Summary

Extract ventilation rates are one of the most frequently misquoted numbers in UK domestic building work — not because the figures are obscure, but because they're easy to misremember room-to-room, easy to confuse with the separate whole-dwelling background ventilation rate, and easy to under-deliver in practice once duct losses are accounted for. A fan rated at the correct l/s figure on its datasheet can still fail to meet Part F on site if the duct run is long, has multiple bends, or uses undersized flexible ducting.

This article is a focused reference on the extract rates themselves — the numbers you need at your fingertips when specifying a fan, checking a spec sheet, or answering a Building Control query on site. For the broader ventilation strategy context (System 1–4, background ventilators, MVHR design and whole-dwelling ventilation rates), see part f ventilation.

The rates matter for two connected reasons. First, they're a Building Regulations compliance requirement — a Building Control inspector or a competent person scheme assessor can and will check them, and getting them wrong risks a failed sign-off. Second, and more practically, they're the actual number that determines whether the room works — inadequate kitchen and bathroom extraction is the single biggest preventable cause of condensation, mould growth and the resulting disputes that end up as fault-finder callbacks months after a job is signed off.

Key Facts

Quick Reference Table

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Room Type Intermittent Extract Minimum Continuous Extract Minimum Run-on Timer Required?
Kitchen — hood/fan adjacent to hob 30 l/s 13 l/s Yes (min. 15 min)
Kitchen — fan elsewhere in room 60 l/s 13 l/s Yes (min. 15 min)
Utility room 30 l/s 8 l/s Yes (min. 15 min)
Bathroom (with bath or shower) 15 l/s 8 l/s Yes (min. 15 min)
En-suite (with bath or shower) 15 l/s 8 l/s Yes (min. 15 min)
Shower room (no bath) 15 l/s 8 l/s Yes (min. 15 min)
WC / cloakroom (no bath/shower) 6 l/s — Yes if mechanically extracted
Wet room 15 l/s 8 l/s Yes (min. 15 min)

Detailed Guidance

Kitchens: why the fan position changes the required rate

Approved Document F sets a noticeably higher requirement (60 l/s vs 30 l/s) for a kitchen extract fan that isn't positioned adjacent to the hob, because a cooker hood mounted directly above the hob captures steam and airborne grease at the source, before it disperses into the room. A wall- or ceiling-mounted fan positioned elsewhere in the kitchen has to move a much larger volume of air to achieve an equivalent moisture-removal effect, because it's extracting already-dispersed, cooled steam rather than capturing it at source.

This has a direct sizing consequence on site: a 100mm axial fan is often adequate for a bathroom but is marginal at best for a kitchen positioned away from the hob — specify a 150mm fan or an in-line centrifugal unit capable of genuinely delivering 60 l/s once duct losses are accounted for, not just the fan's rated free-air figure.

Recirculating hoods do not satisfy Part F. A recirculating cooker hood filters cooking odour through a carbon filter and returns the air to the kitchen — it removes essentially none of the moisture. For new kitchens and kitchen extensions, Part F requires genuine ducted extraction to outside. Where ducting is impractical in a renovation, a separate ducted extract fan meeting the relevant l/s figure must be provided instead of relying on a recirculating hood alone.

Bathrooms, en-suites and shower rooms: one rate, several room names

The 15 l/s intermittent / 8 l/s continuous figure applies to any room containing a bath or shower, regardless of what it's called on the floor plan — bathroom, en-suite, shower room and wet room all fall under the same requirement. The distinguishing test is the presence of a bath or shower, not the room label. A room with only a WC and basin (no bath or shower) is sanitary accommodation, not a bathroom for Part F purposes, and takes the lower 6 l/s figure instead.

Watch for combined rooms — a WC/utility combination, for example, should be assessed against the higher of the two applicable rates (utility room's 30 l/s intermittent generally exceeds the WC's 6 l/s), because the room's function as a utility space with a WC in the corner doesn't reduce the extract requirement driven by the utility use.

Utility rooms: often under-specified

Utility rooms are frequently fitted with an undersized fan because they're treated as an afterthought compared to kitchens and bathrooms, but the required rate (30 l/s intermittent) is the same as a kitchen hood positioned at the hob — utility rooms generate significant moisture from washing machines and tumble dryers venting into the room, and the extract requirement reflects that.

Duct design and why the rated fan number isn't the whole story

A fan's box states its free-air rating — the flow rate achieved with no duct resistance at all. Real installations always have some resistance: duct length, bends, external grilles, and backdraught shutters all reduce the delivered airflow below the free-air figure.

Practical rules of thumb for keeping delivered flow close to rated flow:

Commissioning: proving the rate on site, not assuming it

Approved Document F requires commissioning evidence for new and replacement ventilation systems — this is not paperwork for its own sake, it's the only reliable way to confirm the installed system actually meets the required extract rate given the real duct route, not the fan's catalogue figure.

The standard method is a vane anemometer reading taken at the extract grille, converted to a flow rate using the grille's known free area, and recorded against the room and the required minimum. Build in a margin — aim for 10–15% above the regulatory minimum at commissioning, since fan performance can degrade slightly over time as impellers pick up dust and grease.

When existing rooms need reassessment

Part F applies most clearly to new work — new bathrooms, new kitchens, kitchen extensions, and any new or replacement ventilation system. In existing dwellings undergoing renovation, a like-for-like fan replacement of equivalent or better performance is generally compliant without a full reassessment. However, certain triggers should prompt a fresh look at the extract rate:

Frequently Asked Questions

What's the difference between the intermittent and continuous figures?

Intermittent extract runs only when the room is in use (triggered by a light switch, humidity sensor, or occupancy sensor) plus a run-on period, and must meet the higher of the two rates (e.g. 15 l/s in a bathroom) during that active period. Continuous extract runs constantly at a lower background rate (e.g. 8 l/s in a bathroom) and is typically boosted to the intermittent-equivalent rate during and after moisture-generating activity. Continuous systems (MEV, MVHR) are increasingly the default in new-build and higher-airtightness renovation work because they manage condensation risk around the clock rather than only when someone remembers to switch the fan on.

Does a WC with no bath or shower need mechanical extract at all?

Yes, if it has no openable window, or as standard new-build practice even where a window is present — the minimum figure is 6 l/s intermittent. If the WC has an openable window and building control accepts natural ventilation as sufficient for that specific room in that specific project, mechanical extract may not be mandated, but this should be confirmed with the relevant Building Control body for the job rather than assumed, since practice varies.

My fan's box says 90 l/s — does that automatically satisfy a 60 l/s kitchen requirement?

Not automatically. The 90 l/s figure is almost certainly the fan's free-air rating with zero duct resistance. Once installed with real ductwork, bends, and an external grille, the delivered flow at the room could be meaningfully lower. Specify with a safety margin above the regulatory minimum, and verify the actual delivered flow with an on-site airflow reading at commissioning rather than relying on the box figure.

Do these figures apply to non-domestic buildings too?

No — Approved Document F Volume 1 (Dwellings) sets these specific figures for homes. Non-domestic buildings (offices, commercial kitchens, etc.) are covered by Approved Document F Volume 2, which uses a different methodology (often based on occupancy and CIBSE guidance rather than these fixed per-room l/s figures) and should not be used interchangeably with the domestic figures in this article.

Can I use these same extract rates for a loft conversion en-suite?

Yes — a loft conversion en-suite containing a bath or shower is assessed the same as any other bathroom: 15 l/s intermittent or 8 l/s continuous, with a 15-minute run-on timer if intermittent. The duct route in a loft conversion is often the more challenging part of the job (longer runs, more bends to reach an external wall or roof termination), so pay particular attention to duct sizing and insulation in cold zones to actually deliver the rated flow.

Regulations & Standards