Mass timber in tall residential buildings: three constraints

Mass timber tall building planning proposals in the residential sector run into three constraints, and only one of them is planning. The building regulations restrict combustible materials in the external walls of relevant buildings above a height threshold. The Building Safety Act regime brings buildings of at least 18 metres, or at least seven storeys, containing two or more residential units into the higher-risk category with its gateway process. And warranty providers and insurers apply their own limits, which are commercial rather than statutory and which frequently decide whether a scheme is fundable.

A planning officer cannot resolve any of those. What a planning authority can do is refuse to grant permission for a building that cannot be built, which is why the deliverability evidence matters at application stage.

Why timber structure raises fire questions

Cross laminated timber, glulam and laminated veneer lumber are structural materials that are also combustible. Under fire they char at a broadly predictable rate, and the charring layer insulates the section beneath, which is the basis for designing a mass timber element to retain load-bearing capacity for a stated period.

That is an established engineering approach. The concerns that sit on top of it are different ones: delamination of adhesive lines at high temperature exposing fresh timber, the contribution of exposed timber surfaces to fire growth in a compartment, the behaviour of connections, and whether a fire will self-extinguish once the fuel load burns out.

Those questions are being researched, and the answers are not uniform across products or build-ups. That is the honest position, and an applicant who presents mass timber as a settled matter will get a harder reception than one who acknowledges the uncertainty and explains how the design deals with it.

Regulation and guidance limits by height

The restriction that bites hardest is the prohibition on combustible materials in the external walls of relevant buildings, introduced in 2018 and extended since. Its scope, the building types it catches and the height threshold have all changed, so check the current regulations and the current approved document rather than relying on a summary.

The practical effect for a residential scheme above the threshold is that the external wall build-up has to be non-combustible. Mass timber can still be used for floors, internal walls and cores, provided it does not form part of the external wall, but the detailing at the wall-to-floor junction becomes the critical point and it is not trivial.

Separately, the higher-risk building definition under the Building Safety Act brings a scheme into the gateway process. Gateway One sits at planning and requires a fire statement with the application for relevant buildings. Gateway Two requires the regulator's approval before construction starts, and that is where a novel structural approach is actually tested.

Sequence accordingly. A scheme that clears planning on a fire statement and then fails at Gateway Two has spent a year.

Insurance and warranty constraints

This is the constraint most planning submissions ignore and most funders raise first.

Structural warranty providers publish technical requirements, and several apply restrictions or additional conditions to mass timber in residential buildings, particularly above certain heights or where timber is exposed. Buildings insurance during construction and in operation is priced against the same perceptions, and cover for timber-framed residential has been harder to place.

None of this is a planning matter in the narrow sense. It becomes one when a council asks whether the scheme is deliverable, or when a viability case depends on a build cost that assumes normal insurance terms.

The sensible approach is to engage a warranty provider and an insurance broker at concept stage, before the structural strategy is fixed, and to record the outcome. A one-page note confirming that a provider has indicated it will cover the proposed build-up is worth more in a committee report than three pages on embodied carbon.

The embodied carbon argument, stated properly

The carbon case for mass timber is real and it is frequently overstated.

Report biogenic carbon separately rather than netting sequestered carbon off the headline upfront figure. The recognised assessment methodology requires the biogenic element to be identified, and an assessment that shows a negative upfront number because sequestration has been deducted will be challenged by any competent reviewer.

Report end of life honestly too. Sequestered carbon is only retained while the timber is retained, and the assumptions about what happens at the end of the study period drive the result.

Then make the comparison that is actually useful: the same building in the same form, with a conventional structure, assessed on the same basis by the same assessor. The saving is usually substantial without any accounting adjustments, and presenting it that way survives scrutiny.

The Whole Life Carbon Assessment should state its treatment of biogenic carbon in its first two pages.

Evidence to submit at planning stage

Five documents make the difference between an officer who is comfortable and one who is not.

1. The fire statement required at Gateway One, prepared by someone with the necessary fire engineering competence.
2. An outline fire strategy describing compartmentation, means of escape, the approach to exposed timber surfaces, encapsulation where proposed, and the sprinkler or suppression strategy.
3. A structural strategy note explaining the system, the products, the connection types and the junction detailing at external walls.
4. A deliverability note covering warranty and insurance engagement.
5. The carbon assessment, with biogenic carbon reported separately.

Do not submit a fire strategy that relies on a performance-based approach without saying so. Performance-based fire engineering is legitimate and is scrutinised hard at Gateway Two, and flagging it early is better than having it discovered.

The same evidence set supports the wider fire case on mixed buildings, which is set out in fire strategy for a residential block above commercial units.

Interaction with the fire statement

A Fire Statement is required at Gateway One for relevant buildings under the Building Safety Act framework and the development management procedure order. A licensed or competent professional is legally required to prepare it, and an AI-assisted draft can only be a starting point for that professional rather than a substitute for their judgement or their sign-off.

The statement is about how the building's design affects fire safety, including site access for fire appliances, water supplies, the location of the building on the site and the principles of the evacuation strategy. On a mass timber scheme it should also address the structural material directly, because a statement that does not mention it invites the regulator to ask why.

Where the external wall build-up is the contested point, the interaction with balconies and cladding is covered in balconies, external walls and fire spread, and the internal escape route design in smoke control in common corridors at planning stage.

Where the approach currently works best

Below the height thresholds, in buildings that are not higher-risk, and in non-residential use, mass timber is a straightforward proposition with a good carbon case and a well-understood design route.

Hybrid structures also work: a concrete or steel core with timber floor plates, which keeps the stability system conventional and puts the timber where the carbon saving is largest. Residential schemes of four to six storeys, outside the higher-risk definition and with non-combustible external walls, are being delivered.

Above the thresholds, in residential, the constraint is rarely the engineering. It is the combination of the external wall restriction, the gateway process and the insurance market, and a scheme that has not addressed all three at concept stage is carrying a risk it has not priced.

Enter your postcode on PlanScope to see which reports a residential development is likely to need, including where a fire statement applies.

This is general guidance, not legal advice; check the position with your local planning authority 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. For specialist technical reports, qualified professional input may be required.