In one of the most memorable scenes in the Beatles film Help!, the Fab Four draw up in a car outside a row of terraced houses. Each unlocks his own front door, but this is a ruse to show that they are still down-to-earth boys from Liverpool. For, as the next scene reveals, the houses have been made into a single luxurious bachelor pad. The craze for knocking through was born.

In subsequent decades, the popularity of turning two — or more — houses into an extra-large home may have waned in favour of the sub-division trend. Developers were keen to maximise their profits by creating eight flats from one family house. But now bigger is, once more, officially better, which means that partition walls are out and sledgehammers are in.

Jamie Oliver, the chef who is worth an estimated £40 million, is so enamoured with his home in Primrose Hill, North London, that he has bought the house next door and is knocking the two together to create a £7.5 million superhome. House number two will be painted the same canary yellow as the original, the front balconies will be realigned so that they are on the same level and an adjoining wall will be knocked down to create one huge reception room. The only telltale sign that the house was once two will be the two front doors.

Agents say that knocking through has gained popularity as homeowners who want more space become increasingly wary of moving. Some do not want to uproot their families; others fear annoying new neighbours. Louis Winterbourne, of Savills, has seen “a handful” of such cases in the past year. “There is an ‘improve, don’t move’ mentality that has become much more common over the past 18 months,” he says.

Taking on such a project may seem daunting, but it can cost as little as £30,000 to turn two properties into one if the house next door is a mirror image of your own. Clever planning can get the job done in a couple of months, leaving the most intrusive bit — the knocking down of the central wall — until last. Knocking through will be much more expensive (at least £100,000) and less worthwhile, however, if the two houses are completely different, Winterbourne says, because staircases have to be moved and floors realigned. “About half of such projects look disjointed. Whether they feel natural or not is often down to the original structure rather than any fault with the restructuring design.”

Old Palace Place, a Grade II* listed mansion overlooking Richmond Green, southwest London, and on the market for £7 million with Knight Frank, has an advantage for a buyer hoping to do a conversion of this kind. The house, which has six bedrooms and a 124ft garden, was once joined to the house next door, which, by coincidence, is also on the market, for £2.5 million via Savills. The two were one residence for 60 years and were divided back into two separate homes in 1983, when the second front door was reinstated. The smaller house has four bedrooms, plus a self-contained apartment and an even larger garden than its neighbour. There is no existing planning permission for such a project because no one has owned both houses since the split, but agents think that, given its history, planners should not object.

Gary Bullard, the current owner of the larger house, says that the two would look perfect as one, and that the smaller house would make excellent staff and guest lodgings. However, Mr Bullard, who runs a PR agency for women executives, has no plans to do so himself. “I think it would be a bit too big for just me, my wife and our three cats,” he says.

But will two homes hold their value if they are turned into one? In some cases, properties may even gain in value, depending on their surroundings, according to Dawn Carritt, of Jackson-Stops & Staff. If they are on a street that is lined with unimpressive semis, this will drag down the value of your new home, possibly even below what the two houses would have fetched separately. However, if the other houses on the street are large and yours is the odd one out, it will definitely benefit from an upgrade.

One of the most important things to take into account early on is planning consent. If the property is in a conservation area, making a substantial change to the garden or the façade may be difficult. And it is likely to be much trickier if one of the properties is listed. This is certainly true in the case of Sloane House, a sixbedroom Georgian property just off the Kings Road in Chelsea, which Sir Anthony Bamford, chairman of JCB, the construction equipment group, bought in 2004. The property is now for sale at £79 million, but it comes with Sloane Lodge, the neighbouring cottage, which Bamford originally wanted to knock together with the main house to create a 24,000 sq ft mansion. He bought both properties together for £45 million. Permission to merge the two has been granted — but only to join them at ground and basement level. Aylesford International, the joint agent with Beauchamp Estates, thinks that the two may well be sold separately.

Andrew Langton, chairman of Aylesford International, adds: “The conversion, including the installation of a swimming pool and an overhaul of the plumbing, would probably cost about £15 million.”

Buyers who want more space but not the hassle or expense of building, could always copy Tony Blair’s example in Connaught Square, Mayfair, and buy a neighbouring mews house. This will provide space for your guests but maintain the privacy and integrity of your original home.

CONTACTS: Old Palace Place: Knight Frank, 020-8939 2800. Savills, 020-8614 9108 Beauchamp Estates: 020-7499 7722 Aylesford International: 020-7351 2383

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