Class G flues for biomass and heat network systems

Flue permitted development for a dwellinghouse turns on one measurement: the flue must not rise more than a metre above the highest part of the roof. Get that right, keep the flue off a highway-facing elevation if the house sits on protected land, and most installations on a house need no planning application at all. Flats and maisonettes have no rights here, and a listed building needs consent whatever the Order says.

The General Permitted Development Order 2015 handles flues in two places. Class G of Part 1 of Schedule 2 covers the installation, alteration or replacement of a chimney, flue or soil vent pipe on a dwellinghouse. Separate classes in Part 14, the renewable energy part, cover a flue forming part of a biomass heating system and a flue forming part of a combined heat and power system on a dwellinghouse. Read the current consolidated text on legislation.gov.uk before you rely on either, because the Order is amended often.

What the flue classes permit

The Part 1 right is the general one. A new chimney on a rear elevation, a replacement flue for a gas boiler, a soil vent pipe rerouted during a bathroom refit: all of these sit within Class G on a house.

The Part 14 classes are narrower and tied to the appliance. A biomass boiler or stove flue and a combined heat and power flue each have their own class, and each is limited to one flue per dwellinghouse. That limit catches households installing a wood burner in addition to an existing biomass boiler, since the second flue falls outside the class.

None of these classes apply to a building containing flats. Part 1 of Schedule 2 applies to a dwellinghouse, and the Order defines that term so as to exclude a building containing one or more flats and a flat itself. Our note on why flats and maisonettes have no Part 1 rights explains the consequence for leaseholders, who need a planning application for work a neighbouring house owner could do freely.

The one metre limit above the ridge

The height limit is expressed against the highest part of the roof. On a simple gabled house that is the main ridge. On a house with a chimney stack already standing proud, the stack is part of the roof structure rather than the roof, so measure to the ridge, and take advice if the geometry is unusual.

Three things make this harder than it looks:

• Flat roofs with parapets, where the highest part of the roof and the top of the parapet are not the same thing
• Catslide and mansard roofs, where the highest part may sit well away from the flue position
• Rear extensions with their own lower ridge, which do not raise the limit for a flue on the main roof

The limit matters more for solid fuel than for gas, because combustion appliance standards often want a flue terminal well clear of the ridge to avoid downdraught. A flue that satisfies the appliance manufacturer and the Building Regulations can easily exceed a metre above the ridge, and at that point planning permission is needed.

Restrictions on article 2(3) land

Article 2(3) land means a conservation area, a National Park, a National Landscape (formerly an Area of Outstanding Natural Beauty), the Broads or a World Heritage Site. On that land, the flue classes withdraw the right where the flue would be installed on a wall or roof slope that fronts a highway and forms the principal elevation or a side elevation of the house.

Two points are commonly missed. A footpath or a bridleway can be a highway, so a house facing a public path rather than a road may still be caught. And the restriction is about which elevation the flue sits on, so a flue on a rear roof slope that happens to be visible from the road remains permitted.

Our guide to article 2(3) land and permitted development in protected areas sets out the wider pattern of restrictions across Part 1, which go considerably further than flues.

An article 4 direction can remove the right altogether. Councils use these in conservation areas where roofscape is part of what the designation protects, so check the direction before assuming the class applies.

Listed buildings and why the class does not help

A flue on a listed building needs listed building consent because it alters the building in a way that affects its character. That requirement is independent of the planning position, and installing without consent is a criminal offence under the Planning (Listed Buildings and Conservation Areas) Act 1990.

Conservation officers look at the flue terminal, the material and the route. A stainless steel twin-wall flue climbing a rear elevation is a different proposition from a rendered chimney breast rebuilt in the original position. Where the house has evidence of a former stack, reinstating in that position is the easiest case to argue.

For an unlisted building in the curtilage of a listed building, planning permission may also be needed, and the harm test under national heritage policy applies to the setting of the listed building.

Building Regulations running alongside

Planning and Building Regulations are separate approvals and the Building Regulations do not care whether the flue is permitted development. Approved Document J covers combustion appliances and fuel storage systems, and it governs flue sizing, termination positions relative to openings and roof surfaces, hearth construction, ventilation air supply and carbon monoxide alarms.

Solid fuel appliance installation is usually notified through a competent person scheme, which issues a certificate. If it is not, a building notice to the local authority building control body is the route. Keep the certificate, because a buyer's solicitor will ask for it.

Air supply is the requirement most often designed out and then discovered late. A modern airtight house with a mechanical ventilation system needs a dedicated air supply to a solid fuel appliance, and retrofitting one after plastering is disruptive.

Smoke control areas and solid fuel

Most urban areas of England are smoke control areas under the Clean Air Act 1993. Inside one, you may only burn authorised fuels, or burn other fuels in an exempt appliance approved for that purpose. Defra publishes the lists of authorised fuels and exempt appliances, and they are updated regularly.

Enforcement in England moved to a civil penalty regime under changes made by the Environment Act 2021, so a council can issue a financial penalty for emitting smoke from a chimney in a smoke control area rather than prosecuting. Sales of wet wood and traditional house coal for domestic burning are also restricted.

None of this is a planning matter, and the council's planning team will not tell you about it. Check the smoke control position with environmental health before you buy the appliance.

When a full application is the route

Apply for planning permission where the flue exceeds a metre above the ridge, where the property is a flat, where an article 4 direction has removed the right, or where the flue must go on a highway-facing principal or side elevation on article 2(3) land.

A householder application for a flue is a small submission: forms, a location plan, existing and proposed elevations showing the flue position and height, and a short supporting note. A one-page Planning Statement covering the design rationale, the visual effect and the heating benefit is usually enough at this scale.

The only formal way to confirm that a flue is lawful without permission is a lawful development certificate under section 192 of the Town and Country Planning Act 1990. That is worth the fee where the property is in a conservation area or where a sale is in prospect. Householders installing other equipment face similar questions, and our note on EV charge points under Part 2 covers the nearest equivalent rules.

Enter your postcode on PlanScope to check the designations affecting your property before you commission an installer.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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