Brown Hart Gardens, off Duke Street, with the café pavilion which would be reworked if plans for the garden space, illustrated below, get the go-ahead

IT is known as a backstreet oasis, a secret gem for anybody willing to stray south from Oxford Street.

But the future of Brown Hart Gardens – the raised garden deck sitting on top of an electricity substation in Mayfair – could be the subject of hectic debate next week, with councillors due to decide whether a new restaurant can be built on the site.

Neighbours have warned councillors that more outdoor seats and longer opening hours may lead to extra disturbances for residents in the square. Their objections say noise from an existing café is already intrusive.

The Grosvenor Estate revamped the gardens and opened them up to the public for the first time in decades back in 2007 and visitors are always quick to take photos of the ornate setting and enjoy the hidden garden feel.

The next step in its plans is to partially demolish the existing café pavilion, create the new restaurant, and relandscape the public deck. An increase in the operational hours has been described as “modest” but this has not stemmed concerns held by some of its neighbours.

“This garden space is littered with frequent antisocial behaviour,” said one of the objections sent to the council’s planning department. “The noise even of people talking can be heard in our flats opposite. This is meant to be a community space and should be kept as such, with strict hours and mindful sound, anti-social behaviour monitoring for neighbours. The gates are too low and are frequently jumped over by people wanting to gain access late at night.”

They added: “The garden itself is a nice public space with plants and a calm atmosphere for the area. I do not want a restaurant on my doorstep at all.”

Another opponent to the changes said: “By occupying the space with 10 tables and 30 chairs it will be encroaching on the available space used by the public and local residents who may use the space for some quiet time away from the hustle and bustle of West End living. This is a dawn till dusk access space and should not be extended beyond these hours. The space creates a vortex for sound which resonates affecting those living close by.”

Meanwhile the Victorian Society, a conservation group protecting historic features, has raised concerns, warning that the new structures would clash with the listed site.

“There is ample opportunity to landscape the ­terrace, but it must be inspired by the character of the historic building,” its commentary for planners said.

Westminster city councillors are due to hear the application at a meeting on Tuesday with officials recommending they grant consent for the work.

Brown Hart Gardens can be found off Duke Street where the substation was built in 1902. It replaced gardens which had been used for working-class residents then living in central London.

The Duke of Westminster created the ornamental deck as compensation for the loss of this space. It was locked up in the 1980s by previous owners, the London Electric Board, before being brought back into use by Grosvenor.

Architects Studio Ageli said in an application for Grosvenor’s plans that the space was “hidden and under-utilised”, and added: “This includes the existing café building, which has had no fewer than seven operators in the last 10 years and is currently vacant. This fails to make the best active use of an open space where the original ‘garden’ atmosphere has been somewhat lost over time but could be reimagined in the heart of the West End near Oxford Street.”

It added: “The applicant’s vision is to adapt the existing building on the deck to provide a more attractive restaurant of similar size but with an all-day offering, which can be sustained into the future. Alongside this, a new landscaping scheme… would reinstate the historic garden feel to the rest of the deck, making it a far more attractive place to dwell, improving the city’s green infrastructure and biodiversity while also preserving its primarily open character.”

The application said that “feedback on the proposal was broadly positive, with most responses supporting the vision for the site including the concept of a restaurant use”, adding: “Concerns regarding the potential impact on residential amenity have been taken into account in physical aspects of the scheme design, as well as operational aspects, for example by reducing the closing time which was initially proposed for the restaurant to prevent unacceptable disturbance.”