In short
There is no single legal height, but the council can step in. If a hedge is two or more mostly evergreen or semi-evergreen trees or shrubs, over two metres tall and affecting your enjoyment of your home or garden, you can complain to the council once you have tried to settle it informally. A fee may apply.
This is a job for our Hedge Trimming service.
When the council can help
The government guidance says you can ask your council for a high hedge complaint form if the hedge is all of these: two or more mostly evergreen or semi-evergreen trees or shrubs, over two metres tall, and affecting your enjoyment of your home or garden because it is too tall.
Try to settle it first
You must try to resolve it informally before the council will intervene, and you might have to pay the council a fee to consider the complaint.
Your side of the hedge
You can trim branches that cross onto your property, up to the boundary.
Bringing a hedge back down
If you own the hedge, or your neighbour agrees, a height reduction is usually the answer. Some conifers do not regrow from bare wood, so a hard cut on those needs care. See hedge trimming.