Brown Hart Gardens in Mayfair London is not well-known but once you find it you will cherish it's tranquility. Only a short walk from Selfridges and Oxford Street, Brown Hart Gardens is a raised paved area used by local people, workers and visitors to Mayfair during the day for a quiet lunchtime sandwich or to read in peace.
Brown Hart Gardens is a 10,000 square feet (929 m2) public garden and a Grade II listed structure, managed and maintained by Grosvenor Estates. After being closed to the public for 20 years, Grosvenor Estates completed extensive renovations and reopened Brown Hart Gardens to the public in Autumn 2007.
Information from Grosvenor Estates:

© Laura Porter, licensed to About.com, Inc.Brown Hart Gardens are an amalgam of the former Brown Street on the south side and Hart Street to the north, originally two 18th century streets of small houses. When the houses were pulled down to make way for Chesham and Balderton Buildings in 1886-88, a communal garden was laid down the centre for the benefit of the residents of the flats. This street-level garden was dug up in 1903 in order to construct the sunk electrical power sub-station for the Westminster Electric Supply Corporation that has remained in situ to the present day. Thanks to the prevalence of good taste, the sight to the public was not forgotten and the opportunity was taken to transform visually an industrial artefact into an architectural feature. To the designs of T Stanley Peach, and with George Trollope & Sons as contractors, around the whirring machinery rose an imaginative composition in Portland stone with a raised, paved Italian garden, providing seating along both sides, and enhanced by a domed neo-Baroque pavilion at either end. The raised terrace is for the use of the general public and, in summer, is a pleasant spot in which to while away an hour with one's sandwich and a good book. Not quite a folly, it is a delightful extravagance of classicism, set as it is among a predominance of 19th century red brick, still attracting the admiring interest, rather than otherwise, of the passer-by.
Duke Street Information from Grosvenor Estates:
Duke Street was so named, from its first building in 1724/5, as one of the principal thoroughfares of the new Grosvenor Estate. The street was built to link Oxford Street with the east end of Grosvenor Square by agreements with John Simmons and Augustus Woollaston. By 1735 it had developed as comprising chiefly tradespeople with a few gentry among the tenants. By the end of the 18th century there were three public houses, a wide variety of shops, and a lying-in hospital had already been and gone in 1768.
The street was redeveloped between 1886 when Duke Street Mansions was built, and 1903-5 when the classical styled electricity sub-station was erected in Brown Hart Gardens.
Tradition has been maintained with a preponderance of shops with flats above, together with the addition of a modern hotel and the King's Weighhouse Church of 1889-91 providing a variation in architecture.
How to Find Brown Hart Gardens

© Laura Porter, licensed to About.com, Inc.Nearest Tube Station: Bond Street
Use Journey Planner to plan your route by public transport.
See Streetmap location map.
Entrances: Balderton Street and Duke Street, London W1.
Duke Street entrance is opposite the Ukrainian Church.
Access Note: There are steep steps. Disabled access call: 020 7312 0011.
Opening Times:
May-Sept: 11am-6pm
Oct-April: 11am-3pm
Note: No dogs allowed.
Official Website: www.brownhartgardens.co.uk
Queen Victoria's Elephant House?
Rumors used to abound that the building on Duke Street at the entrance to the courtyard garden was Queen Victoria's Elephant House. Sadly, this is a fun idea but there's no truth in the rumor as the building was built as an electricity sub station and it still has that purpose today.
Before being redesigned in 1903, the site had a communal garden with trees, benches and a fountain but had become a hang out for 'undesirables.' The new structure therefore continued to provide residents with a communal garden while accommodating transformers below.
According to British History Online "the 'garden' is perhaps the only place in London where quarrelling is specifically forbidden by law."