Approved Documents for England: Critical Numbers Across the Main Parts

The Building Regulations 2010 (SI 2010/2214) set the legal requirements for building work in England. The Approved Documents are statutory guidance: following them is a recognised way to show compliance, but they are not the law, and another route can be shown to comply. This guide collects numbers from the main Parts, with the edition each one comes from. All figures are for England.

What are the Approved Documents?

The Approved Documents are published on gov.uk and are marked “for use in England”. The gov.uk collection lists 19 documents (Parts A to T plus the Regulation 7 document on materials and workmanship). Since April 2023 the Building Safety Regulator has a duty to keep the safety and standards of buildings in England under review, including advising government on updates to the Approved Documents. Wales, Scotland and Northern Ireland have their own building regulation systems; confirm which regime applies before using any number below.

Editions behind this guide, from gov.uk and the copies in our library:

Approved Document B amendments are dated. The 2025 amendments took effect on 2 March 2025 (Regulation 38 fire safety information, removal of national classes for reaction to fire and roofs, sprinklers in care homes). The 2026 amendments take effect on 30 September 2026 (a recommendation of more than one common stair in blocks of flats with a storey 18 m or more in height, and design provisions for evacuation lifts). The 2029 amendments take effect on 2 September 2029 (removal of national classes for fire resistance). Earlier editions continue to apply to work covered by the transitional rules in each amendment booklet, so check which edition your building notice or initial notice falls under.

What are the key numbers at a glance?

How does Part A regulate structural dimensions?

Part A covers loading, ground movement and disproportionate collapse. Section 2C gives rules for the thickness of masonry walls in certain small buildings. For cavity walls in coursed brickwork or blockwork, paragraph 2C8 sets leaves of at least 90 mm and a cavity of at least 50 mm, with wall ties at 900 mm horizontal and 450 mm vertical spacing (or equivalent staggered spacing). Paragraph 2C9 is a different rule: a wall should be no thinner than any part of the wall it supports.

Other limits in the same section: a lintel bearing of 150 mm, or 100 mm where the clear span is 1,200 mm or less (2C24); floors supported by a wall spanning no more than 6 m, centre to centre of bearing (2C23); and walls no longer than 12 m between buttressing elements (2C17). Table 3 sets 190 mm as the thickness for the whole height of solid external walls up to 3.5 m high and up to 12 m long. Table 4 sets the imposed loads for which the section is intended: 2.00 kN/m² for floors, 0.25 kN/m² for ceilings, and for roofs 1.00 kN/m² on spans up to 12 m or 1.5 kN/m² on spans up to 6 m. For anything outside the section’s limits, use structural design to the Eurocodes.

How does Part B set fire escape requirements?

Part B covers means of warning and escape, internal and external fire spread, and access for the fire service. The figures below are from the library copy of the 2019 edition with its amendments. Check the amendments table above for the edition that applies to a given job.

• Escape route widths (Vol 2, Table 2.3): 750 mm for up to 60 people, 850 mm for up to 110, 1,050 mm for up to 220, and 5 mm per person above 220. Widths under 1,050 mm are not interpolated. Note 4 allows a narrower gangway between fixed storage racking.
• Dead ends (Vol 2, para 2.28): where a corridor dead end longer than 4.5 m gives access to a point from which alternative escape routes are available, it should be separated by self-closing fire doorsets from the parts of the corridor that provide two directions of escape or that continue past a storey exit.
• Headroom: a minimum clear headroom of 2 m along escape routes (para 5.16).
• Refuges: at least 900 mm × 1,400 mm (para 3.6).
• Escape stair width: not more than 1,400 mm for stairs with a rise over 30 m, unless a central handrail is provided (para 3.10d).
• Emergency escape windows in dwellings (Vol 1, para 2.10): an unobstructed openable area of at least 0.33 m², at least 450 mm high and 450 mm wide, with the bottom of the openable area not more than 1,100 mm above the floor.

How does Part K define stair dimensions?

Part K covers stairs, ladders and ramps, protection from falling, vehicle barriers and loading bays, and protection from impact. Table 1.1 sets rise and going for three stair categories. A private stair is one used for only one dwelling; a utility stair is used for escape, maintenance or other purposes that are not the usual day-to-day route; a general access stair is intended for all users as the normal route between levels. In general, 2R + G should equal between 550 mm and 700 mm.

Flights: if a stair has more than 36 risers in consecutive flights, it needs at least one change of direction between flights (para 1.17). Between landings the maximum is 16 risers for utility stairs and 12 for general access stairs, or exceptionally 16 in small premises where the plan area is restricted (para 1.18b). Minimum headroom is 2 m (para 1.11, Diagram 1.3). Handrails: top at 900 to 1,000 mm (para 1.34a); a circular handrail is 32 to 50 mm in diameter (Diagram 1.13).

What barrier heights does Part K require?

Guarding heights are in Diagram 3.1 of Part K. Guarding is needed at the sides of flights and landings where the drop exceeds 600 mm (para 1.41).

• Single-family dwellings: 900 mm to stairs, landings, ramps and edges of internal floors; 1,100 mm to external balconies, Juliet balconies and roof edges.
• Residential, institutional, educational, office and public buildings: 900 mm to flights, otherwise 1,100 mm.
• Factories and warehouses with light traffic: 900 mm to stairs and ramps, 1,100 mm at landings and floor edges.
• Assembly buildings: 800 mm within 530 mm in front of fixed seating; 900 mm to flights, 1,100 mm elsewhere.
• Retail: 900 mm to flights, otherwise 1,100 mm.
• Glazing at opening windows: 800 mm.

Where children under five may be present, guarding to stairs must not let a 100 mm sphere pass through, and must not be readily climbable (paras 1.39 and 3.3). Vehicle barriers: at floor and roof edges at least 610 mm high, and at ramp edges at least 375 mm (Diagram 4.2). Gangways in assembly seating are at least 1,100 mm wide unless they serve no more than 50 people (para 1.4j); that is a width, not a barrier height.

How does Part M define accessible dimensions?

Part M has two volumes. Volume 1 (dwellings) has Category 1 visitable dwellings (M4(1)), which apply to all new dwellings unless an optional requirement applies, Category 2 accessible and adaptable dwellings (M4(2)) and Category 3 wheelchair user dwellings (M4(3)). Categories 2 and 3 are optional requirements that apply where they have been required, for example by a planning condition.

• Vol 1, Category 1 communal lift (para 1.11): landing at least 1,500 mm × 1,500 mm, door clear width at least 800 mm, car at least 900 mm wide × 1,250 mm deep. Consumer units have switches 1,350 to 1,450 mm above floor (para 1.18b).
• Vol 1, Category 2 communal lift (para 2.16): car at least 1,100 mm wide × 1,400 mm deep, door clear width at least 800 mm. Consumer units 1,350 to 1,450 mm (para 2.30a).
• Vol 2, corridors (para 3.14): unobstructed width of at least 1,200 mm, with passing places 1,800 mm long and 1,800 mm wide at reasonable intervals where the width is under 1,800 mm.
• Vol 2, doors (Table 2): effective clear width, new buildings: 800 mm straight-on, 800 mm at right angles to an access route at least 1,500 mm wide, 825 mm at right angles to an access route at least 1,200 mm wide, and 1,000 mm for external doors to buildings used by the general public.

What do Parts C, F, H, J, L, O and Q require?

• Part C, para 5.5: the damp-proof course of an external wall should be at least 150 mm above the adjoining ground level.
• Part F Vol 1, Table 1.1: minimum intermittent extract rates are 30 l/s for a kitchen with a cooker hood extracting to the outside, 60 l/s for a kitchen without one, 30 l/s for a utility room, 15 l/s for a bathroom and 6 l/s for sanitary accommodation.
• Part H, para 1.3: traps should retain a minimum seal of 25 mm of water under working and test conditions. Table 1 gives the trap sizes and seal depths for common appliances, which are larger for many.
• Part J, Table 2: a closed appliance of up to 20 kW burning smokeless or low-volatile fuel needs a 125 mm diameter flue (or equivalent area, with a minimum dimension of 100 mm for straight flues); a closed appliance above 30 kW and up to 50 kW burning any fuel needs a 175 mm diameter flue.
• Part L Vol 1, Table 4.1: limiting U-values for new dwellings, area-weighted: roof 0.16, wall 0.26, floor 0.18, party wall 0.20, window 1.6, rooflight 2.2 and door 1.6 W/(m²·K). These are limits; the notional-building and target-rate method usually requires better values.
• Part O, Tables 1.1 and 1.2: maximum glazing as a share of floor area by the orientation of the largest glazed facade. For a building with cross-ventilation in a high risk location: north 15%, east 18%, south 15%, west 18%. Without cross-ventilation: north 15%, east 11%, south 11%, west 11%. Paragraph 1.9 adds shading to glazing between north-east and north-west via the south for residential buildings in the high risk location.
• Part Q, Appendix B: door rails, stiles and muntins at least 44 mm thick (B.3), and where a mortice lock and surface-mounted rim lock are used, 400 to 600 mm between the two locking points (B.6).

What mistakes should you avoid?

1. DPC height below minimum. Part C para 5.5 requires 150 mm above the adjoining ground. Landscaping after construction can bury a DPC, so measure from finished ground level.
2. Wrong stair category. A stair serving more than one dwelling is not a private stair. Common stairs in flats follow the general access stair limits (rise up to 170 mm, going 250 to 400 mm).
3. Escape window sized on area alone. Part B Vol 1 para 2.10 needs the 0.33 m² area and at least 450 mm in height and in width.
4. One barrier height for flights and landings. In residential, institutional, educational, office, public and retail buildings, Diagram 3.1 gives 900 mm to flights and 1,100 mm elsewhere.
5. Mixing Part M volumes. Dwelling lift sizes are in Vol 1 (900 × 1,250 mm for Category 1, 1,100 × 1,400 mm for Category 2). Non-dwelling door and corridor sizes are in Vol 2.
6. Using the wrong edition of Part B. A job may fall under the 2025, 2026 or earlier text depending on when its notice was given and when work started. Read the transitional paragraph of each amendment booklet.