Mass timber schemes and the questions planners now ask

Planning officers have started asking sharper questions about engineered timber, and the questions are not really about planning policy. A mass timber planning application succeeds or fails on whether the applicant can show that the building can be built as drawn, within the building regulations that restrict combustible materials, and that the structure will be insurable and warranted. The carbon argument is the reason to use timber. The fire and insurance position is what decides whether the scheme survives to technical design.

So the planning submission has to carry more information than a concrete equivalent, and it has to be clear about the height band the building sits in.

Where timber structures sit in the regulations

Two regimes matter. Regulation 7(2) of the Building Regulations restricts the use of combustible materials in the external walls of relevant buildings above a stated height, with the detail in Approved Document B, and the scope has been amended more than once since 2018. Separately, the Building Safety Act 2022 framework treats higher-risk buildings, defined by reference to height of 18 metres or seven storeys and by use, through the three gateways administered by the Building Safety Regulator.

The distinction that gets lost is between the structure and the external wall. Engineered timber used as internal structure, floors, walls and cores is not caught by the external wall restriction, provided the external wall build-up itself uses materials of the required classification. Many delivered schemes rely on exactly that arrangement.

Check the current text of Approved Document B rather than a summary, including this one, because the amendments have changed both the heights and the building uses in scope.

Height bands and the fire position

Design intent is set by which band the building falls into. In broad terms:

Most delivered mass timber housing in England sits below the higher-risk threshold, and that is a design decision rather than an accident. Pushing a timber scheme through the 18 metre line changes the consenting route, the insurance conversation and the cost at the same time.

For a relevant building, a Fire Statement is required with the application under Article 9A of the Development Management Procedure Order 2015, and it must be prepared by someone with the necessary fire engineering competence. A timber structure makes that document more substantive rather than merely a form-filling exercise. The phasing traps are covered in our note on phased applications and the Gateway One duty.

Encapsulation and exposed timber

Clients want visible timber, because it is most of the point. Fire engineering wants it covered.

The technical positions available are encapsulation with gypsum linings so that the timber does not contribute to the fire, partial exposure with a designed charring allowance under the Eurocode approach to timber in fire, and full exposure in low-rise buildings where the risk profile allows. Each has consequences for the structural sizing, because a charring allowance means adding material that is expected to be sacrificed.

Three practical points that recur:

• connections are the weak link, and protecting steel connectors within timber elements is a detailed design problem rather than a specification note
• compartmentation at floor junctions needs designing around the timber build-up, including cavity barriers and service penetrations
• exposed soffits change the acoustic and the fire strategy at once, so the two consultants need to be in the same room early

At planning stage, show which approach the scheme assumes. An application that shows exposed timber soffits in the visuals and says nothing about the fire strategy invites a Gateway One comment that lands after determination.

Embodied carbon as the counterweight

The reason to accept all of that is the carbon case, and it needs to be quantified rather than asserted. A Whole Life Carbon Assessment to the RICS methodology sets out the upfront embodied carbon, the operational carbon and the end of life position, module by module.

Two points of rigour. Biogenic carbon, the carbon sequestered in the timber, is reported separately rather than netted off the upfront figure, and an assessment that quietly offsets one against the other will be picked apart by a reviewer. And the end of life assumption matters: a timber frame designed for disassembly with mechanical fixings supports a different end of life narrative from one glued and finished in place.

Where a scheme is in London or in an authority with its own carbon policy, the assessment format is usually prescribed, and the reporting template matters as much as the result.

Insurance and warranty questions

This is the question that stops schemes, and it arrives from the funder rather than the council. Warranty providers publish technical requirements for engineered timber, and several restrict exposed structural timber, require specific moisture management during construction, or ask for a design review before work starts. Insurers price construction period risk on a timber frame differently from concrete, and cover for the completed building is a separate conversation again.

None of that is a planning consideration in the strict sense. But an officer who has seen a consented timber scheme rebuilt in concrete under a section 73 application will ask, and a short paragraph in the design statement confirming that warranty and insurance routes have been tested is worth writing. Approach the warranty provider and the insurer before the application, not after.

Evidence to submit at planning stage

A proportionate pack for a mass timber scheme adds the following to the usual documents:

1. A structural approach statement naming the system, the grid, the floor build-up and the assumed fire strategy.
2. The fire statement where the building is a relevant building, prepared by a competent fire engineer.
3. The whole life carbon assessment, with biogenic carbon reported separately and the end of life scenario stated.
4. A construction logistics note, since volumetric or panelised delivery changes vehicle numbers and crane requirements.
5. A moisture management statement covering weather protection during erection, which is where timber schemes most often suffer.
6. An acoustic strategy for the floor build-ups, because lightweight floors need designing for impact sound rather than mass.

Item four is worth more than it looks. Fewer, larger deliveries and a shorter programme is a real planning benefit on a constrained urban site, and it belongs in the construction management documents rather than being left unsaid.

Designing to keep the option open

The pragmatic route on a scheme where the structural material is not yet settled is to design so that either system works: a grid that suits both, floor to floor heights that accommodate the deeper timber build-up, core positions that work for both stability approaches, and a facade design that does not depend on the structural material.

That costs a little in efficiency and it protects the planning permission. A consent obtained on a timber scheme that has to be rebuilt in concrete means a section 73 application, a revised carbon assessment, and an officer asking why the carbon benefit that supported the permission has gone. Whether that variation drags the fire documents with it is covered in our note on whether a section 73 application needs a new fire statement.

One honest uncertainty: the regulatory position on combustible materials has changed several times since 2018 and further amendments are possible, so anything written about the thresholds needs checking against the current Approved Document B and the Building Safety Regulator's published guidance at the time you submit. The report types by category page shows which of the supporting documents PlanScope drafts, and the fire documents are ones where a competent professional is legally required rather than optional.

This is general guidance, not legal advice; the fire strategy must come from a suitably qualified fire engineer. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.