An editor once told me: always look at the loos. It was remarkable, she said, how many grand cultural projets, having spent a fortune on the atrium, the concert hall, the galleries, spent pennies on the bogs.

The smallest rooms at Charles Jencks’s Cosmic House are among the loveliest loos in London with windows on to the garden and a ‘Jencksiana’ mirror over the sink. This was the Baltimore-born writer, critic and landscape designer’s take on the ‘Serliana’ window devised by the mannerist architect Sebastiano Serlio and it recurs throughout this mad and marvellous post-modern house.

Jencks died in 2019 leaving his house on Lansdowne Walk in Holland Park as a stucco memorial. From the pavement, there isn’t much to distinguish No. 19 from its 19th-century neighbours. Inside, it’s another matter. Jencks intended his house as a ‘provocation’. It tweaks the nose and digs the ribs. It is absurd and amusing, solemn and scholarly. Charles’s daughter Lily Jencks describes it as ‘a riposte to people who were fighting against meaning in architecture’. Every cornice has its story. Edwin Heathcote, author and architecture critic of the Financial Times, was anointed by Jencks as ‘Keeper of Meaning’. He has written a delightful guide to the house with a cut-out Jencksiana motif on the cover. The Cosmic House opened to the public last week and the lockers are as good as the loos.

The Cosmic House was the first post-modern house in Britain to be listed Grade I and while it will not be to everybody’s taste (you don’t have to be barmy to live here but it helps) it is an inspiring vision and will, in time, become as cherished a place as the Sir John Soane Museum, Charleston or Kettle’s Yard. Between 1978 and 1985 Jencks remodelled the house with the help of the architect Terry Farrell. Jencks lived here with his wife Maggie, founder of the architecturally uplifting Maggie’s Centres for cancer care, and their children Lily and John.

In Jencks’s day you would have come through the front door (now through the basement) into an oval hall of mirrors supporting an egg-shaped dome. A frieze of philosophers and architects — Imhotep, Pythagoras, Abbot Suger, Erasmus — watches as you wipe your feet. There’s a ‘cosmic loo’ behind a hidden door with a dado rail of changing picture postcards. The ground floor is themed by season. Winter houses some of Jencks’s architectural library (more to come upstairs), Spring looks on to the garden through a picture window framed by Jencks’s ‘London columns’. The illuminated scallop capitals of the columns are intended to bring sun into London’s foggy streets. Summer is the dining room and Indian Summer the kitchen. Instead of conventional Greek Doric triglyphs above the kitchen cabinets, Jencks substituted a ‘spoonglygh’ motif of triple ornamental salad spoons. ‘If you can’t stand the kitsch,’ said Jencks, ‘get out of the kitchen.’ Some of it is marble, much is scumbled MDF. Autumn is the former breakfast room. There’s a sunken ‘sundial’ seating area hung with swagged Cinderella’s ball-dress curtains and an Egyptian room with scarab beetle doorknobs.

The spine of the house is a spiral Solar Stair with its 52 treacherous steps and 365 grooves cut into the risers. At the top is a glorious skylight; at the foot, a black hole mosaic by Eduardo Paolozzi. It is trippy in every sense of the word. With its stepped amphitheatres and Escher stairs that go nowhere, the house would suit the grand old Duke of York: you are always only halfway up and neither up nor down.

The Architectural Library takes its cues from Antonello da Messina’s ‘St Jerome in His Study’. The bookcases are Jerome-via-Jencks with cubby holes for books and papers. Jencks’s slides are housed in free-standing ‘slidescrapers’. The tented ceiling is almost a gothic fan-vault.

Maggie’s study is a simple, white space, now a gallery for changing displays. Otherwise tolerant of baroque bathrooms and ‘universal’ utility rooms, when it came to her study Maggie insisted: ‘Symbolism stops at my door.’ At the top of the Solar Stair is an oculus, which on sunny days lights the house from top to toe. Strange sculptures assembled from desert-rose crystals circle the skylight. (A play on ceiling rose?) The children’s rooms are very much their father’s rooms. Lily remembers never being allowed to pin up so much as a poster.

Then it’s back down to the basement, now an exhibition space, and along a corridor to the Borromini jacuzzi designed by Piers Gough. (‘It never worked,’ says Lily Jencks.) Gough rifled Jencks’s slide library for photos of ‘every dome in Rome’ and, alighting on Francesco Borromini’s dome for San Carlo alle Quattro Fontane, turned it upside down to make a sunken pool. ‘I put the borrow into Borromini’, says Gough in a video about the house.

The garden ends in a mirrored door inscribed ‘The Future’. If you turn back towards the house, you’ll see the ogee roof built over what used to be the garage. An imperial staircase takes you back up to the suite of four seasons rooms. On the day I visited, passion flowers wound around the balustrade. This house is a true passion project. If the Soane is a maudlin, melancholy place, this is the opposite: a happy, Mad Hatter’s home.