How is a property boundary actually determined?

Not by the title plan — which is the fact that resolves, or prevents, most boundary disputes. Registered title plans show only "general boundaries", and are explicitly not a statement of the exact line.

What the title plan actually is. A plan based on Ordnance Survey mapping, showing the general extent of the land. The red line has a real-world thickness of a metre or more at common scales, and the Land Registry states plainly that it does not determine the exact boundary.

So what does? In order of weight:

The original conveyance or transfer deed that first divided the land, and any plan attached to it. This is the primary evidence, and reading it — including its scale, measurements and verbal description — is where boundary questions are actually answered.

Physical features on the ground at the time of that division, which the deed may reference.

Long-established occupation, which can transfer ownership through adverse possession or establish the boundary by conduct.

A boundary agreement between neighbours, which can be recorded.

A determined boundary application, which produces a legally exact line recorded at the Land Registry — the definitive route, and one requiring a surveyor's plan and notice to the neighbour.

The presumptions that are not rules:

"T-marks" on a deed plan indicate responsibility for a boundary feature, where present — they are evidence, not automatic.

The "left-hand fence" convention has no legal basis. It is a widely repeated rule of thumb and is simply not a legal presumption.

The hedge and ditch presumption, which is real: where a hedge sits alongside a ditch, the boundary is presumed to be the far edge of the ditch, on the reasoning that a landowner digging a ditch throws the soil onto their own side.

What to do in a dispute, and the order matters enormously:

Get the deeds — yours and, if possible, the original split.

Talk to the neighbour before anything formal, since boundary disputes are notorious for costing many times the value of the land.

Consider a chartered land surveyor rather than a solicitor first.

Mediation, which courts expect.

Disclose the dispute when selling, which is required and which materially affects saleability.

General information, not legal advice.