Mass timber frames at planning stage: fire and insurance

Choosing a mass timber structure changes what your planning submission has to say about fire, and it changes who will insure the building. Those two consequences arrive early, well before any Building Regulations application, because for a relevant building the fire strategy has to be summarised in a Fire Statement at Planning Gateway One. A scheme that reaches submission with the structural system still open, and the fire and insurance positions unresolved, tends to lose months at the first consultation response.

Cross-laminated timber, glued laminated timber and laminated veneer lumber each behave differently in fire, and the planning-stage question is narrower than the engineering one. It is whether the proposal is deliverable at this height, in this use, with this structure.

Where timber structure meets the planning system

Fire enters the planning process through Article 9A of the Town and Country Planning (Development Management Procedure) (England) Order 2015. For a relevant building, the application must include a Fire Statement, and the Health and Safety Executive is a statutory consultee at that stage. The relevant building definition is tied to the higher-risk threshold of 18 metres or seven storeys with two or more dwellings, so many mass timber residential schemes fall inside it and many timber schools, offices and light industrial buildings do not.

A Fire Statement is not a full fire engineering report. It sets out the principles: how the building will be reached by the fire and rescue service, the strategy for escape and for compartmentation, water supplies, and how the design has taken fire into account at the level of detail available. Where the structure is combustible, that last point is the one the consultee reads carefully.

A Fire Statement under the Building Safety Act 2022 framework has to be prepared by someone with the necessary fire engineering competence, and the same is true of a Fire Safety Strategy written to BS 9991 or BS 9999. A draft is a starting point for that professional rather than a substitute for them.

Height limits and use restrictions to check first

Three separate sets of limits apply, and they are often confused with each other.

Regulation 7 restricts combustible materials in the external walls of relevant buildings above the stated height, and the scope of that restriction has been amended more than once. Check the current Approved Document B and the regulation text at design stage rather than working from a remembered threshold, because the answer determines whether an exposed timber facade or a timber balcony is possible at all.

The frame is a different question from the external wall. A mass timber frame behind a non-combustible external wall build-up is a common and deliverable arrangement. Exposed internal timber soffits and columns are where the strategy gets harder.

Encapsulation and exposed timber in the fire strategy

Encapsulation means lining the timber so that it does not contribute to the fire for a defined period. Full encapsulation of walls, columns and soffits simplifies the fire case considerably and removes most of the argument about charring rates, delamination at glue lines and secondary ignition. It also removes the visible timber that was often the point of the design.

The planning-stage decision is therefore an architectural one with a fire consequence. Decide which surfaces need to be visible, get the fire engineer to test that against the escape and compartmentation strategy, and record the outcome in the Fire Statement. If the answer is that the exposed soffit in the communal stair cannot be justified, better to know before the visualisations are commissioned.

Sprinklers frequently come into this conversation, and the choice has planning consequences of its own around water supply, tank and pump space and access, which is why sprinklers and the planning consequences of the choice is worth settling alongside the structure rather than after it.

Insurer and warranty provider positions

This is the constraint that most often kills a timber scheme, and it sits outside the regulatory system entirely. Insurers and warranty providers set their own criteria for combustible structure: height limits, encapsulation requirements, sprinkler requirements, restrictions on certain uses, and conditions on site fire safety during construction. Those criteria are commercial positions, they differ between providers, and they change.

Two practical steps. Get an indicative position in writing from an insurance broker and from the intended warranty provider before the planning submission, on the actual height, use and structural arrangement proposed. And build the answer into the appraisal, because a requirement for full encapsulation plus sprinklers plus a specified construction-phase regime has a cost that has to be carried somewhere.

The honest uncertainty here is that a position given at feasibility stage is indicative. Underwriting criteria at financial close may differ from those quoted 18 months earlier, and that risk sits with the developer.

Carbon benefits and how to evidence them

The carbon case for timber is real and it is frequently overstated in planning statements. A Whole Life Carbon Assessment prepared to the RICS professional standard is the document that makes the case properly, and the discipline it imposes matters. Report embodied carbon by life cycle module rather than as a single headline figure. Report biogenic carbon separately from fossil carbon rather than netting sequestration off the total, because netting produces a number that cannot be compared with a concrete or steel option.

Then say what happens at end of life. A timber frame's carbon advantage depends on assumptions about module C and module D, and an assessment that assumes reuse without any mechanism to secure it is making an optimistic claim. In London, Greater London Authority guidance sets the expected format for whole life carbon submissions on referable schemes, and boroughs increasingly ask for the same structure.

Coordinating the documents before submission

The sequence that works:

1. Fix the height, use and unit count, then establish whether the building is a relevant building for Article 9A purposes.
2. Get an indicative insurance and warranty position on the structural system at that height and use.
3. Agree with the fire engineer which surfaces can remain exposed and what that requires elsewhere in the strategy.
4. Reflect the outcome in the Fire Statement, the design and access statement and the whole life carbon assessment so the three documents describe the same building.
5. Check the external wall build-up against the current regulation and Approved Document text, particularly for balconies and any timber rainscreen.

Contradictions between documents are what consultees notice. A Fire Statement describing full encapsulation alongside visualisations of exposed timber ceilings, or a carbon assessment claiming sequestration benefit from material the fire strategy requires to be lined, invites a holding objection. The same discipline applies to external wall materials generally, and the choice between BS 9991 and BS 9999 needs making before the strategy is drafted rather than in the middle of it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required: a Fire Statement and a Fire Safety Strategy must be prepared by someone with the necessary fire engineering competence, and an AI-assisted draft is a starting point for that professional rather than a replacement. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.