Mass timber at planning: the fire questions to answer early

Structural material choice is usually a Building Regulations matter and nobody's business at planning. Mass timber is the exception, because for residential buildings of 18 m or more, or seven or more storeys, containing two or more dwellings, article 9A of the Development Management Procedure Order 2015 requires a fire statement with the planning application, and the Building Safety Regulator is consulted on it. That is Planning Gateway One, and it puts the fire strategy on the planning file before a single detail is designed.

For a cross-laminated timber scheme, the questions that arrive at Gateway One are the ones that decide whether the frame survives value engineering later.

Why structural material choice reaches the planning file

Three routes bring it there.

Gateway One itself. A Fire Statement is a validation requirement for relevant buildings, and it has to address the principles of the fire safety design rather than simply confirming that Building Regulations will be met. A Fire Statement is legally required to be prepared by someone with the necessary fire engineering competence under the Building Safety Act 2022 framework, and an AI-assisted draft is a starting point for that person rather than a substitute for their judgement.

Appearance. Where timber is expressed externally, or where the structural grid is visible through glazing, the material is a design matter and belongs in the design statement.

Deliverability. Where an authority is being asked to accept a scheme on the strength of its carbon performance, and that performance depends on a timber frame, the officer will reasonably ask whether the frame is achievable.

Height and use restrictions that affect timber

The regulatory constraint people mean when they say "you cannot build in timber above 18 m" is regulation 7(2) of the Building Regulations 2010, which restricts combustible materials in the external walls of relevant buildings above that height. The categories of relevant building were extended in 2022, and the position is revised, so verify the current wording rather than relying on a summary.

Two distinctions matter and are frequently missed.

Regulation 7(2) is about external walls. A mass timber internal structure with a compliant external wall build-up is a different question from an exposed timber facade, and conflating them kills schemes that were viable.

Height is measured to the top storey floor level under the regulations, which is not the same as the parapet height a planning drawing shows. A scheme "at 18 m" on the elevation may or may not be caught.

Where the building falls below the thresholds, the constraints become those of the functional requirements, the fire strategy and the insurer, rather than a prohibition.

What a fire statement should address for timber structures

The statement is a planning document about principles. For a timber scheme, the principles an assessor looks for are:

• The evacuation strategy, and whether it is stay put or simultaneous evacuation, with the reasoning.
• Whether structural timber is encapsulated or exposed, and where.
• How the structure achieves the required fire resistance period, including charring allowance where timber is exposed.
• Compartmentation, and how junctions between timber elements are detailed to prevent hidden fire spread.
• Suppression, and whether sprinklers or another system form part of the strategy.
• External wall construction, and its relationship to regulation 7(2).
• Access and water supplies for firefighting, including appliance access routes and hydrant positions.

A Fire Safety Strategy to BS 9991 or BS 9999 develops this into the document the design team works to. It is a fire engineering document and requires an author with that competence.

Encapsulation and exposed timber decisions

Exposed timber is usually the reason the client chose timber, and it is the part most likely to be lost.

Encapsulation, typically with layers of fire-rated board, protects the timber and reduces its contribution to the fire load. It also hides the material, adds cost and adds programme. Partial encapsulation, with timber exposed to soffits in some rooms and encapsulated elsewhere, is a common resolution, and it needs to be shown in the fire statement rather than left as an aspiration.

Where timber is exposed, the design relies on a charring allowance: the outer layer chars at a broadly predictable rate and protects the section beneath, so the element is sized with sacrificial depth. That approach is well established in the Eurocode for timber fire design, and it interacts with connection detailing, because steel connectors lose strength faster than the timber around them.

Decide encapsulation early. It changes floor to ceiling heights, which changes building height, which can change whether regulation 7(2) applies at all.

Construction phase fire risk and site management

A mass timber frame is at its most vulnerable during erection, before compartmentation and suppression are in place. Insurers know this, and so do fire and rescue services.

For planning purposes the relevant document is the Construction Management Plan, which should cover hot works control, site security, temporary detection and alarm, water supply for firefighting during construction, and the phasing of protective layers relative to frame erection. The industry joint code of practice on fire prevention on construction sites is the reference point, and a plan that names it and follows it answers most of what a council and an insurer will ask.

Neighbouring occupiers matter here too. A timber frame going up against an occupied terrace attracts objections that a steel frame does not.

Insurance and warranty positions

This is not a planning matter and it will decide whether the scheme is built.

Insurers and structural warranty providers apply their own underwriting positions to mass timber, and those positions are commercial rather than regulatory. They change. A scheme designed around a timber frame without an early conversation with the intended warranty provider and the project insurer carries a real risk of a late structural change.

Raise it at RIBA Stage 2, get a position in writing, and record the assumptions in the design team's risk register. An officer will not ask about it, and a funder will.

Coordinating with the carbon case

Timber's attraction is embodied carbon, and the Whole Life Carbon Assessment is where that is demonstrated. In London, referable schemes are assessed against the Mayor's whole life carbon guidance, and the RICS professional standard sets the method more widely.

Keep the two documents consistent. A carbon assessment modelled on an exposed timber structure and a fire statement describing full encapsulation are describing different buildings, and a consultee who reads both will notice. Encapsulation adds material and therefore carbon, and the assessment should reflect the design that is actually proposed.

Illustrative example: a six-storey residential block at 17.4 m to the top storey floor level, with a CLT frame, encapsulated in circulation cores and exposed to apartment soffits, sprinklered throughout. The scheme sat below the Gateway One threshold, and the fire strategy was still prepared to the same standard because the warranty provider required it. This example is illustrative.

One honest uncertainty: the regulatory position on combustible materials has moved several times since 2018 and may move again. Date every statement of the rules in your design record.

PlanScope drafts planning documents including fire, carbon and construction statements, structured to UK planning policy, with technical inputs flagged for the competent professional to complete. For specialist technical reports, qualified professional input may be required, and the Fire Statement and Fire Safety Strategy both need a qualified fire engineer. Browse the full report list.

This is general guidance, not legal advice. Check the position with your local planning authority, the Building Safety Regulator or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.