In a business-park office in Milton Keynes, early in 2024, someone at the National House Builders Council (NHBC) clicked a mouse, and extending a home jumped in price by tens of thousands of pounds for many Brits.
The NHBC Standards 2024, in force for foundations started from 1 January 2024, were followed later in the year by the enticingly named BS 5837. The updated draft British Standard for “Trees in relation to design, demolition and construction” was closed for consultation in October.
These obscure documents have … cost middle-class Brits with grand designs millions
They sound harmless enough — even tedious. But these obscure documents have, in recent months, cost middle-class Brits with grand designs millions, and, for some, killed their plans altogether.
The new “near-tree” rules (and the tables and calculators inspectors and warranty providers rely on) have pushed foundations to previously unseen depths in suburban gardens.
If an Englishman’s home is his castle, adding an extra turret — or more likely a bedroom, ensuite, or games room — is a dream for many of us.
There is something uniquely exciting and satisfying about designing and building the structure you live beneath. It’s an itch that many of us harbour for years, pondering where to position the kitchen, how to open access to the garden, and how to make it completely our own.
The housing crisis has many villains. Planning, politics, and interest rates. But at its most human-scale, it is, simply, families who can’t fit into their homes yet can’t afford to move.
Extending older houses, which are often on larger plots, is an obvious part of the solution. It is essential, in fact, for creating the denser neighbourhoods we need to avoid carpeting green fields with new estates.
I have a growing family. I grew up in a small town, and my mother still lives here. We wanted to stay here, but family-sized homes now vanish from Rightmove within hours. Our best hope was a small, tatty, post-war retirement bungalow, which we planned to add extra rooms to for our son and new daughter.
But since buying, the economics of extending have collapsed. Material and build costs are up around 30–40 per cent since 2020 because of pandemic inflation. Walls must also now be built thicker, making rooms smaller, because of Net Zero insulation rules.
But the most expensive change, for us at least, and that has not been covered in the press, is foundation depth.
Under the 2024 tree/foundation guidance, designers and inspectors are urged to treat any nearby tree as a structural hazard. On some clay soils, NHBC’s new Building Near Trees chapter can now require foundation depths of around two metres (and in some cases 2.5 metres) depending on species, distance and soil type. If they reach that depth, a structural engineer must design them.
Many readers will never have seen a 2-metre-deep trench in the garden of a semi-detached. And it is quite a sight. It is almost as deep as the walls of my bungalow are high. And the amount of concrete needed to fill it is hard to comprehend, as it is pumped in pipes down the gated alleyways beside your home.
If a war breaks out, I now have the bulk of a nuclear bunker below my bedroom, which would impress Albert Speer.
Our home is far from the worst example. There are many online. And one builder showed me a picture of a garage conversion where, just to replace the garage door with a window, a two-metre trench was needed in someone’s front drive that was more than twice as deep as the window was tall.
For our single-storey extension, the extra excavation and concrete alone added tens of thousands of pounds — all because there is a small Leylandii shrub, roughly five metres away, in the neighbour’s garden, beyond their patio.
Every builder we spoke to thought the proposed foundation depth excessive. Medieval castles, grand churches, and Georgian terraces stand firm on footings a few inches deep. Indeed the rest of my 1950s bungalow stands, without issue, on about one metre.
England’s ground hasn’t changed — Britain’s bureaucracy and inexorable embrace of the precautionary principle.
I am no engineer, and safety matters. But I have worked in policy, with the built environment sector, and other highly regulated sectors. And the more I looked into this issue, the more questions I had.
Yes sound, research shows that subsidence — i.e. the caving in of land — is a growing problem that can’t be dismissed. But the UK already builds to exceptionally conservative structural standards compared to other countries. The new requirements are not, as far as I can see, about preventing collapse, injury, and death. They are about avoiding cracks and long-term ground movement that might one day trigger an insurance claim.
In many other sectors, industry bodies and insurers impose additional conditions like medical exams and security standards.
The British Standards Institution (BSI) is the UK’s national standards body, but it now operates as a private company, selling standards, certification, and assurance services worldwide. The NHBC’s income derives largely from the building industry itself, but notably, it is not-for-profit.
It is reasonable to ask, therefore, if the mechanisms exist to ensure their priorities, and the priorities of underwriters and insurers, are balanced against the priorities of home owners; that cost implications and the impact on the wider housing crisis are properly considered.
Foundations are not the only elements driving up costs. In recent decades, drainage soakaways have got deeper, and steel beams have become more substantial (the beams holding up my roof could support a small skyscraper), and many different regulatory organisations are involved.
But NHBC technical standards are worth focusing on here as they are leading the drive for new foundation standards. And they act as the main warranty provider for around 70 per cent of new homes and inform best practice across the wider building industry, including extensions.
For mortgage lenders, the NHBC or an equivalent ten-year warranty is effectively mandatory for new builds. UK Finance’s Lenders’ Handbook sets out which warranty schemes each bank will accept before releasing funds.
Although not explicitly applied to extensions, local authorities, builders, and insurers often reference the NHBC Technical Standards when assessing compliance with legally enforced Building Regulations.
Together, that reliance makes NHBC’s “guidance” a de facto national code.
The updated guidelines will do some good and stop some future subsidence. But, as far as I can see, no one has tried to assess whether the extra depth will prevent enough damage to justify the extra cost, and what the impact on construction, the housing crisis, and the wider economy could be.
Because these are not laws, of course, there is no statutory requirements for an impact assessment.
Yet, builders I’ve spoken to say the effect on their business is happening now. Some clients are simply cancelling projects.
The added expense tips the balance toward moving instead. And every family that gives up on extending must re-enter the housing market, pushing demand and prices up a little more.
Each abandoned extension means one less opportunity to incrementally increase the housing stock and densify existing streets. It is, potentially, one less family able to stay in the home or town they love, and, critically, one less Brit with the positive freedom to enjoy creating a space of their own.
Self-builds became unrealistic for the vast majority half a century ago. And now, the great British home extension is also slipping away. The sinking bureaucratic black hole of regulation and ever-increasing cost pulls yet another enjoyable, aspirational element of British suburban life out of reach for most middle-class families.
Until we change that, British families will keep pouring their money, literally, into the ground.
