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London's hottest new restaurant wonder is graced by decorous Georgian stone facades with elegant Ionic stone pilasters and large sash windows. Walk through the front door, though, and you feel like Alice stepping through the looking glass.

You are entering a topsy-turvy world of designer mayhem. Out of alcoves emerge the ornate outlines of Louis XVI chairs that have been shrink-wrapped to the wall in taut grey fabric. A few steps along, a stone staircase covered by a cascade of liquid chocolate and caramel, which has oozed down the steps to leave a congealed pool of two-tone brown gloop at its foot. On checking your shoes, it is a relief to discover the flooring is actually hardened resin. The deeper you penetrate the maze of large rooms and halls on the ground and first floors, the more outlandish it gets – the toilets most of all.

Welcome to Sketch, just off Regent Street – London's most outrageous and expensive restaurant for the celeb set, including the Beckhams, Stella McCartney, Rod Stewart and Hugh Grant, where dinner for two could easily knock you back £500. More than a gourmet restaurant, the complex includes a patisserie, brasserie, bar and art-video gallery.

This is the brainchild of millionaire Algerian restaurateur, Mourad Mazouz, who brought together as many as 80 mostly French designers. The thankless task of executive architect was given to the Manser Practice, which describes its role as "like being a circus ringmaster – loosely in control of clowns, acrobats and dangerous animals".

Eating at such expense is pure indulgence and the decor is decadent-chic to match. The building conversion cost some £4.5m, and the total including designer furniture is rumoured at between £10m and £15m.

Mazouz's overall theme is a crazy mix of old and new, with the style switching wildly from one room to the next. The old is inescapable, as the building is grade II listed. Originally built in 1779 to architect James Wyatt's design, it had a large dome-lit gallery and library added a century later when it served as the RIBA's headquarters. As for the new, instead of being complementary or deferential to the old, it is in a very deliberate, ultra-kitsch style that out-Starcks Philippe.

The conversion starts calmly enough in the ground-floor parlour, now the patisserie. Wyatt's exquisite decorative plasterwork on walls and ceiling is an apt backdrop for the dainty cakes on offer. But why are the classical crystal chandeliers and standard lamps sheathed in see-through acrylic cylinders? Because when the lights are switched off, only the modern reflective cylinders designed by Jurgen Bey of Rotterdam are visible.

The main restaurant is anything but calm. Crimsons, purples, golds and browns in richly textured fabrics and spiky sunburst patterns shriek from walls, ceilings and floors, while Wyatt's two demure fireplace alcoves are filled by man-size Aladdin-style urns in patterned brown faïence. The whole scene is Arabian baroque pumped up to Las Vegas garishness, and is enough to bring on indigestion in any diner.

In the West Bar, at the rear of the ground floor, 1960s modernism dominates, with furniture in white plastic, black velvet and stainless steel. Here a white resin floor surface sweeps in bold unbroken curves up the bar and round and over the counter. The walls start white at floor level but gradually meld into dark crimson towards the ceiling. Laser light beams swoop around the room, bounced of revolving acrylic discs.

And now for the toilets. The first pair of toilets, just beyond the entrance hall, are in an over-the-top romantic kitsch that would have Lawrence Llewelyn-Bowen sobbing into his purple pillow. The interiors are barely lit and lined in planks of bevel-edged antique mirror glass. Stylised flower motifs have been etched into the reflective lining of the glass, and these are luridly back-lit in blue for men and crimson for women. Additional and quite superfluous decoration comes in the form of clusters of tiny clear and red Swarovski crystals encrusted on the mirror glazing.

The other toilets are from a different planet. In a large barrel-vaulted hall lie two nests of 12 colossal Alien eggs – all oval, white, shiny and downright spooky. They are free-standing and totally enclosed toilet pods, with the female nest at one end of the hall and male at the other end. Each pod was cast in two glass-fibre segments by Isle of White boat-builder Vision Yachts, which then glued them together, filled the joints and finished them off smoothly in gel and opalescent paint.

Squeezed below these toilets lies yet another bar, a womb-shaped, sci-fi pod related to the Alien eggs, although this time constructed of bent steel poles, expanded metal lath and polished render.

The rebirth of this building as an exclusive restaurant will no doubt continue to draw in those celebrity punters. But it's the plastic pods that really make it a winner.

They are witty, technically advanced, self-contained worlds of their own. And, after indulging in the visual blow-out of Sketch, they remain about the only parts not to leave a cloying aftertaste.