The work required for an underground extensions, he argued, simply may not be worth the money to implement for the subsequent reward in property value.

In Sean’s LinkedIn post, he acknowledged that even with planning approval, developers are tending to throw basements to the wayside as getting them valued and building them out has proved to be too much of a burden.

His post stated: “The logistics involved in creating subterranean accommodation naturally require greater expenditure than traditional, above-ground square footage.

“Unfortunately, in the vast majority of cases, said basement areas then command a lower rate per square foot when complete in terms of value.”

As the market continues to move precariously, with higher interest rates and sluggish house prices, the appeal of a basement may start to wean if these extensions have more potential to hit the pockets in initial development, without any meaningful impact on the projects’ long-term value.

The general price of development also seems to be making an impact; while all construction material costs reduced by 2.3% in November 2023 year-on-year, they remain high.

Sean is not the only surveyor who sees the financial hazards with building basements into a development. Richard Taylor, director at London’s Surveyors and Valuers, agreed that basements may be a thing of the past.

Speaking to DFT about developing basements in the capital, Richard said that the initial stages of developing a basement are already beset with issues: digging downwards runs the risk of disturbing solid foundations or causing structural damage, and dealing with neighbours to do so can also be a hurdle.

He also believes larger factors — such as the war in Ukraine raising the price of steel — have had a significant effect on the viability of basement projects.

“Development costs have gone up considerably, and when people [are] looking to build out these projects, what was an expensive game two years ago is now a very expensive game, and that is affecting people's ability to build them out because it's just so costly — it's just not worth it,” said Richard.

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“Unless it's in a prime central London postcode, you won't get your money back because they are so expensive to build out.

“They're underground, so not prime living space, so if you want to buy cheap flats in London, you buy a lower ground floor or basement flat because they are the cheapest. Why? Because they're the least desirable, so you're talking about developing the least desirable floors in a building.

“It's an expensive, torturous game,” Richard added, explaining that basements are not as popular now due to costs being “so much more”.

Steve Larkin, head of property development at LendInvest, agreed that basement developments were seen as a benefit in London — where space is at a premium — as they can be converted into cinemas, wine cellars, and gyms, where light isn’t a necessity.

“I don't believe they work as well in locations outside of London as you tend to get more space in new-build developments,” Steve commented, “particularly if the area is large enough to accommodate a second storey or an additional annexe for recreational space.”

Steve highlighted the issues developers can face in building them: “In terms of the risks associated, we have seen in the past how precarious these underground basement extensions can be if not undertaken correctly.

“Damage to the buildings above and adjacent to the subject site can cause substantial costs and, therefore, only developers and contractors who have undertaken a number of these successfully should be supported in this regard.”

Chris Gardner, CEO at Atelier, echoed the same sentiment: “We rarely see projects with basements and, I suspect that outside of prime central London, the economics of digging basements just don’t make sense.

“Subterranean space, whether new build or within an existing property, is very expensive to do, and the value and desirability of the space created is lower. Going up rather than down makes more sense to most developers.”

While basements can provide a extra space for additional home utilities, in the current development climate, many see those below-floor spaces as in the dark and out of touch.