Mass timber and the constraints at planning stage

Mass timber planning constraints are mostly indirect, which is why they surprise design teams. Planning does not regulate structural material. What planning does regulate is the fire statement required at Gateway One for relevant buildings, the carbon and circular economy case where local policy demands one, and the drawn envelope, which changes once the fire strategy tells you how much of the timber has to be covered. Each of those routes puts cross-laminated timber and glulam in front of a planning officer, and on a tall residential scheme they arrive together.

The Fire Statement and any fire safety strategy are documents for a competent fire engineer. A draft prepared without that competence is a starting point for the specialist, never a substitute.

Why the structural material reaches the planning file

Three doors, and most schemes come through at least two of them.

The first is Article 9A of the Development Management Procedure Order 2015, which requires a Fire Statement with applications for relevant buildings, and makes the Health and Safety Executive a statutory consultee. The second is local plan policy on whole life carbon and circular economy, which is where the timber argument is usually made and therefore tested. The third is design: encapsulation thickness, compartment sizes and floor-to-floor dimensions all change the massing and the internal appearance being consented.

There is a fourth, less obvious. Where an application promises exposed timber soffits as part of its design quality argument, and the fire strategy later requires them to be boarded, the scheme as built no longer matches the scheme as sold. Officers have started asking about this directly.

Gateway one and the fire statement

Gateway One is the planning stage of the regime introduced under the Building Safety Act 2022 framework. For relevant buildings, an application must be accompanied by a Fire Statement on the published form, and the Health and Safety Executive is consulted as the Building Safety Regulator.

The statement covers the principles of the fire safety strategy at a level appropriate to the planning stage: site access for fire appliances, water supplies for firefighting, the approach to means of escape, external wall construction, and the location of the building relative to its surroundings. For a mass timber building, the external wall construction and the structural fire performance are the parts that attract questions.

Answer them at the level the stage requires. Gateway One is not the place for a full engineered solution, and a statement padded with detail that will change at Gateway Two invites a consultee response asking for consistency you cannot yet give. What it does need is a clear statement that the structure is mass timber, and an outline of how structural fire performance will be demonstrated.

Height, use and the external wall restriction

Regulation 7 of the Building Regulations restricts the combustibility of materials used in the external walls of relevant buildings above a height threshold that was lowered in 2022 and extended to further residential building types. Materials must achieve a high European reaction to fire classification, and timber does not.

The practical effect is that mass timber above the threshold works as an internal structure with a non-combustible external wall build-up, rather than as an exposed timber facade. Read the current definition of a relevant building and the current height threshold before assuming a scheme is outside scope, because both have moved.

Above 18 m, or seven storeys, a residential building is a higher-risk building under the Building Safety Act regime, with the full gateway process and the Building Safety Regulator as building control authority. That is a programme and cost issue rather than a planning one, and it belongs in the design and access statement only as far as it explains the form of the building.

Timber in lower and mid-rise buildings, in schools, offices and light industrial units, faces a much shorter list of constraints, and this is where most delivered mass timber in the country sits.

Insurance and funding as practical limits

Planning cannot refuse a scheme because an insurer dislikes it. Deliverability is a different matter, and a project that cannot be insured or funded will not be built, so the constraint reaches the planning process through viability and through the applicant's own risk appetite.

Three practical points for the design team:

• Warranty providers and insurers apply their own rules to exposed and encapsulated timber, and those rules are more restrictive than the Building Regulations in places
• Construction-phase insurance for a timber frame carries requirements on site fire management, watching briefs and combustible storage that add cost
• Lenders increasingly ask for the fire strategy at an early stage, which means the strategy has to exist before the planning application rather than after it

Test the insurance position before the application. A refusal at the funding stage after permission is granted is the worst sequence, because the redesign then needs a section 73 application or a fresh permission.

Encapsulation and what it does to the design

Structural fire design for timber works either by allowing a controlled depth of charring on exposed members, with the residual section carrying the load, or by protecting the timber so that it does not ignite within the required period. The second route is encapsulation, usually in layers of fire-rated board.

Encapsulation changes four things at once. It removes the visible timber that justified the choice aesthetically. It adds thickness, which either reduces internal dimensions or increases the overall height. It adds weight and cost, eroding part of the programme advantage. And it changes the junction details, because the encapsulation has to be continuous to work.

There is a further technical issue worth stating plainly. Some adhesive systems in laminated panels can lose bond strength in a prolonged fire, exposing fresh timber and hindering self-extinguishment, and this is an area of active research rather than settled practice. A submission that claims timber simply chars and stops overstates the position.

Decide the extent of exposure early and draw it. A building where the soffits are boarded and only the columns are exposed is a different building from the render on the planning application.

The carbon argument, accounted for properly

The carbon case for timber is real and it is frequently overstated. A whole life carbon assessment to the current RICS methodology reports biogenic carbon separately rather than netting sequestration off the upfront figure, because the carbon stored in the timber is released at end of life unless the material is reused.

So present two things: the upfront embodied carbon, which is usually lower than a concrete or steel frame, and the biogenic carbon reported as the methodology requires. Add the assumptions: sourcing and certification, transport distance, and the end-of-life scenario assumed for the panels. A Whole Life Carbon Assessment that shows its workings will survive scrutiny from a borough that reviews dozens of them a year.

Circular economy policy pushes the same way. Demountable connections, avoidance of adhesives and screeds that prevent reuse, and a material passport approach are the things that turn a carbon claim into a commitment.

Illustrative example: a five-storey office of about 4,200 m² with a CLT and glulam frame, exposed soffits in the office floors, fully encapsulated timber in the escape cores, a non-combustible external wall, and a carbon assessment that reports upfront and biogenic figures separately. Details here are illustrative only.

What the submission should say

Be specific and be conservative. Name the structural system. State whether timber will be exposed, and where, and note that the extent may change as the fire strategy develops. Set out the external wall approach and how it satisfies the combustibility restriction if the building is in scope. Explain how structural fire performance will be demonstrated, and by whom.

Then keep the fire documents consistent with each other. Where a Fire Safety Strategy follows the Gateway One statement, the two should not contradict on compartmentation or escape. Our notes on airspace development above flats and its evidence pack and on roof extensions and structural capacity evidence cover two cases where lightweight timber upward extension raises the same questions at a smaller scale.

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