Bathroom Extractor Fan Installation in London: Part F, Wiring and What It Costs

Bathroom Extractor Fans and Building Regulations in London

A properly installed bathroom extractor fan is not merely a comfort feature — it is a Building Regulations requirement for most bathroom and WC installations in London. Inadequate bathroom ventilation is the leading cause of condensation damage and mould growth in London properties, and a compliant extractor fan installation is the most effective and durable solution. This guide covers everything from the specific requirements of Approved Document F to wiring options, duct routing, and the cost of installation in London.

Approved Document F: The Ventilation Requirement

Building Regulations Approved Document F (Ventilation) sets the minimum extract ventilation rates for bathrooms and WCs. For a bathroom containing a bath or shower, the minimum intermittent extract rate is 15 litres per second (l/s). For a WC without a bath or shower, the minimum is 6 l/s. These flow rates must be maintained throughout the duct run, accounting for bends and duct length. A standard 100mm diameter extractor fan with a short, straight duct run will achieve these flow rates comfortably; longer runs with multiple bends may require a higher-rated fan or an inline fan unit to maintain compliance.

Intermittent vs Continuous Operation

Approved Document F distinguishes between intermittent and continuous ventilation strategies. Intermittent ventilation — a fan that operates during and after use — is appropriate for most London residential bathrooms where the bathroom is used periodically throughout the day. The minimum overrun requirement for an intermittent fan is 15 minutes: the fan must continue operating for at least 15 minutes after the light is switched off. This ensures that the moisture generated by a shower or bath is removed before the fan stops. Continuous ventilation at a lower background rate with a boost capability for high-use periods is an alternative strategy used in some whole-house mechanical ventilation systems.

Humidistat Fans: The Preferred Option

A humidistat fan contains a sensor that triggers operation when the relative humidity in the bathroom rises above a preset threshold — typically 70 to 80 percent relative humidity — and switches off automatically when humidity returns to ambient levels. This approach is more energy-efficient than a timer-based fan (it only runs when needed) and more effective than a light-switched fan (it continues running until the moisture is actually removed, regardless of when the light is switched off). For London rental properties in particular, a humidistat fan reduces landlord liability for condensation and mould complaints because it ensures the fan operates appropriately regardless of tenant behaviour.

Wiring Options

Bathroom extractor fans can be wired in three configurations. The most common is a switched live connection from the bathroom light switch, causing the fan to run whenever the light is on and for the programmed overrun period after the light is switched off. This requires a fan with an integral timer. The second option is a separate switched circuit with a dedicated isolator switch outside the bathroom zone — appropriate where the fan is to be controlled independently of the light. The third option for a humidistat fan is a permanent live connection, allowing the fan to monitor humidity and operate independently of the light switch. Each configuration has different wiring requirements; humidistat and timer configurations require a live, neutral, and earth at the fan.

Part P Notification Requirement

Any new electrical circuit in a bathroom is notifiable work under Building Regulations Part P (Electrical Safety in Dwellings). If the extractor fan is connected to an existing lighting circuit — which is the most common arrangement for a replacement fan — this may not constitute a new circuit and may fall within the "minor works" exemption. If a new dedicated circuit is run for the fan, this is notifiable work. Electricians and plumbers who are registered with a competent person scheme (NICEIC, NAPIT, or similar) can self-certify Part P work, satisfying the notification requirement without a separate Building Control application. Always obtain a completion certificate for electrical work in a London bathroom.

Duct Routing: External Wall, Roof Void, Soffit

The extract duct must discharge to external air. The three most common routes in London properties are through an external wall (the most direct and preferred route), up through the floor void and into the roof space then out through a soffit vent or roof tile vent, and directly through a flat roof via a weathered duct terminal. Duct runs should be as short and straight as possible — each 90-degree bend is equivalent to approximately 2 to 3 metres of straight duct in terms of resistance. The duct should be insulated where it passes through an unheated roof void to prevent condensation forming inside the duct.

Duct Size Requirements

The minimum duct diameter for a standard bathroom extractor fan serving a single bathroom is 100mm. Using a smaller duct — 75mm is sometimes tempting where space is restricted — reduces airflow below the minimum required by Approved Document F. Longer duct runs with multiple bends may require 125mm duct to maintain adequate flow rates. Rigid duct is preferred over flexible corrugated duct where possible, as flexible duct has significantly higher resistance and can sag if poorly supported, creating water traps. All external duct terminals should incorporate a non-return valve (backdraught shutter) to prevent cold air, insects, and birds from entering through the duct when the fan is not operating.

Inline Fans for Longer Runs

Where the duct run from bathroom to external termination point exceeds approximately 3 metres of straight duct (or the equivalent in duct with bends), a standard wall-mounted extractor fan may not maintain the minimum flow rate. An inline fan — installed in the duct run itself, typically in the ceiling or roof void — provides higher static pressure capability to overcome the resistance of a longer duct. Inline fans are also quieter in operation, as the noise source is remote from the bathroom rather than immediately overhead. They are the preferred solution for London flats where the bathroom is remote from an external wall and the duct must pass through multiple floor or ceiling voids.

Installation Costs in London

A straightforward like-for-like replacement of an existing extractor fan in London — same location, existing wiring and ducting — costs between £150 and £250 including the fan unit, labour, and connection to the existing switched live. A new installation involving new wiring and duct routing through an external wall or ceiling void typically costs between £250 and £400. Installations requiring an inline fan or extended duct routing to a remote external termination point may cost £350 to £500 or more. These costs include a compliant installation; any electrician who quotes significantly below these ranges for a new circuit installation should be asked to confirm that the work will be Part P certified.

Landlord Obligations and Condensation Prevention

London landlords have an obligation under the Housing Health and Safety Rating System (HHSRS) to ensure rental properties are free from excess moisture and mould, which falls under the Category 1 hazard of damp and mould growth. An inadequate or non-functioning bathroom extractor fan is a contributing factor to damp and mould complaints and can form part of an HHSRS enforcement action. Ensuring all bathrooms in a rental property have a compliant, functioning humidistat extractor fan is one of the most cost-effective steps a London landlord can take to reduce condensation-related maintenance costs and tenant complaints.