Towing a trailer isn’t something you can wing. Between the right licence, the GTW limits, mandatory braking requirements and the documents you need on board, the rules are detailed and mistakes are common. This guide covers everything the Highway Code requires to tow a trailer legally and safely.
GTW, MAM and the Highway Code: the figures to know before you set off
Two key figures come up whenever towing regulations are discussed: MAM and GTW.
MAM (Maximum Authorised Mass) applies to the trailer on its own. It is the maximum mass the trailer can reach when loaded, as shown on its V5C registration document. GTW (Gross Train Weight) covers the entire combination — towing vehicle and trailer together. This is the figure that determines which licence category is required, and which must never be exceeded under any circumstances. Exceeding it can result in a fine and immediate prohibition of the vehicle.
The GTW is shown on the towing vehicle’s V5C. If it isn’t listed, it defaults to the combined MAM of the vehicle and trailer. This is the figure to establish before any journey and before loading. For everything relating to the specific rules that apply at the 750 kg threshold, the article on 750 kg GTW trailer regulations covers them in full.
Category B, B96 or B+E: which licence to tow a trailer?
The licence required to tow a trailer depends directly on the GTW of the combination.
| Combination GTW | Licence required | Additional condition |
|---|---|---|
| ≤ 3,500 kg | Category B | Trailer MAM ≤ 750 kg |
| 3,500 to 4,250 kg | B96 | Trailer MAM ≤ towing vehicle unladen weight |
| 4,250 to 7,000 kg | B+E | Full training course + DVSA exam |
Category B covers the vast majority of everyday towing. Once the GTW exceeds 3,500 kg, B96 is required — obtained after a minimum 7-hour practical training course, with no exam. Above 4,250 kg GTW, towing a trailer requires a B+E licence, involving a full training programme and a practical test with a DVSA examiner. The full breakdown of both extensions is covered in the article on category B96 vs B+E licence.
Trailer brakes: when are they mandatory?
Braking requirements are one of the most commonly misunderstood areas of towing law. A brake system is not always mandatory, but the conditions for exemption are precise.
A trailer can run without its own braking system only if its MAM does not exceed 750 kg and its laden weight does not exceed half the kerbweight of the towing vehicle. If either condition is not met, trailer brakes become mandatory. Above 750 kg MAM, a braking system is required in all cases, regardless of the towing vehicle’s weight.
For light trailers, the overrun brake is the standard solution: it engages the trailer’s brakes automatically when the towing vehicle decelerates. A parking brake is also required on any braked trailer. The article on braked vs unbraked trailers covers these rules in full.
GTW, load distribution and combination stability
Staying within the GTW is not enough on its own. How the load is distributed across the trailer has a direct impact on combination stability, braking behaviour and handling.
A load concentrated too far behind the axle creates a lever effect that lifts the front of the combination and generates a snaking motion that becomes dangerous at speed. Conversely, too much weight pushed forward onto the coupling head overloads the towing vehicle’s transmission and rear suspension. The practical rule: the load’s centre of gravity should sit slightly ahead of the axle, with a minimum of 25 to 50 kg of noseweight on the coupling head.
Lateral distribution deserves equal attention. An unevenly loaded trailer creates an imbalance that makes itself felt under braking and in bends.
Visibility, lighting and mandatory documents
Towing a trailer also comes with specific requirements around visibility and paperwork.
On the lighting side, the trailer must be fitted with rear position lights, brake lights, indicators and approved reflectors. A rear fog light is required above a certain MAM threshold. If the trailer is wider than the towing vehicle, an additional mirror on the offside is mandatory to ensure adequate lateral visibility.
In terms of documents, here is what the Highway Code requires:
- A V5C registration document for any trailer with a MAM above 750 kg
- A number plate matching the towing vehicle for trailers under 750 kg MAM, or a dedicated plate above that threshold
- A tare plate showing the MAM and the load per axle
- A manufacturer’s plate with the name of the trailer builder
- A warning triangle on board in case of breakdown
In summary
Towing a trailer legally comes down to three figures: the trailer’s MAM, the combination’s GTW and the towing vehicle’s kerbweight. These three values determine the licence required, the braking obligations and the limits that must not be exceeded. Everything else — lighting, documents, load distribution — follows from there.