You would be forgiven for walking straight past one of the most expensive properties for sale in London. The restrained red-brick and stone facade of No 38 South Street doesn’t solicit attention, despite its 90ft-wide frontage on this quietly exclusive street between Park Lane and Berkeley Square. Yet the Grade II-listed five-storey mansion, also called Aberconway House, is on the market for £46m, making it the most expensive home for sale in the area, says Archie Riby-Williams, chief executive of Portman Heritage, the estate agency and developer that is marketing the property. It is also in the superleague of the capital’s most extravagant homes, alongside Toprak Mansion, in Hampstead, for sale for £50m.

Built at the southernmost edge of the Grosvenor Estate in 1920, 38 South Street was one of the last private houses of great size to be constructed in this area. It was designed for the industrialist Henry McLaren (later Lord Aberconway), and is generally credited to John Murray Easton, who later designed the Royal Horticultural Hall, in Westminster. Behind the sedate exterior, with its neo-Georgian elevations, tall sash windows and steep pantiled roof, are vast rooms (there is 23,000 sq ft of living space) and polished interiors partly designed by the Edwardian architect Harold Peto.

After McLaren and his family left in 1943, it became the headquarters of the Rank Organisation, the British film-production house. In the 1980s, Westminster council decreed a return to residential use, and in 1991 it was restored as a family home by the Grosvenor Estate.

Its size lends it to entertaining in spectacular style. The entrance hall – 65ft long, with a black marble chimneypiece, pilasters and a cantilevered staircase – oozes drama. The ground-floor dining room seats 30 easily, and the 60ft x 30ft ballroom on the first floor leads off an impressive gallery hall lit by five enormous chandeliers. “The sheer scale makes it unique,” says Sophie Jaggs, associate director at Portman Heritage. “It’s a country estate in the heart of Mayfair.”

The UAE-based owners (who won’t reveal their identity) bought it last summer, along with adjoining properties at Nos 40 and 54, which they have since sold; the three houses were once interlinked at lower-ground level. For two years before that, the house was let to an Arab prince for more than £1.5m a year by another publicity-shy Middle Easterner, who bought it when it was on the market in 2003 for £25m, a sum that included No 40.

“The price is about right,” says Jonathan Hewlett, head of London sales for Savills estate agency, which is unconnected with the sale. “Mayfair has some of the finest houses in London – Crewe House, on Curzon Street, the Saudi embassy, on Charles Street, 100 Park Lane [owned by the Emir of Qatar]. These houses are for people of rank, and there is a high demand for them to come back into residential use, due to expanding global wealth. The scale of the rooms for entertainment suggests that Aberconway House will be bought by someone Middle Eastern.”

So, what do you get for £46m? Thirteen bedrooms, eight reception rooms and a lot of furniture. There are reproduction chairs and side tables, copies of old masters, lots of cigarette cases – and acres of ruched, buttoned and tasselled curtains. From the first floor up, everywhere you tread, your feet sink into stridently patterned carpets. In the two vast master suites on the second floor, no expense has been spared. One is a vision in blue and yellow, with a four-poster bed and a marble ensuite bathroom with gold-railed steps leading to a sunken hot tub. You can imagine Barbara Cartland ensconced in the second, with its ruffle-tastic yellow and pink decor and gilt trimmings.

The warren of bedrooms on the third and fourth floors, along with the generic prints in the hallways, creates a hotel feel. The is echoed in the mansion’s bowels, where, in what was once the staff quarters, glitz is absent: a series of small catering facilities (kitchen is too grand a word) is reminiscent of Stanley Kubrick’s 1980 film The Shining. Creaky service lifts hardly improve matters.

“It may not be to everyone’s taste,” says Jaggs, “but everything is in good working order and the house is structurally perfect – all you need to do is put in a kitchen [there is no main one] and tart it up a bit.” Rewiring is also advised, and Portman Heritage, working with the London-based architects Weldon Walshe, has spent the past year dreaming up a new look. Plans approved by Westminster council and the Grosvenor Estate include the installation of a basement pool, with a loggia to flood the area with light, a massage, spa and gym area, a breakfast/family room and a members’ club-style lounge area and roof garden.

Plans also allow for a panic room, with walls able to withstand sustained gunfire. Riby-Williams estimates that the proposed work would cost about £10m, but that, once it has been completed, the house could sell for £75m.

There’s no off-street parking, however, and gardens at the rear are shared with other houses.

What you get is a serious party space – and a lifetime’s worth of ruched curtains.

38 South Street is for sale with Portman Heritage; 020 7487 4994, www.portmanheritage.com

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