There’s a small cushion on Isabel Goldsmith’s sofa that says: “To be rich is no longer a sin, it’s a miracle.” Isabel is familiar with fabulous wealth. She’s the eldest child of the late Sir James Goldsmith, billionaire and global capitalist, and the great-granddaughter of Bolivia’s “tin king”, Simon Patino. Her mother and namesake eloped to Scotland with Goldsmith when she was just 18 and he was 21. Four months later, she died of a cerebral haemorrhage, hours after her daughter’s birth.

Isabel inherited most of the £75m Patino fortune, and a more modest slice of her father’s £1.6 billion estate. She once featured regularly in The Sunday Times Rich List. Its compiler, Philip Beresford, now estimates her wealth at £30m. So when she turned her Victorian home in South Kensington into a showcase for her pre-Raphaelite art and antiquarian book collections, no expense was spared.

Now it is for sale as Isabel, who also owns an oceanfront estate in Mexico, has itchy feet. “I will keep a flat in London and I’m hoping to get a country house, but in a warm country,” she says. “I want to travel. I have never been to Bolivia.” There are pressing reasons to head to more sultry climes: “When I’m cold I eat too much chocolate.”

The asking price of £8.45m is a far cry from what she paid for it. “I bought the house in 1985 for £625,000, when the area wasn’t as prestigious as it is now. The house belonged to Anthony Shaffer, the playwright. It was very 1960s, very white, with white shag-pile carpet. I had set myself on it, but found myself in a bidding war. I had been to see the house four times, and each time someone was asleep in one of the bedrooms.

Eventually, I went round at eight o’clock one night, found someone awake and begged him to pass on my offer to the owner.”

Isabel got her house, and called in the late Geoffrey Bennison, a decorator whose signature style was boldly luxurious, slightly faded aristocratic. The effect is of a grand country mansion decanted into a London stuccoed semi. “I had one (pre-Raphaelite) painting at the time, and went to see Geoffrey and told him I wanted to do a pre-Raphaelite house. He didn’t do many private houses. I thought he was wonderful. Decorators today are all business: they prefer to do their own design and take the cheque.”

One of the first things to hit the skip was the white shag-pile; the white paint also vanished. Two single doors had led off the ground-floor tessellated hallway into two raised ground-floor reception rooms. Isabel knocked through, blocked off one door and widened the other into double doors, which open into a double-length drawing room, linked by a deep, square arch and flanked by bookcases, stencilled to look like marquetry. An antique fireplace didn’t last. “Everyone who walked by could see in. Of course, it got stolen, ripped off the wall. Geoffrey said he could copy it but he was missing 20 of the 17th- and 18th-century Delft tiles for the inlaid surround.”

Goldsmith scoured antique markets and auction houses for replacements. “It took six months to find them, two here, three there. It took me a year to find the amber shades for the bookcases. I’m an acquisitor, guilty as charged,” she says with a smile. “My maternal grandfather was a collector. That’s where the gene comes from. I am a frustrated artist.”

A small conservatory overlooking the garden has become a domed studio for her art collection; steps from there lead to a gothic folly, added six years ago.

“I had the idea I would sit in there and write letters, but of course I don’t. So many people just send e-mails now,” she says, rather wistfully. At 52, she has a petite grace and her father’s smile, and the resemblance between her and her half-sister Jemima is pronounced.

A scrap of velvet curtain found by Isabel inspired the cut-velvet fabric lining the drawing room; single-panel Victorian net curtains became inner blinds, edged with spindle-shaped trim: “So many people have dropped a note through the door, asking where you can get them.”

Much here is bespoke. Another society decorator, Gabhan O’Keefe, trawled the William Morris archives to find the designs for the studio’s pink and green wallpaper. The sofas in the drawing room are covered in a specially commissioned tapestry. The rugs, some Moroccan, are fashionably faded and almost threadbare. With the velvet-covered, carved Venetian chairs Isabel inherited from her grandfather, the effect is very old money. A lot of it. In all, she estimates she has spent £2.5m on the house.

The garden-level dining room was extended by O’Keefe, who installed gothic glass doors to let in more light. The wall panels were inspired by 19th-century furniture and the plain gold wallpaper is stencilled with a trellis design.

The homely kitchen, though, is Isabel’s creation. “I already had all the (splashback) tiles. I had collected them from all over, including Portobello Road. The tiler was a saint. We had them all laid out on the floor to decide the pattern, and I got the design for the (wooden) floor from a Victorian drawing I’d seen.”

O’Keefe suggested the hall’s Pugin wallpaper (former Lord Chancellor Derry Irvine spent £300 per roll of taxpayer’s money on his); the duck-egg blue carpet was woven to match it.

On the first floor, a front-facing master bedroom became a large bathroom with marble-topped bath and two dressing rooms. What is now the main bedroom overlooks the garden. Its four-poster bed’s canopy is in Bennison’s trademark soft, antique rose shades. The second and third floors have three more bedrooms and bathrooms. It’s a large house, for a woman who has seven half-brothers and sisters, though no children of her own. Isabel’s 1973 marriage to the French adventurer Baron Arnaud de Rosnay ended in divorce after two and a half years.

The house has been such an enormous project that Isabel has found it hard to let go. She worries a buyer will not recognise the importance of Bennison’s work and paint the place white again. For in some ways it’s a shrine — though not to the art movement that inspired it. As Isabel puts it: “This house is in memory of Geoffrey Bennison.”

Isabel Goldsmith’s house is for sale for £8.45m with Aylesford, 020 7351 2383, www.aylesford.co.uk

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