What is a highways structure?

Highway structures are defined as:

Bridges, road tunnels (or underpasses), culverts, underground chambers, pedestrian subways under or over the highway with a composite span of 1 metre or more are classified as highway structures.

Retaining walls, where the height of retained fill measured between lower ground level and upper ground level is 1.37 metres (4' 6") or more are also classified as highway structures.
Not all such structures however are the responsibility of the City Council to maintain, even though they may support the public highway. Many highway structures are owned by other transport undertakers such as Network Rail, London Underground and British Waterways. Bridges and structures on the strategic Transport for London Road Network (TLRN) are maintained by Transport for London (TfL).

Underground chambers such as vaults and cellars which are attached to privately owned premises and extend under the pavement and road carriageway are usually the responsibility of the owner/occupier to maintain in a good condition. Fore more information on vaults and cellars under the highway click on the following link.

Similarly most boundary walls separating privately owned premises and the public highway are usually the responsibility of the property owner to maintain in a good condition.