You want to pull a trailer. When do additional rear-view mirrors have be to attached to your motor vehicle?

Additional rear-view mirrors are required whenever your normal vehicle mirrors no longer give you a sufficient view to the rear and along the sides because the trailer (or its load) blocks the line of sight. The key rule of thumb is: you must be able to reliably monitor what’s happening behind you (especially vehicles approaching from behind) and check before changing lanes, overtaking, merging, or turning. If the trailer body is wider than the car or is a “box” shape (many caravans, enclosed trailers), the interior mirror often becomes useless and even the outside mirrors may not show the full area next to and behind the trailer; then you need extension mirrors.

The size or type of trailer alone is not what triggers the requirement. A fixed width/height threshold (like in answer 2) is not the legal criterion here, because visibility depends on the towing vehicle, mirror design, and the actual trailer shape. Likewise, it’s not automatically required just because it’s a caravan or an agricultural/forestry trailer (answer 3). Those often do require extra mirrors in practice, but only if your rearward view is inadequate without them.

For similar questions, always choose the answer that focuses on “insufficient rearward visibility” rather than specific trailer categories or dimensions.