In 1912, having inherited a fortune at the age of twenty-three, Philip Sassoon celebrated it by building himself a new home. The site – perched high above the vast expanse of Romney Marsh – was glorious. The house, a Dutch Colonial pastiche completed by Herbert Baker in 1914, was – if rather oversized – perfectly nice. Tall chimneys, curvy gables: nothing special.

In 1918, Sassoon decided it was time for a change. What England needed to boost her spirits after the war was an exemplary contemporary synthesis of luxury, style and culture. Belcaire – a name better suited to a rest home than a temple of the arts – was upgraded to Port Lympne. (It currently adjoins the Dinosaur Forest of a wildlife park.) A vast Italianate garden was to be carved out of the cliffs below, ascending to twin swimming pools (they were designed by that ubiquitous 1920s architect Philip Tilden), to float with spectral grace above the marsh. Within the house itself, José Maria Sert’s murals (featuring what an uncharacteristically prudish Herbert Asquith described as ‘elephants in different attitudes’) were joined by a dining-room frieze depicting creamy-hided bullocks and naked Egyptians – rumour has it that loincloths were added before a visit from prim Queen Mary – and the painted Tent Room, the playful creation of Rex Whistler, in which Sassoon’s agreeably eccentric friend Lord Berners was depicted as a small boy clutching a coroneted suitcase while boarding a paddle steamer. Tilden’s Moorish courtyard, often frequented by young airmen – Sassoon was a keen aviator – prompted unkind comparisons to a Spanish brothel. Nobody seems to have commented on the two hundred tons of elephant dung with which Sassoon fertilised an immense herbaceous border.

Adrian Tinniswood’s descriptions of the fantasy world Sassoon created on a Kentish cliff are almost indecently enjoyable. They are matched by his equally tongue-in-cheek depiction of Sassoon’s second country home, Trent Park. Mauve as a drunk dowager when Sassoon took it on, it was swiftly remantled in the rose-red bricks of William Kent’s demolished masterpiece, Devonshire House. And then came the gold: gold for the drainpipes, gold for the forked antlers of stags roaming through the deer park, gold – bright and plenty of it – for the shields, swords and helmets of the assorted lead statues that stood in for a police guard. Modesty was never Sir Philip’s forte.

The point made by Sassoon’s exuberant creations is central to Tinniswood’s splendidly contrary book. Traditionally, the interwar period is seen as marking the death of country-house life on a grand scale (this elegant past is now nostalgically preserved for the curious by the valiant endeavours of the National Trust). Tinniswood does not quite refute this, but he stresses that the demise of many a country house was hastened by an avaricious or hard-up owner’s eagerness to cash in by selling off the surrounding estate. One of his most poignant examples is Wollaton Hall, an Elizabethan prodigy house that today lies buried in Nottingham’s extended sprawl.

Still, there is another side to the story, and a less elegiac way in which to view the past. True, 180 big houses were demolished or left to rot in the first ten years after the Great War. Yet Tinniswood concedes, refreshingly, that many were as dreary as the mock Tudor mansions that swiftly replaced them. Yes, a number of devastated grand families, deprived by war of their male heirs, slowly relocated to more modest abodes. But a change of hands in itself seldom spelled death for an ancestral home: a prime example of survival is Leeds Castle: a ruin in 1927, a glory of the land in its present state. My own grandfather rescued and revived Chirk, the crumbling Welsh castle that he rented for almost forty years. His family, adoring it, regarded it as home.

This theme continues in Tinniswood’s affectionate accounts of successful rescue jobs, such as the Nicolsons’ brave purchase for £30,000 – a huge sum back in 1930 – of Sissinghurst, the ruined Kent tower whose surrounding fields they transformed into England’s best-loved garden. Tinniswood also notes the modest handful of country homes that were actually commissioned between the wars. The Earl of Jersey’s Middleton Park gets short shrift, praised only for an arrestingly mirrored marble and onyx bathroom installed by Robert Lutyens (Edwin’s son) to please Lord Jersey’s ‘Hollywood countess’, the actress Virginia Cherrill. High and Over, a marvellous modernist house still perched like a grounded glider atop a Buckinghamshire hill, is rightly granted more respect than the projected ‘Little Castle’ (thankfully, it was never built) over which the department-store magnate Harry Gordon Selfridge intended to reign. Beyond it, he planned to raise an incredible four-mile stretch of turreted walls.

Houses comprise only part of the story. It is when Tinniswood gets down to what went on in them that his combination of a sharp pen and a squirrel’s eye for detail really pays off. Staying at the future Edward VIII’s Disneylike Fort Belvedere – he later remarked that abdicating hurt less than quitting his beloved home – sounds plain dull: hacking down laurels was the owner’s favoured form of pre-breakfast entertainment, followed by a long, slow round on the Sunningdale golf course, dinner, and a spot of post-prandial needlework by the fire. Such visions of privileged leisure, capturing a time lost for good, are enlivened by Tinniswood’s posh golfing tales. My favourite is of a sloshed Irish peer entreating God’s help in a bunker: ‘And don’t send Jesus. This is no job for a boy.’

Life on the grand scale ground tediously along at Woburn, Welbeck and Chatsworth: Tinniswood lets his teeth show in his account of a sporting day on which the Duke of Devonshire, while aiming at a grounded pheasant, managed to shoot a chef, a guest and a dog (‘My, how everyone must have laughed’). Reports of the high jinks at Madresfield are offered in a gentler spirit: a wistful Lettice Lygon murmurs that perhaps all is ‘not as it should be’ with her flamboyantly bisexual husband, a guest of whom overhears him whispering ‘Je t’Adore!’ to a handsome footman. The astonished guest turns to a quick-witted Harold Nicolson to relate what he’s just heard. ‘Nonsense!’ exclaims Nicolson. ‘He said, “Shut the Door!”’

And then there is Cecil Beaton’s Ashcombe. If there’s one scene from Tinniswood’s irresistibly readable chronicle that I’d give my socks to have witnessed, it’s the all-night Ashcombe fête champêtre of 1937, with Beaton capering about in a jacket tastefully spattered with woolly vegetables and authentic egg whites (‘pretty as daffodils and much more unusual’) and his co-host, the squire of Vaynor, dressed up as Adonis. What the villagers, roped in to perform a rustic maypole dance, may have thought is anybody’s guess.

Tinniswood’s book is erudite, funny and oddly poignant. All I missed were his thoughts about the future. What hope is there for historic houses when today’s super-rich purchasers don’t want beauty and history, but simply a level of showy luxury – state-of-the-art pool, underground spa, panic room, screening room, gym – that even Philip Sassoon might have considered a touch excessive?