The Budget Took Biodiversity Net Gain Off Small Brownfield Sites. It Did Not Change What Is Under Them.

In the Budget on 28 October, the Chancellor confirmed that smaller housing developments on brownfield land will be exempted from Biodiversity Net Gain requirements, in the government's words "making it cheaper and simpler to build".

For anyone trying to bring a small brownfield site forward, that is a real saving. Biodiversity Net Gain has applied as a statutory planning condition since the Environment Act 2021, requiring most development to deliver a measurable uplift in biodiversity. On a former yard, depot or industrial plot, where the existing baseline is often close to nothing and the site is too tight to deliver anything meaningful on it, the requirement has been an awkward fit from the start. Removing it from smaller schemes takes out a cost, a consultant and a delay.

It does not take anything out of the ground.

The two barriers are not the same barrier

Brownfield sites carry two separate problems, and they fail for different reasons.

The first is procedural. Planning conditions, biodiversity obligations, consultation, reports. This is the cost of getting permission, and it is the one the Budget has addressed.

The second is physical. Asbestos in made ground. Hydrocarbon contamination from fuel storage. Heavy metals from former industrial processes. Buried foundations, tanks and services that appear on no drawing. This is the cost of actually building, and nothing announced on 28 October changes it by a penny.

The distinction matters because the second problem is the one that kills programmes. A planning cost is known, quantified and budgeted before anyone commits. Ground contamination is frequently discovered, and discovered is the expensive way round. A site that looked viable when the appraisal was done stops being viable when a machine hits something nobody expected.

Smaller sites, thinner margins

The exemption applies to smaller developments, which is also where this cuts deepest.

A large scheme can absorb a surprise. There is contingency, programme float and enough units across which to spread an unplanned remediation cost. A small brownfield site has none of that. The same discovery that is an irritation on a 200 unit scheme can make a 12 unit scheme undeliverable.

So the sites most helped by the BNG exemption are precisely the sites least able to survive the problem the exemption does not touch. That is not an argument against the measure. It is an argument for spending some of what has been saved on finding out what is actually there.

What the saving is worth

The useful response to this is not to pocket the BNG saving and move on. It is to put part of it into ground investigation at the front end, before the land is bought or the appraisal is fixed.

Intrusive investigation on a small site is not a large number. Set against the cost of discovering asbestos in soils with groundworks already under way, it is close to rounding. The most common reason it does not happen is that it falls in the window where nobody has committed yet and nobody wants to spend, which is exactly when it is worth most.

This is the same point we made when the government expanded brownfield funding: the money was never the only barrier. Easing the planning route has the same blind spot as increasing the subsidy. Both make the paperwork better. Neither makes the ground cleaner.

We go to sites all the time where the desk study said it was fine, and it absolutely wasn't. 300,000 sites with no proper records. That doesn't surprise me at all. The developers who end up in trouble are the ones who cut corners on the ground investigation to save a few quid at the front end. You find it early, you deal with it, and you move on. You find it halfway through groundworks, and you've got a real problem on your hands. It's not complicated.

Worth watching

Two further things from the Budget are worth tracking if brownfield development is your business.

The scope of the exemption has not been fully defined. The Treasury's own summary refers to "smaller housing developments on brownfield land" without setting a threshold, and early reporting has described the change in wider terms than that. Where the line falls will decide how many sites it actually reaches.

Land Remediation Relief was expected to conclude at this Budget, and it has not. The government ran a second consultation that closed on 21 September, having already found that the relief was not fully achieving its objective of incentivising the remediation of derelict and brownfield land. It said it would set out its conclusion at Budget 2026 and legislate through the Finance Bill if it proceeded.

It did neither. Land Remediation Relief does not appear anywhere in the Budget's Overview of Tax Legislation and Rates, the document that catalogues every tax measure announced, including those not going into the Finance Bill.

So the relief continues in its current form for now, which is the good news for anyone with a live appraisal depending on it. The less comfortable reading is that a relief the Treasury has already said is not working has been left unresolved for another year, with the uncertainty that creates for anyone planning remediation spend across that period.