If you got a quote for an extension five years ago and dusted it off today, you would barely recognise the numbers. The cost of building materials in the UK has climbed 56% since 2015. Most of that jump landed in a single brutal stretch between 2021 and 2022, when supply chains seized up and prices went through the roof.
What is surprising is what has happened since. Prices have not kept marching upward. Across the market as a whole they have been broadly flat for three years. So if building work feels more expensive than ever, it is not because everything is still rising. It is because the market has split, and a handful of materials are now moving in completely different directions.
For anyone planning a project, that split is the thing to understand.
The one material running away from the rest
The clearest example is structural steel. In the year to June 2026, the price of fabricated structural steel rose 17.7%. That is the biggest increase of any building material the government tracks, and it is running at nearly three times the rate of the market as a whole, which sat at 6% over the same period.
Steel matters to a lot more projects than people realise. The beam holding up your open-plan kitchen, the frame of a garden room, the supports in a loft conversion. When steel jumps, those jobs get dearer even when nothing else does.
Steel is not rising because of some general inflation. It is a specific story. For three years it was one of the few materials getting cheaper. Fabricated structural steel fell around 16% between 2021 and 2025, dropping back from the extraordinary highs of the supply crunch. Then it reversed, and part of the reason sits with a change in trade rules.
From 1 July 2026, the UK cut the amount of steel that can be imported tariff-free, reducing quotas by around half. Anything brought in above those limits now faces a 50% tariff, double the previous rate. The measure is meant to protect what is left of Britain’s own steel industry, but for anyone buying steel it changes the maths. Imported stock that squeezes past the quota carries a heavy penalty, which pushes buyers toward domestic supply and puts upward pressure on price.
That is where sourcing starts to matter. Domestic metal suppliers, sitting outside the new tariff exposure and often quicker to deliver, look very different on a quote today than they did a year ago. For a homeowner, that mostly plays out invisibly, inside the figure your builder hands you. But it is worth knowing why that figure moved.
Not all steel is behaving the same way
Here is the part that trips people up. Fabricated structural steel is up 17.7%, but concrete reinforcing bar, which is also a steel product, rose just 4.2% over the same year. So a builder quoting for a job heavy on structural sections is facing a very different cost picture than one pouring a reinforced slab. The word “steel” on its own tells you almost nothing now.
While steel climbs, other materials are getting cheaper
Look at the other end of the table and the picture flips.
Cement fell 4.5% in the year to June 2026. Ready-mixed concrete dropped 1.1%. These are materials that soared during the 2021 to 2022 spike and are now drifting back down. So a project built mostly from concrete and blockwork has seen its material costs ease slightly, while a steel-framed job has been hit hard.
Timber tells a similar story. Imported plywood and imported sawn wood have both come down over the past five years, unwinding some of the pandemic-era surge. None of this makes the front page, because “some materials got cheaper” is a harder headline than “costs are soaring”. But it is real, and it matters to what you pay.
This is why a single headline figure like “building costs are up” is close to useless when you are planning actual work. Two extensions on the same street, quoted in the same month, can move in opposite directions depending on what they are made of.
What this means if you are about to build
The practical takeaways are simple enough.
A quote more than a few months old is not reliable, especially on anything steel-heavy. Prices have moved enough that an old figure could be badly out, in either direction.
Ask your builder what is driving the number. If a job leans on structural steel, that is where the recent rise is concentrated, and it is fair to ask whether there is a design that leans more on materials that have held or fallen.
And do not assume everything is expensive now and give up on a project. Some materials have genuinely come back down. The trick is knowing which side of the split your particular job sits on.
The 56% figure makes a good headline, but it hides more than it reveals. The real story of building costs right now is not that they are all rising. It is that they have stopped moving together, and for the first time in years, what your project is made of matters as much as when you build it.