Bathroom Extractor Fans: Building Regulations and Installation

Building Regulations: Approved Document F

The requirement for mechanical extraction in bathrooms is set out in Building Regulations Approved Document F (Ventilation). The key provisions for bathrooms and shower rooms are:

• Bathrooms and shower rooms without openable windows: Mechanical extraction is mandatory. A bathroom with no openable window — which includes most internal bathrooms in London flats, and basement bathrooms in terraced houses — must have mechanical extraction to comply with Approved Document F.
• Bathrooms with openable windows: While natural ventilation via a window can technically satisfy the ventilation requirement, mechanical extraction is strongly recommended in all London bathrooms regardless of window provision. The reason is practical: windows are rarely opened during showering or bathing in cold or wet weather, and London's hard water deposits on surfaces are accelerated by condensation that window-only ventilation fails to prevent.

Approved Document F specifies minimum extract rates. For residential use:

• Bathrooms (bath or shower): Minimum 15 litres per second (l/s) continuous extract rate, or 40 l/s intermittent (fan triggered by occupancy, not running continuously).
• Shower rooms (shower only, no bath): Minimum 30 l/s continuous, or 60 l/s intermittent. The higher rate reflects greater moisture generation from a shower.
• Utility rooms: 30 l/s continuous or 60 l/s intermittent where the room contains clothes drying or washing activities.

These are minimum rates — in bathrooms with particularly high moisture generation (long showers, multiple occupants), exceeding the minimum specification is advisable.

The Overrun Timer Requirement

One of the most frequently omitted specifications in bathroom fan installations is the overrun timer. Approved Document F requires that extract fans in bathrooms have an overrun period — the fan continues to run for a set time after the room is no longer occupied. The purpose is to remove residual moisture that has accumulated during use but has not yet been extracted.

The required overrun period is a minimum of 15 minutes after occupancy ends. A fan that switches off at the same time as the bathroom light (a common wiring arrangement in older properties) fails to extract the moisture left behind after bathing. This is one of the primary causes of persistent condensation damp in bathrooms that "have a fan" — the fan has no overrun and stops before its job is done.

The overrun timer is built into most modern bathroom fans — it is not a separate component. When selecting a fan, confirm the timer is included and can be set to 15 minutes or longer. Fans with a humidity sensor (which continue to run until the relative humidity in the room drops below a set threshold) are even more effective than fixed-period timers, as they operate in direct response to actual moisture levels.

Types of Bathroom Extract Fan

Axial Wall Fan

The most common type for short duct runs — the fan motor and impeller are mounted directly in the wall aperture, with a short duct through the external wall or cavity to outside. Axial fans are appropriate for runs up to approximately 1–1.5 metres. Beyond this length, an axial fan's static pressure capacity is typically insufficient to overcome the duct resistance and maintain the required extract rate.

Inline Duct Fan

The fan motor sits within the duct run (not at the wall aperture or ceiling) and can be located remotely — in the loft or ceiling void — while the duct itself runs from the bathroom to the external termination point. Inline fans are suited to longer duct runs and to situations where the fan noise needs to be away from the bathroom (the fan in the loft is much quieter from the bathroom than a fan mounted in the wall). They are the standard solution for London flats where the duct must run horizontally through a ceiling void to reach an external wall.

Centrifugal Fan

Centrifugal fans generate higher static pressure than axial fans, making them suitable for high-resistance duct runs — long horizontal runs with multiple bends, or runs to a communal shaft where there is back-pressure from other units. For complex duct configurations in London flats, a centrifugal fan (or an inline fan with sufficient static pressure) is often necessary to achieve the required extract rate at the outlet.

Part P: When Is Fan Installation Notifiable Work?

Under Building Regulations Part P (Electrical Safety in Dwellings), electrical work in bathrooms is classified as special location work and has specific requirements:

• Installing a new fan with its own dedicated circuit or new switched spur: This is notifiable work under Part P and must either be carried out by a registered competent person (an electrician registered with NICEIC, NAPIT, or similar) who self-certifies, or be notified to the local Building Control authority.
• Replacing an existing fan like-for-like in the same position: This is generally not notifiable if it is simply swapping one fan for another on an existing circuit without any wiring changes. However, even for replacement, using a qualified electrician is advisable in a bathroom environment.

For new builds, extensions, or bathroom renovations involving new electrical work, the fan installation must be notified as part of the broader Building Regulations submission.

Installation in London Flats: Duct Routes

In London flats, the duct route for a bathroom fan is a significant practical constraint. Options include:

• External wall termination: A short duct run through the external wall to a grille visible on the exterior. This is the simplest route but requires the bathroom to adjoin an external wall — which is not always the case in London apartment layouts where bathrooms are frequently internal rooms.
• Communal shaft: Many London purpose-built blocks of flats have communal ventilation shafts into which individual flat ventilation ducts connect. Check with the building management company or review the lease before connecting to a communal shaft — there may be building rules about connections, and the shaft may require cleaning as part of building maintenance.
• Horizontal run through ceiling void to external wall: An inline duct fan running through the ceiling void is the typical solution for flats where the bathroom does not adjoin an external wall. The duct may run five to eight metres through the void before terminating at an external grille. This route requires adequate static pressure from the fan to maintain extract rate at the duct length involved.

Note: ducting into the roof space without external termination is not a compliant installation — moisture-laden air discharged into a roof void causes condensation damage to the building fabric. The duct must terminate outside the building envelope.

Cost

Supply and fit of a bathroom extractor fan in a London property, including duct and external grille, typically costs £150–£250. This covers a standard axial fan with overrun timer for a straightforward through-wall installation. An inline duct fan for a longer run in a flat, including duct materials and loft or ceiling void access, typically costs £200–£350. Where Part P notification is required, the notifiable work certificate adds a modest additional cost that is typically included within the electrician's fee.