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Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom, covered by around 375,000 listings.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites, and in more recent times a road crossing (Abbey Road) and graffiti art (Banksy 'Spy-booth') have been included.

In England and Wales there are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures (at the discretion of local authorities) using A, B and C designations.

In Scotland three classifications are also used but the criteria are different. There are around 47,500 Listed buildings.
Category A (8%)- generally equivalent to Grade I and II* in England and Wales
Category B (51%)- this appears generally to cover the ground of Grade II, recognising national importance.
Category C (41%)- buildings of local importance, probably with some overlap with English Grade II.

In Northern Ireland the criteria are similar to Scotland, but the classifications are:
Grade A (2.3%)
Grade B+ (4.7%)
Grade B (93%)

Read more at Wikipedia Link

Designed in 1839, The Royal Crescent is one of the most architecturally interesting Nineteenth Century developments in Holland Park. Evidently inspired by its older namesake in Bath, it differs from the Bath crescent in that it is not strictly a true crescent but rather two quadrant terraces each terminated by a circular bow in the Regency style. The plan of the Royal Crescent was the design of the planner Robert Cantwell, and it was the need for the newly fashionable underground sewers which caused the crescent to be designed in two halves rather than any consideration for architectural aesthetics. On early maps it is marked as Norland Crescent.
The stucco fronted crescent is painted white, in the style of the many Nash terraces which can be seen elsewhere in London's smarter residential areas. Today many of these four storey houses have been converted to apartments, although a few remain as private houses. The Royal Crescent is listed Grade II* Link