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The one renovation that could double the value of your house (and make an enemy of your neighbours)

Neighbours Jimmy Page (left) and Robbie Williams (right, with wife Ayda) have been locked in a fierce battle over Williams' plans to build a basement extension at his Holland Park home (Getty/ Rex/ Benett)
Neighbours Jimmy Page (left) and Robbie Williams (right, with wife Ayda) have been locked in a fierce battle over Williams' plans to build a basement extension at his Holland Park home (Getty/ Rex/ Benett)

It may not make you popular but creating a basement under your home could double its value in some parts of London, research has revealed.

Estate agent Foxtons calculated the average cost of adding subterranean storeys and the typical increase to the resulting selling price in each borough.

It found that domestic diggers in Kensington and Chelsea could increase the net value of their properties by 96 per cent.

The leafy west London district is the best place for adding an underground floor, according to the study, because of the large footprint and high price of its homes.

Camden came in second in the value-add table, with an average 84 per cent uplift, while residents in every borough can expect at least a 35 per cent increase in value.

Across the capital as a whole, the typical full-storey basement costs £167,955 to build but adds £459,653 to sale price, according to the study. This represents a net gain of £291,699, or more than half of the average property value.

Of course these are average figures from aggregated data and costs can vary wildly for a variety of reasons. There are also other fees that can quickly mount up during the design, planning and fit-out phases.

But Eric Morson, managing director at specialists consultancy Basement Masters, said basements didn't have to be the preserve of the rich.

"A young couple who can dig under their ground-floor flat could double their floor space without doubling their costs," he said.

The five districts where basements typically add most value

Borough

Estimated construction cost for average basement

Estimated uplift in property value from basement

Net increase as percentage of average house price

Kensington and Chelsea

£271,500

£1,424,738

96%

Camden

£240,000

£923,075

84%

Islington

£205,500

£785,203

81%

Westminster

£193,500

£950,197

78%

City of London

£205,500

£819,543

76%

Source: Foxtons analysis. Based on full one-storey basements below average house in each borough, using cost estimates per square metre and assuming value uplift in line with floor space added

Those with plenty of available land in premium postcodes can add significant extra value with a basement, Morson added.

Decisions to dig have to be made on localised factors such as ground conditions, existing floor type, planning rules and desired use, he said.

Homeowners should allow a minimum of 18 weeks for the construction phase and be prepared to move out for part of that in most circumstances.

Can you still do a basement extension in London?

Of course, before excavation can begin, there is the design process and the thorny issue of planning.

Many London boroughs have cracked down on basement construction in recent years after a spate of controversial and locally-opposed mega-digs.

Kensington and Chelsea brought in new guidance in 2016 after raising "great concern" about the impact of large excavations on neighbours.

Basements in the borough can generally only be one floor deep and have a maximum footprint equal to the house and half its garden. Construction method statements must be submitted alongside planning applications.

Guidance issued by Camden Council in 2021 states that basements should in most cases not extend beyond the footprint of the building above or be more than four metres deep. The north London local authority requires specific impact assessments to be carried out at planning stage.

Queen guitarist Brian May co-founded a campaign group to ban mega-basements following his own experience of neighbours' deep digs when he lived in Kensington.

"This is a behaviour so anti-social that action clearly now needs to be taken to bring the perpetrators to justice," May told the Independent on Sunday in 2015. "If hooded young people were causing such destruction of people’s quality of life, they would be behind bars by now.”

As May lamented, basement extensions have long been popular in Kensington & Chelsea. His fellow rocker Jimmy Page objected to his neighbour, Robbie Williams' renovation plans for his Holland Park home, which include a basement swimming pool in a planning battle that lasted over four years.

Foxtons chief executive Guy Gittins conceded that basement excavation can be divisive.

"It may make you a tad unpopular with your neighbours due to the lengthy and extensive construction process required," he said.

"However, for those who can execute such a mammoth renovation project, it can be well worth it. Not only will your home command a much higher price when you do come to sell, but it will hold far higher appeal in the eyes of prospective buyers when compared to the competition.

"Of course, it’s a complicated process and one that requires a lot of planning and a sizable budget to achieve."