Kitchen Extraction Requirements: Cooker Hoods, Ducting & Building Regs Part F

Quick Answer: Building Regulations Approved Document F (2021 edition) requires kitchen extract fans to achieve minimum 30 l/s (108 m³/h) adjacent to a hob. Cooker hoods must be capable of achieving a minimum of 60 l/s (216 m³/h) for intermittent operation. All extraction must either duct to outside or use a recirculation filter — but recirculation does not comply with Part F's ventilation requirements for moisture removal. Ducting must terminate externally with a weatherproof grille.

Summary

Kitchen extraction is one of the most frequently misunderstood areas of building compliance in residential work. Many kitchen installers fit a recirculation hood (charcoal filter, no duct) and consider the job done — but this does not comply with Building Regulations Part F, which requires actual air removal to the outside. Recirculation hoods remove grease and odour but return moisture-laden air to the kitchen, failing to address condensation and humid air that causes mould growth and building deterioration.

The 2021 revision of Approved Document F introduced new requirements that significantly increased the minimum ventilation rates and added requirements for background ventilation (trickle vents) in kitchens. Understanding what is actually required — and being able to explain it to customers — is increasingly important as building control inspections become more common.

Key Facts

Quick Reference Table

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Hood Type Compliance with Part F Notes
Ducted chimney hood Yes — if ducted to outside Most common compliant option
Ducted integrated (in unit) Yes — if ducted to outside Must maintain 60 l/s through the unit
Ceiling extract (island hob) Yes — if ducted to outside Needs roof penetration or long duct run
Recirculation hood No (Part F) Allowed, but does not count as ventilation
Downdraft extractor Yes — if ducted to outside Duct runs below worktop; complex routing
Extractor fan (no hood) Yes if 30 l/s minimum Can be in ceiling or wall adjacent to hob

Detailed Guidance

Part F Requirements in Detail

Building Regulations Approved Document F (2021, applicable to work in England) sets two types of requirement for kitchens:

1. Extract ventilation (Table 1.2 of Part F) Kitchens must have extract ventilation capable of removing humid air and cooking odours. The rates are:

2. Background ventilation In dwellings with new or replacement windows, background ventilators (trickle vents) are required. In kitchens: equivalent to 8000mm² equivalent area (EqA). If the kitchen is a habitable room in an open-plan layout, the requirement increases.

3. Humidity-controlled extract Humidity-controlled extract fans (those with a built-in humidistat) can operate continuously at lower rates and intermittently at higher rates — they are an acceptable alternative to a standard intermittent fan.

Scotland and Wales: Wales follows its own version of Approved Document F. Scotland has separate Building Standards. The principles are similar but the specific rates vary — check the applicable regulations for the project location.

Cooker Hood Height and Position

Above gas hobs: the minimum clearance from hob surface to underside of hood is 650mm. This is a gas safety requirement — getting the hood too close to a gas flame risks igniting grease deposits in the hood or duct. Always check the hob manufacturer's instructions too — many specify 750-800mm minimum.

Above electric/induction hobs: minimum 450mm; typical install height 600-700mm. The clearance is less critical for electrical safety but affects extraction performance — too high and grease vapour spreads before being captured.

Canopy width: the hood should be at minimum the same width as the hob; ideally 100-150mm wider each side. A wider hood captures more cooking fumes before they escape the capture zone.

Duct Routing

The duct from the cooker hood to the external grille is where most extraction systems lose performance. Key principles:

Keep it short: every metre of duct reduces airflow due to friction. Every 90° bend is equivalent to approximately 1m of straight duct. A system with 3m of straight duct and two 90° bends has an equivalent duct run of 5m.

Duct sizing by run length:

Duct Equivalent Length Recommended Diameter
Up to 3m 125mm
3-5m 150mm
5-7m 150mm (check hood specification)
Over 7m Specialist calculation; may not be achievable

Duct type: rigid aluminium round duct is best for performance. Rectangular duct fits between joists and ceiling voids but has higher resistance. Flexible corrugated duct should be used only for short connectors (less than 0.5m) and final connections — never for full runs, as the corrugations add significant friction.

Condensation in the duct: where the duct passes through a cold space (unheated loft, external wall cavity), condensation can form inside the duct and drip back into the kitchen. Insulate the duct in cold voids. Route the duct on a slight fall towards the external end so any condensate runs out rather than back.

External Grilles and Terminals

The external termination must:

For grilles through external walls: use a standard wall vent with back-draught shutter. Round-to-rectangular adapters are available for aesthetic brickwork grilles. Keep the external face velocity low (2 m/s or less) to prevent wind back-pressure defeating the extraction at the fan.

Flat roof termination (island hoods): roof cowl with bird guard; ensure the cowl is sealed to the roof structure and a fire-stopping collar is used where the duct passes through a fire-rated element.

Recirculation Hoods — When They Are Used

Despite not meeting Part F requirements for new and notifiable work, recirculation hoods are widely used in:

In these cases, a recirculation hood may be the practical solution, but the installer should document that Part F compliance has been raised with the customer and that the ventilation strategy needs to address moisture removal by other means (e.g. whole-house MVHR, passive ventilation, humidity-controlled bathroom extract).

Frequently Asked Questions

Does every kitchen need a hood?

Part F does not specifically require a hood — it requires extract ventilation. A ceiling-mounted or wall-mounted extract fan adjacent to the cooking area, capable of 30 l/s, complies. However, a fan without a hood is generally less effective at capturing steam and grease vapour directly above the hob.

Can I duct into a chimney breast?

No. Ductless hoods (recirculation only) can be fitted in fireplace alcoves, but hoods must not be ducted into existing chimney flues unless the flue has been decommissioned and is solely used for the purpose. Active or potentially active flues create a fire and fume risk. A standalone duct through the chimney breast wall is acceptable if correctly installed.

What if the hob is on an island with no wall access?

Options for island extraction: 1) Ceiling-mounted canopy hood ducted through the ceiling void and out through the roof or soffit; 2) Downdraft extractor (integrated into the worktop beside or behind the hob, ducted below the floor); 3) Overhead pendant island hood ducted to ceiling. All require careful duct routing planning before the kitchen is installed — this is very difficult to retrofit.

Does Part F apply to a kitchen renovation?

Part F applies to new builds, and to extensions and material changes of use in existing buildings. A straightforward kitchen refurbishment (replacing units, worktops, appliances) in an existing dwelling generally does not trigger a full Part F compliance obligation, but it is best practice to bring the ventilation up to Part F standard. Installing a new extraction system always gives the opportunity to do it right.

Regulations & Standards