Notes on carbon, compliance and construction.

Any supplier bidding on a UK central government contract worth more than five million pounds a year now has to submit a Carbon Reduction Plan alongside their bid. This catches out more bidders than it should, usually because the plan they produce is not in the format the government actually requires.

It is not an open format document

A Carbon Reduction Plan follows a specific government template. It is not a general sustainability report, and a strong general carbon footprint assessment, however well done, does not satisfy the requirement on its own unless it is repackaged into the correct structure.

The plan is designed to be quickly comparable across competing bidders, not to showcase how much work went into producing it.

What the template actually requires

• A stated commitment to achieving Net Zero, with a target year no later than 2050
• Baseline year emissions, covering Scope 1 and 2 in full, plus the specific Scope 3 categories the guidance requires
• Current year emissions reported against that same baseline
• A list of completed carbon reduction initiatives already in place
• A list of planned future initiatives
• A formal declaration, signed off by a director or equivalent senior leader, confirming the plan meets the reporting standard

Where this trips people up

The most common mistake is submitting a general emissions report instead of the mandated template. A detailed footprint assessment is genuinely valuable, it is the working behind the numbers, but it is not the document a contracting authority is scoring the bid against. The two are related but not interchangeable, and treating them as the same thing is usually what costs a bidder marks it should not have lost.

The second common mistake is scale. The government template is short by design, so padding it out with methodology and appendices does not strengthen a submission, it obscures the specific things an evaluator is checking for.

Before a business can set a credible reduction target, or produce a Carbon Reduction Plan worth the name, it needs an honest baseline. Here is what that actually involves, using a regional contractor as the working example, not a services business, since that's the shape most readers here actually need.

Standards and boundaries first

A proper assessment is built to the GHG Protocol Corporate Accounting and Reporting Standard, using the UK Government's most recent GHG Conversion Factors. Before any numbers are calculated, the boundaries need setting, an operational control approach covering everything the business has control over, split into Scope 1 direct emissions, Scope 2 purchased energy, and Scope 3 everything else in the value chain.

Where the data actually comes from

Consider a typical regional contractor, roughly 40 staff, a mix of office and site based work, plant and machinery on hire, materials procured project by project. The total comes to around 199 tonnes of CO2 equivalent across the year. The shape of that number is the useful part, not the total itself.

Scope 1 lands at roughly 26 percent, almost entirely diesel burned directly in plant, machinery and generators, not office gas. That's a real, structural difference from a services business, where Scope 1 is usually negligible. A contractor's own fuel use is a genuine, direct part of the footprint, not something buried three tiers down the supply chain. Scope 2 stays small, under 3 percent, office and site electricity combined. Scope 3 still carries the largest share, around 72 percent, but the categories driving it look nothing like a services firm's, materials procurement and plant hire, not flights and homeworking.

The real value of an assessment is not the headline number, it is knowing where to act, and for a contractor that's rarely the office.

Not every number carries the same confidence

The single largest category in this example is purchased construction materials, calculated using a spend based method, a category split specifically because a pound spent on ready-mix concrete and a pound spent on office software carry very different carbon behind them. This is worth being direct about, spend based estimates are the least reliable way to calculate emissions, since they assume a fixed relationship between money spent and carbon produced, which does not hold well across different suppliers. A proper assessment says this plainly rather than presenting every figure with equal confidence. The fix is not a better formula, it is better data, an Environmental Product Declaration from the concrete or steel supplier directly, in place of the spend based estimate.

Hotspots, not just a total

In this example, five categories account for around 89 percent of the entire footprint, purchased construction materials, plant and machinery diesel, waste sent to landfill, staff commuting, and plant and equipment hire, in that order. That's where a reduction plan should focus first, not spread thin across twenty categories with roughly equal effort. Worth noting too, waste sent to landfill outweighs everything else being recycled or composted by a wide margin, in this example alone, that's often the single fastest, cheapest reduction available, before touching material specification or plant fuel at all.

Embodied carbon gets raised on most projects eventually. The problem is timing. By the time it reaches the estimating desk, the structural design is often already fixed, and the only thing left to do with the number is report it, not improve it.

When this actually belongs in the process

Embodied carbon for a concrete substructure is a design-stage question, not a post-tender reporting exercise. Once the mix, the reinforcement schedule and the structural form are locked, what's left is measurement, not reduction. Raising it during pre-construction review or concept design, while the structural engineer can still change the specification without a redesign, is the only point where a bid can genuinely compete on the number rather than just disclose it.

By the time embodied carbon reaches the estimating desk as a line item, the decision that would have moved it has usually already been made.

The lever that actually moves the number

Cement replacement is the single biggest lever available without a structural redesign. Ground granulated blast-furnace slag, GGBS, or pulverised fuel ash, PFA, substituted for a portion of the Portland cement in a mix reduces the embodied carbon of that concrete meaningfully, since cement production is the dominant carbon source in a standard mix, not the aggregate or the water. A 30 percent GGBS replacement is a commonly specified starting point for a substructure mix, higher replacement rates are achievable where the structural engineer confirms the strength and curing time requirements still hold, since higher GGBS content typically slows early strength gain, which can matter on a tight programme.

This is worth raising as a genuine design conversation with the structural engineer, not a box to tick after the fact, since the achievable replacement rate depends on the specific mix design, exposure class and programme, not a single number that applies everywhere.

What to actually ask your concrete supplier for

• An Environmental Product Declaration, EPD, for the specific mix being quoted, not a generic industry average figure
• The cement replacement percentage already built into their standard mixes, and what a higher replacement rate would cost and what it would do to strength gain timescales
• Confirmation of which modules, A1 to A3 typically, their published figure actually covers, since a number without a stated boundary isn't comparable to anything

A Carbon Reduction Plan is only as good as the data behind it, and most of that data doesn't sit with the main contractor, it sits three or four tiers down the supply chain, with people who have never been asked for it before. Here's how to actually go and get it without it becoming its own project.

Who to actually go to first

Don't start with every supplier on the project, start with the few that actually move the number. Rank your supply chain by spend, then by category, upstream transportation and distribution and purchased materials are usually where the real weight sits on a construction project, not the smaller service contracts. A handful of suppliers, your top five by spend, is a genuinely sensible place to start, since chasing forty suppliers for the same information rarely beats getting real data from the five that matter most.

What to ask for, and in what order

1. An Environmental Product Declaration for the specific product supplied, the single best piece of evidence a supplier can hand over
2. Failing that, their own carbon reporting, a Carbon Reduction Plan or sustainability report with product or activity level detail, not just a corporate headline figure
3. Failing that, direct activity data, fuel and energy use tied to the specific order, which can be converted using the same government conversion factors used elsewhere in the assessment

Asking every supplier the same generic question gets a generic answer. Asking for a specific document, an EPD, usually gets a real one.

When a supplier genuinely doesn't have an answer

This will happen, and the honest response is to say so, not to quietly paper over it with an invented number. Where primary data isn't available, a documented spend based or generic industry factor is a legitimate fallback, provided the plan states plainly which figures are confirmed and which are estimated. A Carbon Reduction Plan that's honest about its weaker data points is a stronger document than one that presents every figure with equal, unearned confidence.

"Timber has a lower carbon footprint than steel" is repeated often enough on site to sound settled. The published research says something more specific, and more useful, than that.

What the studies actually show

A widely cited life cycle assessment by Allan and Phillips, comparing mid and low-rise buildings, found CLT structures showed a 30 to 40 percent reduction in global warming potential compared to equivalent structural steel buildings. That's a genuine, meaningful difference, and it's the figure behind most of the general claims made about timber versus steel.

A 30 to 40 percent reduction is real. It is also an average across a body of studies, not a guarantee for any specific building.

The finding that complicates the simple story

A UK life cycle assessment published in 2026, studying a modular light-gauge steel social housing scheme, found a normalised embodied carbon intensity of 366 kgCO2e per square metre across modules A1 to A5, describing this as comparable to recent high-performing CLT buildings. In other words, a well-optimised, fabric-efficient steel scheme closed most of the gap the broader literature reports, not through a different material, but through better design and specification within the material it already used.

This matters more than it might first seem. The 30 to 40 percent figure is a comparison of typical outcomes, not a fixed physical law. A poorly specified CLT building and a well-optimised steel one can land closer together than the headline percentage suggests, and the reverse is just as true.

What this actually means for a real decision

Material choice is a genuine, significant lever, and the general pattern in the literature does favour CLT over steel by a meaningful margin on average. But the honest conclusion isn't "always specify timber," it's that the specification quality within either material matters enough to change the outcome substantially. A frame system your team already builds well, executed carefully, can outperform a lower-carbon material poorly specified. The only way to know which applies to a specific project is to actually model both options against the real design, not assume the answer from a general industry statistic.