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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

Downing Street is a street in central Cambridge. It runs between Pembroke Street and Tennis Court Road at the western end and a T-junction with St Andrew's Street at the eastern end. Corn Exchange Street and St Tibbs Row lead off to the north. Downing Place leads off to the south.
To the north at the western end is the New Museums Site of the University of Cambridge. To the south is the Downing Site of the University. Many of the University's departments are located here.
Wiipedia: Link

The New Museums Site is a major site of the University of Cambridge, located in the centre of the city, on Pembroke Street and Free School Lane, sandwiched between Corpus Christi College, Pembroke College and Lion Yard. The smaller and older of two university city-centre science sites (the other is the Downing Site), the New Museums Site houses many of the university's science departments, lecture halls and examination rooms, as well as two museums.
Formerly the site of the university Botanic Garden (which is now between Hills Road and Trumpington Road in the south of the city), the New Museums Site is an eclectic mixture of grand Victorian buildings erected between 1870 and 1909, such as the Old Cavendish Laboratory; yellow-brick buildings from the 1930–40s, largely utilitarian with the exception of the Mond Building; and modernist glass-and-concrete buildings dating from the 1970s, such as the Materials Science and Metallurgy tower.