Part A
Approved Document A: Structure
The statutory requirement covering structural safety of buildings in England (Wales has its own Approved Document A). Approved Document A sets out acceptable methods for meeting Part A: foundation design (type, depth, width, concrete grade), wall and roof structural calculations, stability against disproportionate collapse, and combined dead/imposed/wind loading. For extensions, Approved Document A para 2E4 gives a minimum depth of 0.45m to the underside of strip foundations to avoid frost and, on shrinkable clay, at least 0.75m, 0.9m and 1.0m on low, medium and high shrinkage clay, deeper near trees (NHBC Chapter 4.2 uses the same figures), which is why 1m is the common working depth on clay. It also covers strip foundation sizing and steelwork design. Your structural engineer designs to Part A and building control checks against it.
Also known as
Part AApproved Document AAD A
What Part A requires for a home project
The Part A requirements and terms the buildwiz guides rely on, with paragraph references where the document gives one. Check the current edition on gov.uk before you build to it.
Limits and figures
- BS EN 845-1 joist hanger product standard
- Mandatory product standard for joist hangers used in UK construction, superseding the older BS 6178. Proprietary hangers from Simpson Strong-Tie, Expamet (BAT), BPC Fixings and similar carry BS EN 845-1 certification with declared characteristic load capacities. Site-fabricated hangers cut from flat steel are not certified and will not be accepted by building control. Required for compliance with Approved Document A and for NHBC and LABC sign-off.
- C25/30 concrete: 28-day characteristic cylinder strength
- Minimum characteristic cylinder compressive strength at 28 days for a BS 8500 C25/30 mix (30 N/mm² on a cube). Ordered as C25/30 or as the designated concrete RC25/30; there is no GEN 4 (the GEN grades stop at GEN 3, which is C16/20). The grade most structural engineers specify for domestic extension strip and trench-fill foundations; Approved Document A's minimum for plain foundations in non-aggressive ground is lower, GEN 1 or ST2. Also the usual minimum for on-site concrete padstones carrying steel beam end bearings.
- Cavity wall tie maximum horizontal spacing
- Maximum horizontal spacing between stainless steel wall ties in a cavity wall, per Building Regulations Part A. Combined with the 450mm vertical limit, this gives a minimum density of 2.5 ties per square metre. Ties must slope slightly downward toward the outer leaf to drain water away from the inner leaf.
- Cavity wall tie maximum vertical spacing
- Maximum vertical spacing between stainless steel wall ties in a cavity wall, per Building Regulations Part A. Tightens to 300mm vertical spacing within 225mm of openings, corners, and edges. Combined with the 900mm horizontal limit, gives a minimum 2.5 ties per m² in the field.
- Extension foundations minimum depth to firm ground
- Commonly cited minimum depth for extension trench-fill foundations in normal ground conditions. Approved Document A sets a performance requirement (reach undisturbed load-bearing strata below the zone of seasonal moisture movement), not a fixed figure, but 1m is the working baseline for most domestic extensions on stable ground away from trees. Clay soils and tree proximity push this deeper.
- Heavy lateral restraint strap: minimum section
- Cross-section of a heavy lateral restraint strap. Approved Document A paragraph 2C35 asks for tension straps conforming to BS EN 845-1 with a declared tensile strength of at least 8 kN, and its Diagram 15 shows a 30 x 5 mm galvanised mild steel (or other durable) strap at least 1200 mm long. Straps of another section are acceptable where the maker declares the 8 kN strength.
- Holding-down strap maximum centres at wall plate
- Maximum spacing between galvanised holding-down straps tying the wall plate to the inner blockwork leaf. Resists wind uplift on the roof. Approved Document A para 2C36 calls for vertical straps at least 1m long at eaves level at no more than 2m centres (they may be omitted on some tiled roofs of 15 degrees or more), to BS EN 845-1 in galvanised steel or better.
- Holding-down strap minimum length
- Minimum length of a galvanised holding-down strap securing the wall plate to the blockwork inner leaf (Approved Document A para 2C36: vertical strapping at least 1m long, at no more than 2m centres). NHBC gives 30x2.5mm as the minimum strap section. The strap is turned and nailed over the wall plate.
- Horizontal wall chase maximum depth as fraction of wall thickness
- Maximum depth for a horizontal chase cut into a single-leaf masonry wall under Approved Document A. On a 100mm block this gives 16mm. Horizontal chases carry more structural risk than vertical ones because they cut across the wall's load path.
- Lateral restraint hanger spacing on long masonry walls
- Approved Document A Diagram 15c requirement for masonry walls exceeding 3 m in length: lateral restraint must be provided to the wall by restraint-type joist hangers (or hold-down straps) at maximum 2 m centres along the wall. Standard joist hangers carry vertical floor load only; restraint hangers additionally tie the joist to the wall and prevent the wall bowing outward under wind or live load. The structural engineer specifies which hangers must be of the restraint type: site-substituting standard hangers for restraint hangers is a structural defect and a building-control reject.
- Lateral restraint strap: maximum centres
- Maximum centre-to-centre spacing for lateral restraint straps at eaves level, along roof slopes, and at gable bases under Approved Document A. The 2 m rule covers buildings up to three storeys (England and Wales) or two storeys (Scotland). Straps prevent walls being pushed outwards by roof spread and stop the roof lifting under wind uplift. Positions are specified on structural engineer drawings and checked by the BCO at pre-plaster stage.
- Lateral restraint strap: minimum declared tensile strength
- Minimum tensile resistance required for lateral restraint straps under Approved Document A when used in domestic construction. Straps must be CE marked to BS EN 845-1 with a declared strength of at least 8 kN. Simpson Strong-Tie, Sabrefix and BPC heavy-duty straps all meet this with margin. Light-duty straps used for vertical wall-plate restraint are a separate specification with lower declared loads.
- Lateral restraint strap: minimum masonry downturn
- Common manufacturer and warranty-provider detail for the bent end of a lateral restraint strap: a downturn of at least 100 mm turned over an uncut block of the inner leaf and built in as the wall goes up. Approved Document A Diagram 16 shows the strap turned over an uncut block but does not give a length. Without the downturn the strap relies on face fixings, which lack the capacity of a built-in strap.
- Lateral restraint strap: minimum truss span
- Minimum number of structural timber members a lateral restraint strap must span to distribute tensile load adequately. Straps fixed to only one or two trusses concentrate load and defeat the distributed-restraint principle Approved Document A is built on. Straps must fix to noggings between trusses, or to the truss members themselves, not to longitudinal bracing across the truss tops, a common but non-compliant builder shortcut.
- Standard aerated block compressive strength
- Compressive strength of standard-grade aerated concrete (aircrete) blocks used for inner-leaf cavity wall construction in UK domestic buildings up to two storeys. Approved Document A confirms 3.6 N/mm² is acceptable for this application. Higher-strength 7.3 N/mm² variants (Thermalite Hi-Strength 7, Celcon equivalent) are specified above 2.5 storeys or for higher load paths.
- Strip foundation minimum concrete thickness
- Minimum thickness T of a plain concrete strip foundation under the Approved Document A Section 2E rules: 150 mm or P, whichever is the greater, where P is how far the strip projects beyond the face of the wall it carries. The width of the strip comes from Table 10 (load per metre run of wall and the type of ground), so heavier walls on weaker ground get a wider strip, a larger P and therefore a thicker strip. There is no separate figure by number of storeys: a two-storey wall reaches a greater thickness only through its wider strip. A structural engineer's design can specify more.
- Usual concrete grade for habitable extension foundations
- Grade most structural engineers specify for strip, trench-fill or raft foundations under habitable extensions, in line with NHBC practice. It is not the regulatory minimum: Approved Document A (para 2E2) allows GEN 1 or ST2 concrete for plain foundations in non-aggressive ground, and sulfate or aggressive ground needs a designated FND class. Building control checks the delivery ticket against the engineer's specification, so ordering C20 against a C25/30 specification can get the pour refused.
- Vertical wall chase maximum depth as fraction of wall thickness
- Maximum depth for a vertical chase cut into a single-leaf masonry wall under Approved Document A. On a 100mm block this gives 33mm. Deeper chases remove too much wall section and weaken the structural capacity, particularly near lintel bearings or corners.
- Wall tie minimum density across wall face
- Minimum number of wall ties per square metre of cavity wall face required by Approved Document A and BS EN 845-1, derived from the 900mm horizontal × 450mm vertical maximum spacing. Less than 2.5 per m² fails the regulation regardless of how the spacing is measured. The BCO may not count every tie but will scan the cavity at multiple points to confirm the pattern.
Terms explained
- Back Boxes (Metal)
- Galvanised steel boxes recessed into the wall to house the wiring connections behind socket and switch faceplates. Available from 16mm to 47mm deep. Standard-profile sockets, switches and dimmers fit a 25mm box; USB sockets, cooker switches and slim or screwless faceplates need 35mm, and smart modules 47mm. Must be earthed.
- BS EN 1990
- Eurocode 0 - Basis of structural and geotechnical design. The head Eurocode that sets the principles and requirements for safety, serviceability and durability of structures, the partial-factor (limit state) method, and the rules for combining actions used across the rest of the Eurocode suite. Approved Document A of the Building Regulations cites the Eurocode series (BS EN 1990 onwards) as a route to demonstrating structural adequacy.
- BS EN 1995-1-1 (Eurocode 5)
- The European standard for the design of timber structures, with its UK National Annex and the BSI guidance PD 6693-1. Approved Document A (para 2B1) names it, with BS 8103-3, as alternative guidance to the TRADA span tables for sizing floor and roof timbers. An engineer works to it; a homeowner never needs to open it.
- Concrete Blocks 100mm (Lightweight/Aerated)
- Lightweight aerated concrete blocks (e.g. Thermalite, Celcon) used for inner leaf of cavity walls. Better thermal performance than dense blocks and easier to cut and handle.
- Concrete Blocks 140mm
- Wider concrete blocks used where additional structural strength or wall thickness is required, common in foundation walls and below DPC level.
- Concrete Mixer
- A rotating drum mixer for combining cement, sand, aggregate, and water on site. Used for mixing mortar for bricklaying and small batches of concrete. For foundation pours, ready-mix delivery is more practical. Usually hired.
- End Grain Preservative
- Brush-on preservative for re-sealing cut ends of treated timber. Factory treatment only penetrates a few mm; cutting exposes bare wood that must be re-treated. Apply two coats minimum. Barrettine is the most common UK brand.
- Engineering Bricks
- High-strength, low-porosity bricks used below DPC level, in manholes, and wherever moisture resistance is critical. Class A is stronger and more expensive than Class B; Class B is sufficient for most domestic extension work.
- Joist Hangers
- Galvanised steel brackets that support the end of a timber joist where it meets a wall or beam. Available in various sizes to match common joist dimensions. Must be fully nailed with the correct number of fixings as specified by the manufacturer; building control will check this.
- Kilonewton
- The unit of force a structural engineer works in. One newton (N) is a small push or pull, so loads are quoted in kilonewtons (kN), where 1 kN is 1000 N, roughly the weight of 100kg. A beam carrying 45 kN is supporting about four and a half tonnes. Loads spread along a length are written kN/m and over an area kN/m2.
- Lintel
- A horizontal structural element placed above openings (doors, windows) to transfer the load from above to the sides. Can be concrete, steel, or stone. Building control inspects that lintels are correctly sized and installed over every opening.
- Ready-Mix Concrete C20
- General-purpose ready-mix concrete (GEN 3, strength class C16/20) for shed bases, paths, haunching and other non-structural work; extension foundations are normally C25/30. Delivered by lorry and pumped or wheelbarrowed into place. Minimum order usually 1m³.
- Ready-Mix Concrete C25
- Medium-strength ready-mix concrete commonly specified for domestic extension strip foundations. The most commonly used strength grade for residential foundation work.
- Ready-Mix Concrete C30
- Higher-strength ready-mix concrete used where the structural engineer specifies greater load-bearing capacity, such as reinforced foundations, pad foundations for steels, or ground floor slabs subject to heavy loads.
- Structural steel
- Steel beams used to support loads in building construction, including lintels, ridge beams, and support frames for openings in walls.
- Timber Connectors
- Galvanised steel straps, clips and plates used to tie structural timber elements together and restrain them against wind uplift. Restraint straps connect the roof structure to the walls, and truss clips connect individual trusses to the wall plate. Building control will specifically check these are installed.
- TRADA Eurocode 5 span tables
- The published tables of section sizes and spans for solid timber joists, rafters and similar members in dwellings up to three storeys (excluding trussed rafter roofs). Approved Document A has contained no span tables since 2004 and points to these as one way of meeting requirement A1, without naming an edition. The tables are paywalled; a structural engineer uses them or a calculation to size joists, which is why the leaf gives no joist size.
- Treated Timber (Tanalised)
- Timber pressure-treated with preservative (typically tanalith) to resist rot and insect attack. Essential for any timber in contact with or close to the ground, external use, or built into masonry near DPC level. Has a distinctive green tint when new.
- Wall chase
- Channel cut into masonry to conceal pipes and cables within the wall. Approved Document A imposes strict depth limits: vertical chases max one-third of single leaf wall thickness, horizontal chases max one-sixth. Central to both first fix plumbing and first fix electrics.
- Wall tie
- Austenitic stainless steel connector (Approved Document A para 2C19 requires BS EN 845-1 stainless ties) embedded in the mortar joints of both leaves of a cavity wall, transferring lateral load and stabilising the outer leaf. Spacing and embedment depths are set by NHBC 6.1 and BS EN 845-1; typical spec is 2.5 ties per square metre at 450mm vertical and 900mm horizontal centres, with closer spacing at openings.
Pages where Part A comes up
16 guides reference Part A, grouped by build phase.
Planning
Pre-construction
Structure
First fix
Completion
Want the full picture?
The buildwiz build guides walk through a UK extension in sequence and link to Part A (and every other Approved Document) at the exact stage it bites. That is usually a more productive way to use this material than reading the document cover to cover.