Guide
Extract and ventilation in period flats
The extract route is one of the few things in a kitchen or bathroom that the building can simply refuse to provide.
Why it is settled first
In a new-build the duct route is a detail. In a period flat it is a constraint that shapes the layout, because there may be only one wall it can honestly leave through — and sometimes none. Deciding it late means either a hole somewhere regrettable or a hood that does not vent.
So the route is agreed at survey, while the old kitchen is still standing: which wall, what run length, how many bends, where the terminal lands and what it looks like from outside.
What a conservation area controls
Conservation-area designation concerns the appearance of the area, so it reaches the terminal rather than the fan. A grille on a principal elevation — the front of the house, or any elevation prominent from the street — is the case most likely to attract objection, and on a listed building the position is stricter still.
Internal bathrooms and shared ducts
An internal bathroom with no external wall needs mechanical extract with a real route to outside. What it must not have is a fan discharging into a ceiling void or a roof space, which simply moves the moisture somewhere it will do damage quietly.
In mansion blocks there is often a shared duct serving stacked bathrooms. Connecting into it is usually the right answer and usually needs the freeholder's consent, because it is common fabric. Where an existing connection is being reused, it is worth checking it is still clear: decades of accumulated grease and lint are common.
When recirculation is the right answer
Where no acceptable duct route exists, a recirculating hood with carbon filtration is the honest specification. It handles smell and grease reasonably well and does nothing for moisture, so it works best in a kitchen with an openable window.
We would rather specify that openly at survey than duct into a void and describe it as extraction. Filters need replacing on a schedule; that is a maintenance point worth knowing before you choose it rather than after.
Questions
Sometimes, in a top-floor flat, though it needs a proper terminal and a route that does not run through insulation or a cold void where condensation will form in the duct. It also usually reaches common fabric, so in a leasehold building it becomes a consent question. It is worth investigating where a wall route is unavailable, but it is not a simple substitute.
A great deal. Every bend and every metre reduces what the fan actually moves, and a long convoluted run through cupboards can leave a powerful hood performing poorly. That is why route length is part of the layout decision rather than something worked out afterwards by the fitter.
The work this applies to
Where this comes up
Related guides
- Renovating in a conservation area: what consent actually covers
Much of North West London sits in a conservation area, and the rules bite far harder on the outside of a building than on the rooms inside it.
- The rear-return kitchen: layouts that work
The standard conversion kitchen in NW6, NW5 and N8 is a narrow rear return, and its width decides almost everything about it.