Whether or not your trailer is attached to your insured vehicle, it will be automatically covered for civil liability. The sole condition is that it must usually be attached to your vehicle.
If you only need coverage for damage that you cause to others, you don’t even need to add it to your insurance policy.
If you wish to insure your trailer against collision or theft, you must add your trailer to your insurance policy. The types of coverage offered are the same as those offered for your car, i.e.: all perils, collision and hit-and-run accidents, specified perils and all perils other than collision.