How Changes to Biodiversity Net Gain Could Affect Your Self-Build Project

If you are planning your self-build project, you may not be aware of new changes to the Biodiversity Net Gain regulations which may impact your project. Custom and self-build development has, until now, sat outside mandatory BNG entirely. But, from the end of July 2026, that exemption will disappear, putting self-build and regular projects on the same footing — but there are still exemptions which may apply to you.

If you are currently assembling a self-build application, or you are a contractor advising a client through the process, the date on which you submit could determine whether Biodiversity Net Gain applies to the scheme at all. Below, we set out exactly what is changing, who it affects, and how to plan around it.

01 — The backgroundWhat Biodiversity Net Gain actually requires

Biodiversity Net Gain became mandatory for most planning permissions in England in February 2024. Initially, it was rolled out to larger schemes (10+ dwellings), leaving smaller projects to continue without these requirements, and smaller sites followed in April 2024.

In simple terms, it requires a development to leave biodiversity measurably better than it found it — a minimum 10% uplift, calculated using Natural England's biodiversity metric, secured for at least 30 years through a habitat management and monitoring plan.

For a housing estate on a greenfield site, that is a significant but manageable design exercise. For an individual building a single home on a modest plot, it has often been disproportionate: commissioning an ecologist, running the metric, and legally securing habitat commitments can cost several thousand pounds and add weeks to a programme — for a scheme whose ecological footprint might be a garden and a driveway.

Recognising this, the original BNG regulations provided exemptions specifically for small-scale self-build and custom-build development. That exemption is the now being removed.

02 — What is changingThe self-build exemption is being withdrawn

Following a government consultation and response published in April 2026, Defra has confirmed a package of BNG reforms aimed at reducing the burden on smaller schemes with broader and simpler terms. Whilst this is likely to be welcomed news to Small and Medium Enterprise housebuilders, some self-builders may fall into a BNG trap.

Both changes — the new 0.2 hectare exemption and the removal of the self-build exemption — are expected to be introduced together, by secondary legislation, before 31 July 2026, subject to parliamentary timetabling. Until that legislation is in force, the current rules and the existing self-build exemption still apply.

03 — The practical effectDoes this actually change anything for your plot?

For many self-build plots, there may be no changes to your development project. Government's own assessment is that this new threshold will keep the majority of small residential schemes, including most self-builds, out of scope. Some Council's require legal agreements to be signed to ensure that the development will be self-build and therefore the BNG requirements do not apply. Assessing both self-build and regular developments the same may well remove this requirement, however some Councils may still seek legal agreements as part of the overall planning assessment.

However, if your development plot is in excess of 0.2ha, you will be required to meet the Biodiversity Net Gain requirements. As described above, you will need to demonstrate a 10% increase in the net gain of biodiversity.

The risk sits with plots at the margins: larger self-build or custom-build plots, smallholdings, paddock purchases, and rural exception sites that exceed 0.2 hectares once access, gardens, and any accompanying land are included within the red line. These schemes lose their current protection and gain nothing to replace it — they will need to demonstrate a 10% net gain like any other development, unless a further exemption applies.

04 — If your scheme falls outside the exemptionWhat BNG compliance actually involves

If your self-build plot exceeds 0.2 hectares, or a priority habitat is identified on site, you will need to work through the standard BNG process rather than assume it away. In outline, that means:

1. A baseline ecological assessment — your first step will be establishing the pre-development habitat value of the site using the statutory biodiversity metric.
2. Testing the biodiversity gain hierarchy — on-site habitat creation or enhancements on or off site
3. Securing the 10% net gain — either through on-site habitat works, purchasing off-site biodiversity units, or, statutory biodiversity credits.
4. A legal agreement — a planning condition or a section 106 obligation, committing to a habitat management and monitoring plan for 30 years.

Importantly the same regulations which take the self-build exemption away, also make it easier to secure off-site biodiversity gains compared to on-site gains. Off-site credits are often preferred by self-builders as it doesn't require the loss of any garden land and does not require management plans which can come with their own complications for small projects.

None of this is insurmountable for a self-build project — but scoping and costing this additional requirement before the application goes in, will help you navigate the planning process.

05 — TimingWhy the date you submit could matter more than the site itself

Because the current self-build exemption remains in force until the new regulations formally take effect, there is a genuine window in which a scheme's BNG position depends on when it is validated, not just where it sits. Whilst the wait the actual legislation, a larger self-build plot submitted and validated before the changes come into force may still benefit from the existing exemption; the same scheme submitted a few weeks later, once the regulations are in force, would not.

06 — Practical next stepsWhat we would suggest doing now

• Measure your red line properly. Do not estimate — get the site area within the application boundary confirmed precisely, including access and any garden or paddock land.
• If you are near the 0.2 ha threshold or above, get a BNG cost and programme estimate now, rather than after validation — it changes what the build is likely to cost and how long the planning stage will take.
• Build the answer into your pre-application conversation with your local authority, particularly if your build is not ready to submit until after the summer.

Planning Review vs. Site Appraisal — which do you need?

A Planning Review answers a specific, defined question in writing, usually within days — anything from the questions to "what do I need to submit?" A Site Appraisal is a fuller feasibility assessment of what a plot can support: capacity, constraints, access, heritage and ecology risk, and an indicative development strategy. If you're still deciding whether a plot is worth buying, or what it could realistically carry, start with a Site Appraisal instead. If you use a Planning Review or Site Appraisal and you instruct Cedar Planning for a pre-application or planning application, a portion of your fees will be credited towards future work undertaken within 30 days.

Biodiversity Net Gain reform is moving quickly, and further changes — including a possible brownfield residential exemption and revisions to the biodiversity metric itself — are expected later in 2026. We will keep this page updated as the secondary legislation is confirmed. If you would like a second opinion on a specific plot, get in touch with Cedar Planning — we cover self-build and custom-build projects across Suffolk, Norfolk, Cambridgeshire and nationwide.