This guide provides general information about Approved Document B and its application to timber cladding in England. It does not constitute fire engineering or legal advice. Building regulations compliance must be confirmed with your building control body or a qualified fire engineer for your specific project. Timber Cladding Specialists is a material supplier, not a fire safety consultant.
- For most UK houses and extensions below 11m — timber cladding can be used subject to boundary distance rules. No Euroclass minimum is prescribed for the cladding material itself.
- For residential buildings with a storey at 11m or more — the 2022 amendments introduced a prohibition on combustible materials in external walls. Timber cladding requires a fire-engineered alternative approach agreed with building control.
- Boundary distance rules apply to all buildings — combustible cladding counts as an unprotected area. The closer the building is to the boundary, the more limited the unprotected area permitted.
- The 2026 amendments (effective 30 September 2026) focus on escape stair provisions in blocks of flats — they do not change the 11m combustibility rules for external cladding.
- Fire-rated timber cladding achieving Euroclass B-s1,d0 is available — factory-applied fire retardant treatment — as an alternative approach for higher-risk applications.
Approved Document B is the statutory guidance document that sets out how to meet Requirement B4 of the Building Regulations — external fire spread. For timber cladding, Approved Document B is the primary compliance framework that determines whether a proposed cladding specification is acceptable, and under what conditions. This guide explains the key provisions relevant to timber cladding — the 11m threshold, combustibility rules, boundary distance requirements, Euroclass ratings, and the 2022, 2025, and 2026 amendments — in plain language aimed at architects, self-builders, and developers specifying timber cladding on UK projects. For a broader overview of building regulations and timber cladding see our timber cladding building regulations guide and our self build cladding guide.
What Is Approved Document B?
Approved Document B (AD B) is the statutory guidance document published by the Secretary of State giving practical guidance on how to meet the fire safety requirements of the Building Regulations 2010 for England. It covers Requirements B1 to B5 of Schedule 1 to the Building Regulations, addressing means of escape, internal fire spread, structural fire resistance, external fire spread, and fire service access.
AD B is published in two volumes. Volume 1 covers dwellings — houses and flats. Volume 2 covers all other building types — commercial, educational, healthcare, industrial, and mixed-use buildings. For most timber cladding projects on residential new builds, extensions, and conversions, Volume 1 is the primary reference. For commercial and mixed-use projects, Volume 2 applies.
The current edition is the 2019 edition incorporating amendments made in 2020, 2022, 2025, and forthcoming changes scheduled for 2026 and 2029. The provisions relevant to external cladding are primarily found in Requirement B4 — External fire spread — and the associated Sections 10 and 11 of Volume 1, and Sections 12 and 13 of Volume 2.
Amendment History — What Has Changed Since Grenfell
The 11 Metre Rule — The Most Important Threshold for Timber Cladding
The most significant provision of Approved Document B for timber cladding is the height threshold introduced by the 2022 amendments — commonly referred to as the 11 metre rule. This rule applies to residential buildings (purpose groups 1 and 2 — dwellings and residential institutions) with a storey at 11 metres or more in height above ground level.
For residential buildings (including houses converted to flats and blocks of flats) with a storey at 11m or more above ground level, the external walls must not contain combustible materials. Untreated timber cladding is a combustible material. The 11m measurement is to the floor level of the highest storey, not to the top of the building. A four-storey block of flats will typically trigger this requirement.
Below 11m — for the vast majority of UK houses, two-storey and three-storey residential buildings, extensions, and garden rooms — this prohibition does not apply. Timber cladding can be used on buildings below 11m subject to the boundary distance rules described in the next section.
| Building type | Height | Timber cladding permitted? | Conditions |
|---|---|---|---|
| House / extension | Below 11m (typical) | ✓ Yes | Subject to boundary distance rules |
| Garden room / outbuilding | Below 11m | ✓ Yes | Usually exempt from Building Regs entirely |
| Flat conversion (up to 3 storeys) | Below 11m typically | ✓ Yes | Subject to boundary distance rules |
| Block of flats (4+ storeys) | 11m or more | ✗ Restricted | Combustibility prohibition applies — fire-engineered alternative approach required |
| Commercial building | Any height | ✓ Generally yes | Boundary distance rules apply — Vol 2 Sections 12 and 13 |
| Mixed-use residential over 11m | 11m or more | ✗ Restricted | Residential portions subject to combustibility prohibition |
Boundary Distance Rules — Unprotected Areas
For buildings below the 11m threshold where timber cladding is permitted, the key compliance issue is the relationship between the building and its site boundary. AD B Section 11 (Volume 1) and Section 13 (Volume 2) set out rules about unprotected areas — parts of the external wall that do not provide the required fire resistance, including combustible cladding materials and window and door openings.
The closer a building is to the boundary, the less unprotected area is permitted. This reflects the fire safety principle that a building close to the boundary poses a greater risk of fire spreading to adjacent buildings — requiring more of the wall to be of protected (fire-resisting) construction. Buildings with more than 1 metre of distance to the boundary have progressively more flexibility in unprotected area.
For most detached and semi-detached houses set back from the boundary, the permitted unprotected area will comfortably accommodate timber cladding across all elevations. For terraced houses, buildings on tight urban plots, and extensions close to the party wall boundary, the unprotected area calculation may limit the extent of timber cladding — particularly on the elevation facing the boundary. Confirm with your building control officer at design stage, not after materials are ordered.
The detailed calculation methods for acceptable unprotected areas are set out in Appendix B of both volumes of Approved Document B, using the Enclosing Rectangle method or the Aggregate Notional Area method. These calculations are typically carried out by a fire engineer or experienced building control officer — they are not something most self-builders or contractors need to calculate themselves, but it is important to understand that they exist and to ensure your building control application addresses them.
Regulation 7(2) — The Combustibility Ban
The 11m combustibility prohibition described above is implemented through Regulation 7(2) of the Building Regulations 2010, which was amended in 2022. Regulation 7(2) requires that where a residential building has a storey at least 11m above ground level, specified attachments to the external wall — including cladding — must be of a European Classification A1 or A2-s1,d0 or better. These are the non-combustible classifications under the European reaction to fire system.
Timber, even when fire-retardant treated, does not typically achieve A1 or A2 classification. Fire-retardant treated timber achieves Euroclass B-s1,d0 at best — a combustible material with limited flame spread. This means that strictly under Regulation 7(2), fire-retardant treated timber cladding does not satisfy the prohibition for residential buildings over 11m.
However, AD B acknowledges that alternative approaches to compliance are possible where agreed with building control. A fire-engineered approach — typically involving a fire engineer's assessment demonstrating that the overall external wall system achieves an equivalent level of safety to the prescriptive requirement — may allow Euroclass B-s1,d0 treated timber to be accepted on residential buildings over 11m in specific circumstances. This is not a blanket permission — it requires case-by-case assessment.
Euroclass Reaction to Fire — What the Classifications Mean
The Euroclass system classifies materials by their reaction to fire — how they behave when exposed to fire. It replaced the previous UK national class system (Class 0, Class 1, Class 2, Class 3) following the 2025 amendments to AD B. The classifications relevant to external cladding are:
| Euroclass | Description | Typical materials | AD B significance |
|---|---|---|---|
| A1 | Non-combustible — no contribution to fire | Concrete, brick, steel, mineral wool | Satisfies Regulation 7(2) — buildings over 11m |
| A2-s1,d0 | Non-combustible — limited contribution | Some coated metals, mineral boards | Satisfies Regulation 7(2) — buildings over 11m |
| B-s1,d0 | Combustible — very limited flame spread | Fire-retardant treated timber | Does not satisfy Reg 7(2) — may be accepted via fire-engineered alternative approach |
| C-s2,d0 | Combustible — limited flame spread | Some treated timber products | Does not satisfy Reg 7(2) — limited suitability for high-risk applications |
| D and below | Combustible — normal contribution to fire | Untreated timber, many plastics | Not suitable for buildings where fire performance is a key requirement |
The s1,d0 suffix in the Euroclass designation refers to smoke production (s1 = very limited smoke) and flaming droplets (d0 = no flaming droplets). Both suffixes are important for external cladding — smoke production and flaming droplets both affect the safety of building occupants during evacuation.
For guidance on obtaining Euroclass B-s1,d0 fire-retardant treated timber cladding see our fire rated timber cladding guide, our Euroclass B cladding guide, and our fire retardant treatment service.
Practical Application — Common Project Types
Single and two-storey houses and extensions
The vast majority of UK residential timber cladding projects — new build houses, house extensions, and conversions — are below 11m in height and not subject to the Regulation 7(2) combustibility prohibition. Timber cladding can be used freely subject to the boundary distance rules in Sections 10 and 11 of Volume 1. For most houses with reasonable setback from the boundary, compliance is straightforward and does not require a fire engineer. Confirm with your local building control officer at design stage. Our most widely specified species for residential projects are Siberian larch and ThermoWood — both naturally durable, available in contemporary profiles, and suitable for all standard residential applications below 11m.
Garden rooms and outbuildings
Most garden rooms, garden offices, and domestic outbuildings are either fully exempt from Building Regulations or subject only to limited requirements. A detached building not used as sleeping accommodation with a floor area under 30m² is generally exempt. For buildings over 30m², boundary distance rules may apply — but the unprotected area calculation for a typical garden room is unlikely to cause any issue with timber cladding given the typically generous setback from boundaries in garden settings. For planning considerations see our planning permission for timber cladding guide.
Blocks of flats and residential apartments
This is the most complex category for timber cladding under AD B. Blocks of flats with a storey at 11m or more are subject to the Regulation 7(2) combustibility prohibition, effectively ruling out untreated timber cladding. Where a fire-engineered alternative approach is being considered to allow Euroclass B-s1,d0 treated timber, a qualified fire engineer must be engaged at early design stage — not after planning approval. Browse our fire rated cladding range for available treated products.
Commercial buildings
Commercial buildings are governed by Volume 2 of AD B. The 11m combustibility prohibition under Regulation 7(2) applies specifically to residential buildings — commercial buildings are not subject to the same blanket prohibition. However, boundary distance rules under Sections 12 and 13 apply, and for some high-occupancy or high-risk commercial uses additional fire performance requirements may be appropriate. Confirm with building control at design stage. Our Nordic spruce and charred timber ranges are widely used on commercial projects.
Fire-Rated Timber Cladding — When and How to Specify It
Factory-applied fire retardant treatment can achieve Euroclass B-s1,d0 on a range of timber cladding species and profiles — including Siberian larch, ThermoWood, and Nordic spruce. This treatment is applied in a controlled factory environment under controlled conditions and is certified by an accredited testing laboratory. It is not a site-applied paint or coating — it requires factory treatment as part of the production process.
Fire-retardant treated timber cladding is appropriate for:
- Residential buildings over 11m where a fire-engineered alternative approach is being considered
- Commercial projects where the fire engineer or building control specifies improved fire performance
- Buildings near boundaries where unprotected area calculations are tight
- Projects where the client or architect wants enhanced fire performance for insurance or risk management reasons
- Some planning conditions in sensitive locations that specify fire performance requirements
Fire-retardant treatment adds approximately 10–20% to the material cost of the cladding and typically adds 3–6 weeks to the lead time. It should be specified at early design stage to allow sufficient time for production. See our fire retardant treatment service, our factory coatings service, and our fire rated cladding for house extensions guide for full details.
For the vast majority of UK timber cladding projects — houses, extensions, garden rooms, and commercial buildings below 11m — Approved Document B does not prohibit timber cladding. The key requirements are: satisfying boundary distance rules for unprotected areas (confirm with building control); using correctly installed ventilated cavity construction (satisfies Approved Document C moisture requirements); and using A4 stainless steel fixings throughout. For buildings over 11m, engage a fire engineer at early design stage before specifying materials.
What does Approved Document B say about timber cladding?
Approved Document B Requirement B4 sets out the rules for external wall materials. For residential buildings with a storey at 11m or more, the 2022 amendments introduced a prohibition on combustible materials in external walls. For buildings below 11m, timber cladding can be used subject to boundary distance rules. The relevant sections are 10 and 11 (Volume 1) and 12 and 13 (Volume 2).
What is the 11 metre rule for timber cladding?
The 11 metre rule, introduced by the 2022 amendments to AD B, requires that residential buildings with a storey at 11m or more above ground level must not use combustible materials in external walls. This effectively prohibits untreated timber cladding on tall residential buildings. Fire-retardant treated timber achieving Euroclass B-s1,d0 may be considered via a fire-engineered alternative approach, subject to agreement with building control.
What Euroclass rating is required for timber cladding?
For residential buildings over 11m where a fire-engineered alternative approach is agreed with building control, timber cladding must achieve a minimum of Euroclass B-s1,d0. For buildings below 11m, no specific Euroclass minimum is prescribed for the cladding material itself, though boundary distance rules must be satisfied.
Does Approved Document B apply to garden rooms and outbuildings?
Most garden rooms and domestic outbuildings are exempt from Building Regulations entirely. A detached building not used as sleeping accommodation with a floor area under 30m² is generally exempt. For buildings over 30m², boundary distance rules under AD B may apply, but a typical garden room with reasonable setback from boundaries is unlikely to face any issue with timber cladding.
What are the boundary distance rules for timber cladding under Approved Document B?
AD B Sections 11 and 13 set limits on unprotected areas of external walls relative to the boundary. Combustible cladding counts as an unprotected area. The closer the building is to the boundary, the more limited the permitted unprotected area. The calculation methods are in Appendix B of both volumes and are typically carried out by a fire engineer or building control officer.
Fire-Rated Timber Cladding — Euroclass B-s1,d0 Treatment Available
We supply timber cladding with factory-applied fire retardant treatment achieving Euroclass B-s1,d0 — available in Siberian larch, ThermoWood, and Nordic spruce across a range of profiles. We can provide full product test certificates and fire performance data for building control submission. Contact our team to discuss your project requirements.