Approached up a sweeping driveway that runs through manicured parkland, Compton Bassett House, which sits in 70 acres in the foothills of the Marlborough Downs, in Wiltshire, looks every inch a grand country house. The honey-coloured exterior exudes confidence, tradition and history.

First impressions can be deceptive, however: gone are the days when a country house meant chintzy armchairs, animal trophies and gloomy family portraits. Behind its period facade, this eight-bedroom, 19,913 sq ft house is air-conditioned, has mood lighting in every room and underfloor heating that takes the chill off the marble and stone floors. On the lower ground floor is a leisure complex, with a pool, a hot tub, a sauna, a steam room – and a sophisticated air-recycling system, which the agents promise “will prevent the usual swimming-pool smells or mildew”. Nearby is a purpose-built hangar to house the helicopter.

The property, for sale for £8.5m, is one of a growing number of top-tier houses, from Cornwall to the Cotswolds, with the kind of features hitherto confined to the most expensive London penthouses. “The top end of the market has moved on so much,” says Crispin Holborow, head of farms and estates for Savills estate agency, which, along with its rivals Knight Frank, is selling Compton Bassett House. “We are building up a different set of values for houses that have been finished to a very high standard. There is a gap between your average country house and one in super condition. You know when you walk in somewhere and there is an indefinable something that just says ‘wow’. ”

It is a far cry from the days when the typical English country house was furnished with a jumble of moth-eaten inherited pieces and assorted floral homages to Laura Ashley. Agas are being replaced by commercial-quality stainless-steel numbers, roll-top baths by mosaic-encrusted wet rooms, billiards rooms converted into high-tech media rooms with cinema screens, and carpets and thread-bare rugs consigned to the staff quarters. The antlers that once adorned the walls, a sign of hunting prowess and aristocratic legitimacy are being varnished and remodelled into chandeliers. Holborow has even come across a boot room that is now used solely for cleaning the pets – well, you can’t have dirty paws mucking up the decor, can you?

Standerwick Court, a beautiful Queen Anne house in Wiltshire, 14 miles from Bath, is another example of what happens when you add modern features to a traditional country mansion. The 10-bedroom, Grade II*-listed property has been given a 21st-century makeover by Avia Willment, 46, owner of Universal Marina, near Southampton.

Willment, who competes in yacht races around the world, has spent four years and more than £1m restoring her home. She has installed new central heating and more than half a mile of water mains, and the property has been WiFi’d. It has marble bathrooms throughout, including what her children – Jonty, 13, and Vita, 10 – call their James Bond shower, which is covered in a gold-veined yellow marble with pressure jets and a double head.

“We wanted to keep it true to period detail, but have added modern comforts,” says Willment, who has retained the central staircase, thought to have been won in battle against an 18th-century Spanish ship by the property’s then owner, Admiral Edgell. “Everybody loves it,” she says. “They are all surprised by how immaculate and comfortable it is.”

Many more owners are bridging the gap between period status and 21st-century creature comforts. Symm, an upmarket builder that specialises in restoring historic properties for wealthy clients, including Russians, Arabs and royalty, has just completed the restoration and enlargement of what it describes as a “very substantial” 19th-century house outside Oxford. There are a few traditionally decorated rooms, which are all very Country Life, but the top floor is more like a penthouse, complete with cinema.

In another Grade II-listed Victorian mansion, Symm has installed a cantilevered stone staircase with glass balustrading, limestone floors throughout and a suede-panelled library. “Most projects require the latest technology, with rooms carefully integrated into the period fabric of the building,” says Aidan Mortimer, group chief executive. “A lot of money is being spent. Media rooms, wet rooms, underground garaging, swimming pools and spas are increasingly common. The appearance of the quintessential English country house is maintained, but all manner of surprises await inside.”

The designer Kate Bingham has installed a top-of-the-range bowling alley in Roman Abramovich’s £12m estate in West Sussex. It goes with the go-kart track, clay-pigeon shoot, trout lake and rifle range. Igor Falkovsky, a co-director of the Fulham-based architects Studio DAR, is also increasingly finding work outside the M25. One of his latest projects is a Grade II-listed house in Buckinghamshire: the Russian owners wanted the same sleek design in their Regency country house as in their homes in London and abroad.

“It was well maintained, all very traditional in style, with heavy curtains and fancy cornicing,” Falkovsky says. “We went for the complete opposite, ripped some walls out, changed the layout and remodelled six bedrooms to create four bedrooms, four bathrooms and four dressing rooms.”

The 18-month conversion included the construction of a luxury bungalow in the grounds, for guests who value their privacy – the staff are in the gate lodge – and reinforcing the floors to take the weight of the bath, which is carved out of a single block of marble. “It is a proper country mansion with views of rolling hills,” Falkovsky says. “It is only when you look out of the Georgian window that you realise you are not in London.”

The top-end developers who have made their names in the upper echelons of the capital’s property market are also moving to the countryside. Last summer, Mike Spink, who specialises in projects in Kensington and Chelsea, bought Grade II-listed Park Place, a decaying pile set in 570 acres on the river near Henley-on-Thames, in Oxford-shire, for a record-breaking £42m. He is spending three years and several million pounds restoring the property into a country house fit for one of the super-rich who have made a home in Britain.

“There’s a certain type of person who wants to recreate London in the country,” says Rupert Sweeting, head of country houses for Knight Frank. “We are beginning to see state-of-the-art kitchens, gas fires rather than log fires, walk-in wine cellars and lots more bathrooms. There is also a fascination with gyms in outbuildings.”

Yet there is a limit: wealthy owners may lavish hundreds, or even thousands, of pounds on doing up each square foot of their London property, but the sheer size of country houses means the overall quality of the gadgetry and the finish is unlikely to be so high. Redo too much of the interior and you may destroy the charm of the place.

“The more traditionally furnished country house still sells more easily,” Sweeting warns. “The market for such cutting-edge design is still new. A London-styled contemporary interior in a country house may give pleasure, but it does not necessarily translate into big profits.”

Compton Bassett House is for sale for £8.5m with Knight Frank (020 7629 8171, www.knightfrank.com) and Savills (020 7499 8644, www.savills.com). Standerwick Court is for sale for £4.5m with Savills (020 7409 8823)

How to do penthouse-pile chic

Out
Trophy animal heads
Wallpaper: floral sprigs, tiny hummingbirds and green trellis
Stables for horses
Billiards rooms
Dusty family portraits

In

Big, big elk antlers
Wallpaper: large patterns
Stables for swimming pools and spas
Media rooms
Underfloor heating
Warhol-style renderings of your pets

Times recommends

• Second homes: credit crunch casualty or money-spinner?
• How I built my own sliding house
• Top 10 incentives for house buyers