Cross laminated timber in a residential planning application

Cross laminated timber can be used in a residential planning application in England, but above 18 metres the external walls of a relevant building cannot contain combustible material, and that single rule shapes the whole design. Regulation 7(2) of the Building Regulations 2010 restricts what materials may become part of an external wall on relevant buildings over that height, and the 2022 amendment widened the building types caught. So a cross laminated timber residential scheme either stays below the threshold, or keeps the timber out of the external wall build-up, or becomes a hybrid.

Planning officers rarely ask about any of this directly. What they ask about is height, floor-to-floor dimensions, the carbon case and, on anything at 18 metres or seven storeys, the fire statement. All four are consequences of the structural choice, which is why the material decision belongs at the start of the design rather than after the massing is fixed.

Where mass timber is and is not permitted in residential buildings

Regulation 7(2) applies to external walls and specified attachments on relevant buildings, which include blocks of flats above the height threshold. Timber that forms part of an external wall on such a building is caught. Timber that sits inside the building, in floor plates, cores and internal structure, is not caught by regulation 7(2), although it still has to satisfy the fire resistance and surface spread requirements in Approved Document B.

That distinction produces the common solution on taller schemes: a timber structural frame and floors with a non-combustible external wall system, or a concrete core with timber upper floors. It is a compromise and worth describing honestly in the design statement rather than presenting the building as a timber building when the envelope is not.

Below the threshold the design freedom is much wider, and this is where most of the completed residential mass timber work in England sits. Four to six storeys of cross laminated timber over a transfer structure is a well-understood arrangement.

Height restrictions and their source

Two height numbers matter and they are not the same.

Where a scheme crosses either line, the planning submission needs a Fire Statement and the Health and Safety Executive becomes a statutory consultee on the application. A fire statement is a report type where fire-engineering competence is legally expected: it must be prepared by someone with the necessary competence, and an AI-assisted draft is a starting structure for that person rather than a substitute for them.

Government has consulted on changes to the combustible materials rules, including the height at which they bite. Check the position in force on gov.uk at the point of design freeze, because a scheme designed to an announced but uncommenced change is a scheme with a problem.

Encapsulation and exposed timber surfaces

Architects choose mass timber partly for the exposed soffit, and that is the detail most likely to be lost. Exposed timber contributes to fire load and to surface spread of flame, so the fire strategy will usually require encapsulation of timber surfaces in escape routes and often in habitable rooms, with limited areas left exposed under a reasoned argument.

Decide early how much exposure the strategy can support, because the answer changes the planning drawings. An encapsulated soffit raises the floor zone, which raises the building, which can breach a height limit in the local plan or push the scheme into the fire statement threshold. Officers notice when internal visualisations show bare timber and the sections show a plasterboard build-up.

Structural depth and floor to floor heights

A cross laminated timber floor plate carrying the same span as a concrete flat slab generally needs more depth once the acoustic build-up for Approved Document E and a services zone are added. Timber floors are light, which makes them good for impact sound only after mass is deliberately added back, usually as a screed or a loose fill layer.

The compounding effect is what catches schemes out. An extra 150 millimetres per floor across seven storeys is more than a metre of building height, and that metre has to come from somewhere: a lower ground floor, a reduced parapet, or an application that asks for more height than the policy allows. The arguments about daylight to neighbours follow directly, so test the massing for daylight to neighbours before committing to the structural depth. The practical questions officers have begun asking about these buildings are set out in the note on mass timber schemes and the questions planners ask.

Carbon case in the planning balance

Mass timber's planning advantage is embodied carbon, and it has to be quantified to carry weight. A Whole Life Carbon Assessment following the RICS methodology gives the comparison a defensible basis, with the sequestration and end-of-life treatment stated rather than assumed.

Be careful with the sequestration argument. Reporting biogenic carbon as a negative figure without the corresponding end-of-life module invites a challenge from any reviewer who knows the methodology, and in London the Greater London Authority's guidance on whole life carbon sets out how it expects the modules to be reported for referable schemes. State the comparison in the terms the assessment allows: upfront embodied carbon against a stated baseline, with the assumptions listed.

Illustrative example. A six storey, 32 flat scheme compared as a timber frame and as a reinforced concrete frame. The timber option showed lower upfront embodied carbon, a shallower foundation design because of reduced loads, and a longer floor zone. The planning case led on the first two and dealt with the third by accepting a lower top floor rather than asking for extra height. Figures are illustrative, but the trade-off is real and the officer respected the fact that it was stated plainly.

Documents to submit

1. Design and access statement describing the structural system, the external wall build-up and which surfaces are exposed
2. Fire statement where the building is 18 metres or seven storeys or more, prepared by a competent fire engineer
3. Whole life carbon assessment with the biogenic carbon treatment set out
4. Sections at a scale that shows the real floor build-up, not a notional slab
5. A structural note confirming the frame type, transfer structure and foundation implications
6. Acoustic design intent for separating floors, referencing Approved Document E

Keep the structural note short and factual. It is not a structural survey of an existing building, and it does not need calculations at planning stage, but it should say what the frame is, how loads reach the ground and what the floor zone is. Warranty and insurance terms for mass timber are a commercial matter to settle with the funder early, and what belongs on the planning file is covered in the note on structural warranties and the planning file.

One honest uncertainty: the regulatory position on combustible materials has moved more than once since 2018 and there is live policy work on it. Design to the rules in force, record the date you checked them, and keep a hybrid fallback for any scheme sitting within a metre or two of 18 metres.

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.