You can see the problem: you have several television series and a host of cookery books to your name, and an estimated fortune of £40m. You want a mega-mansion that reflects your new-found wealth. But you love your house and are reluctant to leave your exclusive corner of north London. What do you do? If you’re Jamie Oliver, naked chef and school-food campaigner, the answer is simple — you buy the pukka house next door.

Oliver, 32, who has a four-storey terraced house in trendy Primrose Hill, has just acquired next door, too. He has applied to planners for permission to knock them into a 19-room superhouse — providing plenty of room for the chef, his wife, Jools, and their daughters, Poppy, 6, and Daisy, 5. Oh, and the couple also want to built a large extension at the back.

Camden borough council is due to rule on the application in the next few weeks. If approved, it will not only provide them with lots more space, but could add substantially to the value of their properties, too. While individual houses on the street sell for £2.5m or so, local agents estimate such a super-house would be worth more than £7m.

Oliver is just the latest in a line of celebrities who have outgrown their homes and sought to buy next door. Chelsea FC’s owner, Roman Abramovich, has done it; Michael Palin, the former Python turned professional traveller, has done it; so have Alastair Campbell, the former spin doctor, and the actress Helena Bonham Carter. If you’ve already bought your prestige townhouse with basement conversion, but need extra space for your staff, a pool and a cinema room, sometimes the only option is to expand sideways.

“A person’s house needs to grow with their wealth,” says Jonathan Hewlett, head of London sales at the estate agent Savills. “In London, it is often difficult to find one house that gives the volume of space and number of bedrooms required. People see it as easier than moving house. If it’s a prime location and they really like that street, they’ll buy next door instead.”

Robert Bailey, founder of the buying agency Robert Bailey Property, agrees. “Generally, the super-rich would spend every weekend in their main house in the country,” he says. “Nowadays, due to pressure of work and longer hours, they are tending to downsize in the country or sell up and live most of the year in London, perhaps taking longer holidays abroad.” Typically, his clients are looking for 7,000-8,000 sq ft of space, with housing for four children, nannies and any extra staff.

Such conversions, although relatively rare, are most often found in the priciest parts of prime central London, such as Kensington and Chelsea, or further north in Belgravia and St John’s Wood. The scale of work varies enormously: sometimes it is just a matter of knocking through a few adjoining doors, but more ambitious projects can involve removing the party wall or even rebuilding the inside completely, so little more than the facade remains.

Whatever the work, you must obtain consent from planners, who are not always happy to see two normal-sized houses turned into one super-sized one — as Madonna found last year, when she was refused permission by Westminster city council to knock through her two houses in Marylebone.

The conversion process may also be far from straightforward, depending on the layout of the two properties. You may think buying the house next door will double your floor space to 8,000 sq ft, but if you are required to keep all existing hallways, you may end up with only 6,000 sq ft that is usable. There are more potential problems if the house is listed or you own only the leasehold. If the Crown Estate has the freehold, as can be the case in the area near Regent’s Park, that can cause problems.

“Some houses feel like two separate properties with interconnecting doors,” Hewlett says, “and others flow well. Often, you can’t take down a party wall, chimney or staircase due to planning laws. It is seldom possible to create one spectacular room across one floor.”

Despite such problems, the marriage value created by joining two properties can be considerable. The estate agent Knight Frank, for example, sold adjoining houses on Walton Street, in Knightsbridge, for £11.5m last year — a single house on the same road would have gone for about £4m.

In other cases, the whole may not be worth more than the two parts. A laterally converted property on Upper Cheyne Row, an exclusive pocket of Chelsea, is a case in point.

With 5,319 sq ft of space over four storeys, it has two double drawing rooms, five bedrooms, a staff flat, a sauna and a wine cellar, and is on the market for £7.9m. It is Grade II-listed, however, meaning the original floor plans of the houses had to remain.

Howard Elston of the estate agent Aylesford, which is selling the properties jointly with Savills, acknowledges that a single house on the same street could cost almost £4m — meaning there may be little or no marriage value. “Because both these homes are Grade II-listed, the ground, first and second floors must remain intact,” Elston says. “They are joined only on the landing levels, with a connecting door. It is tricky to sell because you need to find a buyer who wants that particular layout.”

Robin Ellis, one of London’s most upmarket builders, specialises in upgrading the homes of the super-rich. To ensure you create a house that maximises floor space and realises the potential of your investment, he says, you need to do your research.

Some houses in areas such as Notting Hill and Primrose Hill — which used to be considered “inner city” — had been modified before the gentrification of the 1980s. “If a property was butchered in the 1960s and 1970s, when planning laws were more relaxed, then alterations can be more flexible,” Ellis explains.

The trick, he says, is to seek out houses where the roof level may have been changed or where the basements have, at some point, been turned into separate flats, accessible only from the street. “A lot depends on the condition of the property and how many period features remain,” he advises. “If the property was at some stage converted into an office and all the features were knocked out, there’s a good chance you can do some serious renovations.”

An alternative approach, favoured by some who live in Kensington or Belgravia, is to buy the mews cottage behind their house. This follows the example of Tony Blair, who in 2004 paid £3.65m for a Grade II-listed Georgian townhouse on Connaught Square, in Westminster, then last year bought the mews house behind it on Archery Close for an estimated £800,000.

“The ideal scenario for a London house would be, as with Tony Blair, to buy the neighbouring mews,” says Rupert des Forges, a partner at Knight Frank. “They are usually separated only by a garden or terrace, are more feasible in terms of cost and give garaging, ancillary guest rooms above — and, of course, security at the back of your property.”

Lateral thinkers

What works for multimillionaires in the most expensive parts of central London will probably not be worth it for the rest of us out in the suburbs or elsewhere in the country, where property values are substantially lower. In other words, think again before making that offer on the semi next door.

Bailey says there is a danger of “overcapitalising” if property values are not high enough in your street. “You might be better moving to a better road than buying two houses next to each other in a secondary location,” he says.

Ellis agrees: “In the middle market, it might not be worth the expense,” he says. “But at the high end — global prime London — the marriage value may be huge. People really will pay for an exceptional house in the right area.”

n 28 and 28a Upper Cheyne Row is for sale for £7.9m through Aylesford (020 7349 2628, www.aylesford.com ) and Savills (020 7730 0822, www.savills.com )

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