There is something, well, almost fraudulent about the tranquillity of the Sir John Soane Museum, about the delight in knowing that this unfathomable live-in folly of a residence with its hidden passages and sliding panels actually belongs to us. There is something disingenuous about the glowing warmth of the place, put together as it was by an obsessive, self-absorbed martinet who left this structure and its extraordinary contents to the nation to spite his son, partly because he refused (or was not good enough) to follow his father’s profession and become an architect.

Poor George – ‘a gaunt, sad man, earning a precarious livelihood as a minor poet and playwright’ recalled George Sala at the end of the nineteenth century – was a victim of his father’s devotion to the next generation of architects. When the ‘Act of the Settling and Preserving of Sir John Soane’s Museum’, introduced by the radical MP John Hume, became law four years before Sir John’s death, his son protested that his father had been ‘improperly importuned and persuaded’ to take a step ‘which would leave himself and his family destitute’. William Cobbett presented a petition in the House of Commons, saying it was morally wrong ‘for a man to divert his estate from his family’, and Sir Robert Peel thought he should let the whole thing go to the British Museum to save the nation the burden of its upkeep.

But the Act of the Settling and Preserving of Sir John Soane’s Museum went through, George and his children did not starve, and we became the heirs to the most extraordinary, delightful and aesthetically confused private home in London, which is about to yield to us probably its last secrets.

It is not so much its physical obscurity in a terrace on the north side of Lincoln’s Inn Fields that makes Sir John Soane’s Museum ‘London’s best kept secret’ any longer – 120,000 find it each year – as the mysteries within, and the seven million-pound renovation which started in February and will take three years will reveal many more.

We know Soane through his home now, but in his day he was not only the pre-eminent architect of his day he was enormously influential, taking his role as professor of architecture at the Royal Academy with great punctilio. He continued to be an influence long after his death, and his own tomb at St. Pancras was the inspiration for Giles Gilbert Scott’s red telephone box of the 1920s.

Soane was also a fanatical collector: of architectural drawings for the benefit of his students but also the nick-nacks of art and curiosity that tickled his insatiably inquisitive fancy, with which he at first packed his home and around which he then had to rebuild the residence. Many of the objects were also acquired for the benefit of his students for whom he created a drawing studio (in such a way as to make it either visible or audible from any part of the establishment, forestalling mischief) and whom he expected to remain diligently at their drawing boards for at least twelve hours a day.

He had bought antique marble sculptures, Hogarth’s series The Rake’s Progress, Canaletto’s The Grand Canal, a thirteenth-century wooden boss from Westminster Abbey, a Roubiliac medallion portrait of Handel, Sir Robert Walpole’s desk, a Turner, a Reynolds and others paintings by friends and contemporaries, and the entire architectural library of his hero Robert Adam. Everywhere are the wildly extravagant architectural fantasy drawings Soane got Joseph Gandy to draw in their scores. In 1823, when the British Museum had unaccountably turned it down, he snapped up the Belzoni Sarcophagus, the coffin of the Egyptian King Seti I. He seems to have had little idea about what he had paid the enormous sum of two thousand pounds for, writing of it ‘This marvellous effort of human industry and perseverance is supposed to be at least three thousand years old’.

When he died in 1837 he left the three houses – by that time he had added Nos.13 and 14 Lincoln’s Inn Fields to No.12 – to us with the proviso of giving free access ‘to Amateurs and Students in Painting, Sculpture and Architecture’, and the trustees have kept everything that Soane collected, the director, Tim Knox, told me. ‘We’re cleaning, restoring and redisplaying hundreds of objects left to the nation, and we’ll recreate the rooms and spaces that Soane carefully designed’.

One of the first acts of the trustees after his death was to strip out the private apartments at the top of the house, including Soane’s bedroom and bathroom, his wife’s drawing room and their private oratory, to turn into offices. Today, working to paintings of the rooms Soane had commissioned in the 1820s, these rooms are all being restored to the way he knew them, the colours and fabrics all exhaustively researched. Added to that, Tim Knox is installing a new exhibition gallery and conservation studios, and a lift to allow full disabled access at last. The work will be completed in 2014, the bicentenary of the building of No.13.

Over the last hundred and sixty years successive curators have added elements and made mostly discreet alterations. At one point No.14 was sold and let. Seven years ago the museum was able to buy it back, then four years later a lease ran out so that it came under the director’s control. Now it gives the museum the space to return once again to Soane’s vision.

The restorers have been immeasurably helped by the chance discovery on the shelves of the British Library eight years ago of a book of a hundred and forty-four prints, drawings and watercolours bound into a quarto- sized book that also comprised a hundred and one pages of text devoted to the buildings closest to Soane’s heart, including the Lincoln’s Inn Fields buildings. Most of the illustrations had been unknown to scholars but give a unique insight into how Soane planned the buildings he loved – his Lincoln’s Inn Fields house, his country home, Pitshanger Manor (which Soane always spelt ‘Pitzhanger’) at Ealing, and Soane’s wife’s tomb.

Many of the illustrations were made using the then latest technology of lithography. It had taken the place of engraving in Soane’s office and may have been introduced to him by his last pupil and later assistant, Charles James Richardson, who was to become an eminent architectural historian himself. Richardson had been given the task of copying objects and drawings by Soane or Gandy, but everything he did and the items from which he worked were meticulously filed away by George Bailey, Soane’s senior assistant and later curator of the museum. Fittingly, it was an architecture student studying for a master’s degree who uncovered the slim volume while researching Pitshanger Manor. It appears to be a companion to another, larger, collection of drawings and commentaries about Soane’s public commissions which is now in the Victoria and Albert Museum and has long been familiar to scholars. The British Library version is a much more personal record. The house is full of invention and little whimsies. There is an obsessive attention to light quality which was admirably brought out in the 1999 Royal Academy exhibition devoted to Soane, co-curated by MaryAnne Stevens and Tim Knox’s predecessor Margaret Richardson, that they chose to entitle John Soane, Architect: Master of Space and Light. The light channels running from the roof which he used to illumine the interiors are being restored and a hitherto undiscovered basement tunnel has been found which will provide a new link between collections storage space and a new court.

In legend there had been a hermitage on the site and so in one of the tiny courtyards Soane built a folly, ‘Monk’s Yard’, its floor decorated with the bottoms and tops of port bottles the holy man was supposed to have comforted himself with, and an elaborate monument to the Soane family dog, Fanny. In the other courtyard he built a pasticcio, an extraordinary thirty-foot monument to ‘Architecture, ancient and modern’; but in a lack of attention to the site’s past that he would have found outrageous had he known, he built it over what had been a well. In 1896 it collapsed, with a third of it smashed beyond salvation. That has already been restored, and this time a toilet is being returned to the use Soane had given it: a small sculpture gallery. A room added in the later nineteenth century is to become a ‘cabinet of curiosities’ with some of the oddities Soane had amassed, such as the brooch Charles I wore at Naseby, Mrs Soane’s jewelled gloves, and a mummy’s head.

One of Soane’s greatest delights was his picture gallery, an ingenious small salon which is enlarged by screens folding out of the walls revealing more and more pictures, of varying quality. The Rake’s Progress, all twelve paintings, are concealed under one fold; on the opposite wall the screens fold out twice to reveal a recess which contains the model of the building Soane considered his masterpiece, the Bank of England, demolished in 1937. Pride of place goes to the Canaletto, and above it, barely visible,

is a small, undistinguished painting of A Hen Defending her Chickens. The artist is Sir Francis Bourgeois who commissioned Soane to build the Dulwich Picture Gallery and the genial Sir John would have acquired the work as a gesture to a friend rather than finding it a significant example of Georgian fine art. ‘Some of them are works of art, some of them are just curiosities, some of them are really rather dull,’ says Simon Jervis, chairman of the Soane and former director of the Fitzwilliam Museum. ‘But they’re all Soane.’