Building Regulations Part F (Ventilation) for London Extensions and Conversions

**The regulatory framework**:

• Part F of the Building Regulations is published as Approved Document F (ADF). As of the 2021 revision (which took effect in June 2022), ADF has two volumes:
• •*Volume 1*: Dwellings (houses and flats converted into dwellings) — the relevant document for most London residential projects
• •*Volume 2*: Buildings other than dwellings — for HMOs and commercial properties

The 2021 revision introduced several changes from the previous (2010) version that are relevant to London extensions: 1. *Ventilation rates increased* for dwellings — whole-house extract ventilation and purge ventilation requirements are more stringent 2. *New home ventilation requirements* (Section 2): for new dwellings and for extensions/conversions that result in a new dwelling (e.g., a house-to-flat conversion), whole-dwelling continuous mechanical ventilation (MEV or MVHR) may be required rather than just room-by-room extract 3. *Air quality considerations* formalised — CO₂ levels as a proxy for poor air quality; provision for monitoring in certain circumstances 4. *Ductwork commissioning* now requires a test report to be provided to Building Control (for MEV and MVHR systems)

**Part F ventilation types**:

ADF identifies three types of ventilation that must be provided:

*1. Background ventilation (trickle ventilators)*: Purpose: low-level continuous ventilation to provide fresh air exchange when windows and doors are closed. Provided by trickle ventilators (small slots in the head of window frames, or through-wall ventilators). The minimum equivalent area (EA) required per room depends on the room type and the whole-dwelling background ventilation calculation.

Typical trickle ventilator sizes: 4,000mm² EA per habitable room (kitchen/diner, living room, bedroom) for most dwellings. Note: 'equivalent area' is a measure of the open area of the vent — 4,000mm² = approximately a 50×80mm slot.

• *2. Purge ventilation (openable windows)*:
• Purpose: rapid ventilation of individual rooms to purge pollutants, moisture, or cooking odours. Provided by openable windows (and/or external doors where applicable). The ADF requires minimum openable area:
• •Habitable rooms: minimum 1/20 (5%) of the floor area of the room, with the opening part of a window or door
• •Wet rooms (kitchens, bathrooms, utility rooms): minimum 1/20 (5%) of the floor area, or provision of the specified extract ventilation rate

• *3. Extract ventilation (fans)*:
• Purpose: removal of moisture and cooking odours at source in 'wet rooms'. Required in:
• •Kitchen (where cooking appliances are present)
• •Bathroom (with bath or shower)
• •Utility room (where washing machine or tumble dryer are installed)
• •Shower room
• •WC (if internal — no windows)

**Room-by-room extract ventilation rates (ADF 2021, Volume 1)**:

| Room | Minimum intermittent extract rate | Minimum continuous extract rate | |---|---| | Kitchen (with hob) | 60 l/s adjacent to hob (or 30 l/s elsewhere) | 13 l/s | | Utility room | 30 l/s | 8 l/s | | Bathroom (with bath or shower) | 15 l/s | 8 l/s | | Shower room | 15 l/s | 8 l/s | | WC (internal) | 6 l/s | — (not applicable) |

These rates are for intermittent extract fans (the fan runs when the room is occupied and for a run-on period after) and for continuous extract (the fan runs continuously at low background rate, boosting when the room is occupied). The fan specification must be selected to meet these rates.

**Duct routing for extract fans**:

• All kitchen and bathroom extract fans must duct to the outside — not to a roof space or ceiling void (this creates condensation problems in the void). The duct must:
• •Be as short and straight as possible — each bend reduces airflow
• •Be minimum 100mm internal diameter for kitchen extract (150mm or larger for high-capacity kitchen extract); minimum 100mm for bathroom
• •Terminate with a weather-proof louvred grille or wall cap on the external facade
• •Be insulated where it passes through uninsulated spaces (roof space, under-floor void) to prevent condensation within the duct

**Kitchen ventilation — the most frequently under-specified item in London extensions**:

Kitchen ventilation is the most commonly under-specified and most immediately problematic element of Part F compliance in London extensions. The requirements depend on whether the kitchen has a hob (gas or induction):

• *For kitchens with a hob (the normal case)*:
• •Minimum extract rate: 60 l/s (litres per second) when the fan is adjacent to the hob (within 300mm) — i.e., a cooker hood directly over a hob
• •Or: 30 l/s for a wall-mounted extract fan not adjacent to the hob (this is a lower-performance option and not recommended for a kitchen with regular cooking)

• *What 60 l/s means in practical terms*:
• •60 l/s = 216 m³/h — this is a significant extraction rate; most cheap cooker hoods (the type sold with kitchen cabinets) have recirculating filters, not extraction — these do NOT count for Part F compliance
• •A real extract cooker hood must duct to outside (not recirculate), must be rated at minimum 60 l/s (216 m³/h), and must be installed with a continuous smooth duct (as short and straight as possible) to the external wall

• *What 'recirculating' hoods do NOT provide*:
• •Recirculating hoods pass air through a grease filter and a carbon filter and return it to the kitchen — they remove grease particles and odours but do not remove moisture. Moisture from cooking steam is the primary cause of condensation and mould in kitchen areas. Recirculating hoods do not comply with Part F.

• *Ducting a cooker hood in a rear extension*:
• •The most common route is through the rear wall of the extension below the eaves line, using a 150mm–200mm diameter duct
• •Alternatively, through the flat roof (a vertical upward duct through the roof insulation to a roof grille) — but this creates thermal bridging and a penetration in the flat roof membrane (a potential leak point)
• •The duct should be sleeved with a 150mm core drill through masonry walls and finished with a wall cap externally

• *Gas hob additional requirement*:
• •If a gas hob is installed in the new kitchen, Building Regulations also require compliance with Part J (Combustion appliances) — specifically, provision for combustion air and flue gases from the gas appliance. ADF and ADA (Approved Document J) requirements must both be met — typically a minimum ventilation opening of 650mm² per kW of the combined gas appliance rating.

**Whole-dwelling ventilation — when does the extension trigger a whole-house requirement?**:

For existing dwellings being extended (not a new dwelling), ADF 2021 applies 'consequential improvements' requirements when the extension is significant enough to trigger them:

• *The trigger test (ADF 2021, Section 3)*:
• If the extension creates a 'new or replacement ventilation system' or if the extension results in the creation of a 'wet room' (kitchen, bathroom, utility room) that did not previously exist, the whole-dwelling ventilation must be checked. In practice, for a typical single-storey rear extension:
• •If the extension creates an open-plan kitchen where there was previously a separate kitchen with extract ventilation, the new open-plan kitchen must have extract ventilation to Part F standards
• •The extension itself must have trickle ventilation (background) and purge ventilation (openable windows) at the standard rates
• •If the rest of the house already has adequate ventilation (trickle vents in windows, extract fans in bathrooms), no whole-dwelling upgrade is required

• *When whole-dwelling ventilation IS triggered*:
• •New-build dwellings (not extensions to existing): ADF 2021 requires MEV (mechanical extract ventilation) or MVHR (mechanical ventilation with heat recovery) in new dwellings above a certain air-tightness level — specifically, dwellings tested at q50 < 5 m³/(h·m²) must use MEV or MVHR rather than passive/trickle ventilation
• •House-to-flat conversion (creating a new dwelling): the new flat must meet the ADF 2021 standards for new dwellings — typically requiring mechanical ventilation

**Loft conversion — Part F requirements**:

A loft conversion typically creates 1–2 bedrooms and 1 bathroom. For each room:

• *Bedrooms*:
• •Trickle ventilators in the roof windows (Velux or dormer windows) — typical specification: 4,000mm² equivalent area trickle ventilator in each roof window; check with the Velux/window manufacturer that the window includes adequate trickle ventilation
• •Purge ventilation: the roof window must be openable (fixed roof windows in bedrooms do not satisfy purge ventilation)

• *Bathroom in the loft*:
• •Extract fan to outside at minimum 15 l/s (intermittent) — routing the duct to outside is more complex in a loft (must exit through the roof or through a gable wall; cannot terminate in the roof space)
• •Typical solution: 100mm duct through the roof covering via an in-line roof tile vent (for pitched roofs) or via the flat roof of a dormer cheek
• •Trickle ventilation: if there is a window in the loft bathroom, include trickle vent; if no window, the extract fan provides the required background rate

• *Staircase and landing*:
• •Part F does not specifically require ventilation to the staircase and landing, but Building Regulations Part B (Fire safety) may require a protected staircase in a 3-storey house after a loft conversion — see `loft-conversion-building-regulations`

**Mechanical Ventilation with Heat Recovery (MVHR) — when is it appropriate for a London extension?**:

MVHR is a whole-dwelling ventilation system that: 1. Extracts stale air from wet rooms (kitchen, bathrooms, utility room) via continuously running extract fans and ducts 2. Passes the extracted air through a heat exchanger within a central MVHR unit 3. Draws fresh outside air through the heat exchanger, warming the incoming fresh air with the heat from the extracted stale air (heat recovery efficiency 75–95%) 4. Supplies the warmed fresh air to habitable rooms (bedrooms, living rooms) via a supply air duct system

• *MVHR advantages*:
• •Recovers 75–95% of the heat in the extracted air — reduces heating bills in well-insulated dwellings
• •Provides continuous fresh, filtered air to all habitable rooms without draughts from opening windows
• •Reduces condensation and mould (continuous low-level extract keeps moisture levels controlled)
• •Virtually silent in normal operation (central unit in loft, roof space, or utility area)

• *MVHR for London extensions — when is it worth it?*:
• •MVHR is most cost-effective in new, highly air-tight dwellings — it requires good air-tightness (q50 < 3 m³/(h·m²) or better) to justify the installation cost and to avoid short-circuiting (air leaking in through the building fabric rather than entering through the controlled supply)
• •In a typical London Victorian house extension (masonry cavity walls, existing house with inherent air leakage), MVHR is not typically cost-effective — the existing house is not air-tight enough for MVHR to function correctly, and the extension alone is too small to justify the ducting investment
• •MVHR is worth serious consideration for: complete refurbishments where the entire building envelope is being upgraded and air-tightness can be achieved; new-build annexes or garden rooms designed to near-Passivhaus standards

*Cost of MVHR*: Supply and installation in a 3-bedroom London house: typically £6,000–£15,000 depending on ductwork complexity.

**Commissioning and documentation for Part F**:

The ADF 2021 introduced new commissioning and documentation requirements:

• *What must be commissioned*:
• •All intermittent and continuous extract fans must be tested for airflow rate (using an anemometer or flow hood) and must be confirmed to meet the minimum extract rates specified in ADF Table 1.1
• •MEV and MVHR systems must be balanced (supply and extract airflows measured and adjusted to within 10% of the design rates at each terminal) and a commissioning report produced

• *What must be given to Building Control*:
• •A commissioning certificate confirming the ventilation system has been installed and tested
• •For MEV/MVHR: a full commissioning report showing measured airflow rates at all supply and extract terminals
• •The Building Control inspector may ask for the commissioning certificate before issuing a completion certificate

**Common Part F compliance failures in London extension work**:

*1. Kitchen cooker hood recirculating (not extract)*: As above — the most common failure. The kitchen hood must extract to outside; recirculating filters are not compliant for moisture removal.

*2. Extract fan ducted to roof space rather than outside*: Bathroom or kitchen extract fan duct terminated in the roof space (loft) rather than through the roof or external wall. This introduces warm, moist air into the roof space and creates ideal conditions for mould, timber decay, and condensation on the roof structure.

*3. Trickle ventilators absent or sealed*: Windows installed without trickle ventilators, or homeowners sealing trickle ventilators (to reduce draughts) — both reduce background ventilation below the Part F requirement.

*4. Duct cross-sectional area too small*: Kitchen extract duct with 100mm internal diameter where the cooker hood requires 150mm or larger to achieve the required extract rate — the airflow is limited by the duct, not the fan motor.

*5. Duct length or bends excessive*: A cooker hood with four 90° bends and 6m of duct may achieve half its rated airflow or less — the duct resistance exceeds the fan's capacity. Each 90° bend is equivalent to approximately 1.5m of straight duct for resistance purposes; total equivalent duct length should ideally not exceed 6–8m for a standard domestic cooker hood.