rather than a motor one. Lending a trailer to another business, or hiring yours out, puts your property in somebody else's custody and usually needs the insurer's agreement. Both arrangements are common in the trades and both sit outside a standard motor and tools policy unless somebody has asked.
Questions people ask about trailer insurance
Is my trailer covered by the van's insurance?
Usually only for third party liability while it is attached. Damage to and theft of the trailer, and liability while detached, are separate questions.
What security do insurers require?
Hitch locks, wheel clamps, storage behind a locked gate or an immobilised position, and often a tracker on higher value trailers.
Is a hired trailer covered?
Not by motor cover. The hire agreement makes you responsible for it, which is a hired in plant exposure with its own section.