- 30 m³ is an aggregate internal volume limit, not a single-dormer size.
- It is one of several conditions — not a stand-alone permission.
- Meeting the volume figure does not by itself make a project exempted development.
- Where unclear, a Section 5 declaration from the local authority is the formal route.
Where the 30 m³ figure comes from
The 30 m³ figure appears as part of the 27 July 2026 update to the planning exemptions relating to certain dormer developments. It is an aggregate internal volume limit — that is, the combined internal volume of the dormer additions to the roof, not the size of a single dormer taken alone.
Why the number is often misunderstood
In conversation, 30 m³ is easy to picture: a box roughly 3 m by 4 m by 2.5 m. That mental picture makes it feel like a simple allowance — build anything up to that size and it is exempt. In practice the exemption is a set of conditions, and this is only one of them.
The other conditions matter equally
The exemption also refers to design considerations, positioning relative to the ridge and eaves, distances to boundaries, roof pitch, dormer type and how the works interact with the front elevation of the dwelling. Front dormers and dormers on Protected Structures typically fall outside the exemption altogether.
Aggregation across the roof
Because the 30 m³ is aggregate, it applies whether the works are a single large dormer or two smaller dormers on the same roof plane. Adding a small side dormer today and a rear dormer next year, for example, would be considered together for the purpose of the condition.
Meeting the number is not the same as being exempt
This is the most important point to take from this guide. A proposed dormer that measures under 30 m³ can still fall outside the exemption for other reasons — for example, because of its position, because the dwelling has already used part of its allowance, or because the property has protected status. The volume figure narrows the field; it does not by itself create the right to build.
Building Regulations still apply
Whether or not planning is required, Building Regulations apply to the works. That includes structure, fire safety, escape routes, ventilation, energy performance and drainage. Exemption from planning is not exemption from Building Regulations. See Building Regulations versus planning permission for a fuller explanation.
How we typically use the 30 m³ condition in practice
On a home survey, we take measurements that let us estimate the internal volume of any proposed dormer against the aggregate allowance. Where the numbers are comfortably within the limit and every other condition is met, the project may be capable of being progressed as exempted development. Where any condition is borderline, we generally recommend either a design adjustment or a Section 5 declaration for formal confirmation.
When a Section 5 declaration is worth it
A Section 5 declaration is a formal request to the local authority to confirm whether specified works constitute exempted development. It is a paper exercise — not a planning application — and it provides written certainty before construction begins. Where any part of the assessment is close to a threshold, it is usually the sensible route.
What this means for your project
Every home is different. Use this guidance to ask the right questions and to shape your brief — but confirm what applies to your specific property with a home survey and appropriate professional input.
Frequently asked questions
- Does 30 m³ mean any dormer under that size is exempt?
- No. The 30 m³ limit is one condition among several. Design, location, roof slope, distance to boundaries, and whether the property is a Protected Structure or in an Architectural Conservation Area all apply as well.
- How is the volume measured?
- The condition refers to the internal volume of the dormer additions in aggregate. The measurement is done from finished internal surfaces. This is a technical calculation and should be done or checked by the design team.
- What if I have two dormers?
- The condition is aggregate, so multiple dormer additions to the same roof plane are counted together against the same limit.
- How can I be certain a project is exempt?
- The only formal certainty comes from a Section 5 declaration from the local authority. It confirms in writing whether specified works are exempted development.
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