Part F
Approved Document F: Ventilation
The part of the Building Regulations covering ventilation: the extract ventilation kitchens and bathrooms need, plus the background and purge ventilation a habitable space needs to control moisture and air quality. It is closely tied to Part L, because making a building more airtight raises the importance of adequate ventilation.
Also known as
Part FApproved Document FAD FVentilationApproved Document F Volume 1AD F Vol 1
What Part F requires for a home project
The Part F 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
- Background ventilator area for a habitable room added to an existing dwelling
- Where the existing room next to a new habitable room already has at least 5,000 mm2 of background ventilators, Approved Document F para 3.17 asks for 12,000 mm2 equivalent area between the two rooms and another 12,000 mm2 between the new room and the outside.
- Bathroom extractor fan run-on where the room has no openable window
- Approved Document F Volume 1 para 1.51: where a bathroom or other wet room has no openable window, an intermittent extract fan should have a 15-minute overrun after the room is vacated, usually wired through a delay-off timer. Where the room has an openable window, the paragraph does not set a run-on, though a humidistat or timer still helps clear moisture after a shower.
- Bathroom intermittent extract rate
- Minimum intermittent extract rate for a bathroom under Approved Document F Volume 1 (Table 1.1). A loft ensuite priced without the fan or the duct to outside is incomplete against this requirement.
- Extractor hood clearance above electric/induction hob
- Typical manufacturer minimum distance from the surface of an electric or induction hob to the underside of an extractor hood. The hood and hob manufacturers' installation instructions govern; where none is specified, Approved Document F para 1.21 gives 650mm to 750mm above the hob surface.
- Extractor hood clearance above gas hob
- Typical manufacturer minimum distance from the surface of a gas hob to the underside of an extractor hood. The hood and hob manufacturers' installation instructions govern; where none is specified, Approved Document F para 1.21 gives 650mm to 750mm above the hob surface.
- Part F minimum trickle vent count per dwelling
- Approved Document F Volume 1 paragraph 1.57 (new dwellings): the total number of background ventilators in the habitable rooms and kitchens should be at least four if the dwelling has one bedroom and five if it has more than one bedroom. The count is a new-dwelling rule. An extension follows Section 3 (paragraph 3.17), which sizes the new room's background ventilators and checks the existing room it opens off.
- Part F new-build EA: single-storey habitable rooms
- Minimum equivalent area per habitable room required in a single-storey new dwelling or extension adding a habitable room under natural ventilation, per Approved Document F 2022 (Table 1.3). Higher than the replacement-window standard: the most commonly missed specification when ordering windows for an extension.
- Part F purge ventilation: minimum openable area ratio
- Minimum openable window area expressed as a fraction of the served floor area, to satisfy purge ventilation requirements under Approved Document F Volume 1 (2021). The openable area must be at least 1/20th of the floor area of the room it serves, measured as the clear opening area when the window is at its maximum opening angle. The 1/20 figure applies to hinged or pivot windows opening 30 degrees or more, sash windows and external doors; windows opening 15 to 30 degrees need 1/10, and windows opening less than 15 degrees are not suitable for purge ventilation.
- Part F replacement-window EA: bathrooms
- Minimum equivalent area required per bathroom when replacing windows in an existing dwelling with no mechanical ventilation system, under Approved Document F 2022 (Table 1.3). Lower than the habitable room figure: specify the correct value per room type.
- Part F replacement-window EA: habitable rooms
- Minimum equivalent area required per habitable room when replacing windows in an existing dwelling with no mechanical ventilation system, under Approved Document F 2022 (Table 1.3). The figure homeowners must specify when ordering replacement windows to ensure compliance.
- Sanitary accommodation intermittent extract rate
- Approved Document F Vol 1 Table 1.1 intermittent extract rate for sanitary accommodation (WC only). A bathroom is 15 l/s, already in the graph. Confirm with the surveyor which category a loft shower room falls into.
- Utility room intermittent extract rate
- Minimum intermittent extract rate for a utility room under Approved Document F Table 1.1. On continuous extract the high rate is 8 l/s (Table 1.2).
Terms explained
- Acoustic trickle vent
- A trickle vent variant with noise-attenuating baffles, recommended in Approved Document F paragraph 1.54 for windows on noisy facades. Acoustic ratings typically 32–40 dB. Accepted as equivalent background ventilation.
- Activated carbon filter
- A replaceable filter cassette filled with activated carbon granules, fitted inside a recirculating cooker hood to adsorb cooking odours. Does not satisfy Part F ventilation requirements on its own. Replacement interval: every 6 weeks (heavy use) to 6 months (light use).
- Continuous mechanical extract ventilation
- One of three recognised ventilation strategies under Approved Document F. When CMEV is installed, trickle vent equivalent area requirements per room drop significantly compared to the natural ventilation default. Distinct from MVHR.
- Cooker hood
- The appliance category for kitchen extraction with multiple sub-types (chimney, canopy, island, downdraft, integrated), each requiring different duct routing approaches and affecting wall unit layout.
- Duct cowl
- The external termination fitting for a ducted extraction run. Non-return valve type prevents back-draught and rain ingress when the extractor is off.
- Duct insulation wrap
- Insulation applied to ducting that passes through unheated ceiling voids or loft spaces. Prevents condensation forming inside the duct due to temperature differential.
- Ducting
- Extraction ducting for cooker hoods and mechanical ventilation. 150mm rigid duct is the industry standard; flexible duct is for short transitions only because its ribbed interior cuts airflow by roughly 25m³/h per metre. External runs need a non-return cowl to stop back-draught, and runs through unheated voids need insulation wrap to prevent internal condensation.
- Equivalent Area
- The regulatory measurement standard under Approved Document F 2022 for quantifying trickle vent performance. Equivalent Area = physical open area × discharge coefficient (Cd), so EA is always smaller than geometric open area. Every compliant trickle vent must have its EA value marked on it.
- Extraction rate
- The measure of how much air a cooker hood removes per second (l/s) or per hour (m³/h). Approved Document F specifies minimum extraction rates: 30 l/s when adjacent to the hob, or 60 l/s when located elsewhere in the room.
- Flexible ducting
- Flexible ducting for extraction runs. The ribbed interior creates turbulence and reduces airflow by approximately 25m³/h per metre. Should only be used for short transition sections (max 300mm), not as a substitute for rigid ducting throughout a run.
- Humidity-activated trickle vent
- A trickle vent variant that modulates its opening in response to relative humidity: wider when humidity rises, narrower in cold dry conditions to reduce draughts. Addresses the primary homeowner complaint about cold draughts while maintaining Part F compliance.
- MVHR
- A whole-house ventilation system that extracts stale air from wet rooms and recovers heat to warm incoming fresh air. Increasingly required under Part F where high airtightness levels make natural ventilation insufficient.
- MVHR commissioning certificate
- Document required by building control at final inspection when MVHR is installed. Confirms the ventilation system has been commissioned to Part F flow rate requirements. Often forgotten by homeowners, delaying the completion certificate.
- Pitched roof window
- Standard pivot-type roof windows (Velux GGL, Fakro FT series, RoofLITE equivalent) for pitched roofs. Require a specific flashing kit matched to tile type, structural trimmer rafters, and a manufacturer-specified underfelt collar. Available in standard size codes from builders merchants with 1-2 week lead times for stock sizes.
- Purge ventilation
- Rapid ventilation through openable windows, required by Approved Document F. Minimum openable area of 1/20th of floor area. Distinct from trickle (background) ventilation and extract ventilation. BCO checks compliance at final inspection.
- Recirculating hood
- A cooker hood that filters air through carbon filters and returns it to the kitchen rather than ducting it outside. Does not comply with Building Regulations Part F for new kitchen extensions: building control will reject it as sole ventilation.
- Rigid ducting 150mm
- The standard-diameter rigid duct pipe used for cooker hood extraction runs. 150mm is the industry-standard diameter for performance; substituting smaller diameter (e.g. 120mm) degrades airflow significantly.
- Sanitary accommodation
- The Approved Document F term for a room containing a WC but no bath or shower. It has a lower extract rate than a bathroom (6 l/s against 15 l/s), so a loft room with a shower counts as a bathroom and a WC-only room does not.
- Soffit vent terminal
- The external termination fitting for a cooker hood extraction duct, installed in the soffit board or as a purpose-made tile vent in the roof slope. Must include a non-return flap to prevent backdraughts and wildlife ingress. 150mm bore matches standard kitchen extraction ducting.
- Trickle vent
- A small continuous ventilation slot in a window or door frame, required under Approved Document F to provide background ventilation to habitable rooms. The BCO checks new windows include trickle vents or equivalent ventilation.
- Utility room
- A room or alcove for the washing machine, tumble dryer, sink and often the boiler, separate from the kitchen. Approved Document B para 2.11 allows it as an inner room; Approved Document F Table 1.1 sets a 30 l/s intermittent extract rate (8 l/s continuous high rate). Its size is a design choice: no Approved Document sets a minimum for an extension.
Pages where Part F comes up
15 guides reference Part F, grouped by build phase.
Planning
Pre-construction
Structure
Specification
Second fix
Completion
Want the full picture?
The buildwiz build guides walk through a UK extension in sequence and link to Part F (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.