Category B: Vehicles up to 3.5 tonnes MAM and with up to eight passenger seats
Category B vehicles may be coupled with a trailer up to 750kgs MAM (allowing a combined weight up to 4.25 tonnes MAM) or a trailer over 750kgs MAM provided the MAM of the trailer does not exceed the unladen weight of the towing vehicle, and the combination does not exceed 3.5 tonnes MAM.
A vehicle with an unladen weight of 1.25 tonnes and a MAM of 2 tonnes coupled with a trailer with a MAM of 1.25 tonnes could be driven by the holder of a category B entitlement.
This is because the MAM of the combination does not exceed 3.5 tonnes and also the MAM of the trailer does not exceed the unladen weight of the drawing vehicle.
Whereas, the same vehicle with an unladen weight of 1.25 tonnes and a MAM of 2 tonnes when coupled with a trailer with a MAM of 1.5 tonnes would fall within category B+E.
This is because although the combined weight of the vehicle and trailer is within the 3.5 tonnes MAM limit, the MAM of the trailer is more than the unladen weight of the drawing vehicle.
Vehicle manufacturers normally recommend a maximum weight of trailer appropriate to their vehicle. Details can usually be found in the vehicle's handbook or obtained from car dealerships.
The size of the trailer recommended for an average family car with an unladen weight of around 1 tonne would be well within the new category B threshold.