Flues and soil vent pipes on a front roof slope

A flue on the front roof slope of a house in a conservation area is not permitted development, and careful detailing does not change that. Everywhere outside designated land, Class G of Part 1 of the GPDO 2015 allows a chimney, flue or soil and vent pipe on a dwellinghouse without an application, provided it does not rise a metre or more above the highest part of the roof. Those two rules settle most questions about flue front roof slope permission before anyone opens a drawing.

The difficulty starts when the appliance manufacturer, Approved Document J and Class G each want the terminal in a different place.

What Class G actually permits

Class G of Schedule 2, Part 1 to the Town and Country Planning (General Permitted Development) (England) Order 2015 covers the installation, alteration or replacement of a chimney, flue or soil and vent pipe on a dwellinghouse.

Part 1 applies to houses. It does not apply to flats or maisonettes. A boiler flue or a soil pipe on a converted building of flats therefore needs a full planning application rather than a householder one, even for a 100 mm stainless terminal on a rear wall.

Class G is also lost in the ordinary ways. An Article 4 direction can withdraw it across a conservation area or along a single street. Article 3(4) of the Order means permitted development cannot authorise anything carried out in breach of a condition, so an estate where Part 1 rights were removed by condition on the original permission has no Class G at all. Dwellings created through prior approval routes such as Class Q or Class MA frequently have Part 1 restricted as well. Read the planning history before you assume anything, because the deeds rarely mention it and the vendor almost never does.

Positions on a wall follow the same class but behave differently in practice, and the rules on flues, vents and stoves on a house elevation cover that case.

The one metre measurement, and where people misread it

Class G.1(a) removes permission where the height of the chimney, flue or soil and vent pipe would exceed the highest part of the roof by one metre or more.

Two words in that sentence do a lot of work. The first is "highest". The comparison is with the highest part of the roof of the dwellinghouse, not with the piece of roof the flue happens to pass through. On a two-storey semi with a single-storey rear addition, a flue rising from the flat roof of that addition is measured against the main ridge, which usually leaves a generous allowance. Plenty of householders assume the opposite and apply when they did not need to.

The second is "or more". A terminal 950 mm above the ridge is within Class G. One at 1,000 mm is not. Building tolerances and a cowl added later can move a compliant flue over the line, and the breach is measured as built rather than as drawn.

Where the works also involve taking down an existing stack, the position is different again, as set out in the rules on removing a chimney.

Designated land and the elevation test

Class G.1(b) is the restriction that catches front roof slopes. On article 2(3) land, a chimney, flue or soil and vent pipe is not permitted development if it would be installed on a wall or roof slope which fronts a highway and forms either the principal elevation or a side elevation of the dwellinghouse.

Article 2(3) land means conservation areas, National Landscapes (formerly Areas of Outstanding Natural Beauty), National Parks, the Broads and World Heritage Sites.

Both halves of the test have to be satisfied before the restriction bites.

That last row is where the money is. A house set behind a green, a courtyard or a wholly private access may have a principal elevation that fronts no highway, in which case a front-slope flue can still be permitted development. The status of the access is a question of fact. Adopted highway records sit with the county or unitary highway authority, and the householder technical guidance takes a wide view of what counts as a highway, so an unadopted lane carrying a public right of way may still defeat you.

Soil vent pipes, section 55 and the internal route

Not every soil pipe is development at all. Section 55 of the Town and Country Planning Act 1990 excludes works that affect only the interior of a building, or that do not materially affect its external appearance. A stack boxed inside the house, terminating through a rear slope with a small vent tile, is a weak candidate for enforcement even before Class G is considered.

Approved Document H sets the constraint that usually forces the pipe outside. A ventilating pipe open to the air has to finish clear of openings into the building, and the figure most designers work to is 900 mm above the top of any opening within 3 m. Air admittance valves to BS EN 12380 can replace an open stack inside the dwelling, though the drainage system still needs at least one ventilated discharge stack, normally at the head of the run. On a terrace where the front slope is the only roof facing the right way, that single ventilated stack is the one you cannot hide.

So the honest sequence is: design the drainage so the open vent lands on a rear or flank slope, then use valves internally for the rest.

When the appliance dictates the position

Approved Document J governs flue outlet heights for combustion appliances, and it does not care about Class G. For solid fuel, a flue passing through the roof within 600 mm of the ridge is normally required to terminate at least 600 mm above it, with greater heights where the flue emerges further down the slope. Gas appliances have their own terminal clearances from openings, boundaries and adjoining property. Verify both against the current edition rather than an older printed copy, because the diagrams have changed.

Biomass and wood-burning appliances push the numbers upward. Dispersion, downdraught from a nearby taller building and the chimney-height requirements that apply in a smoke control area under the Clean Air Act 1993 can all demand a terminal above the metre allowed by Class G. At that point the choice is a planning application or a different appliance. Note that a smoke control area also restricts which appliances may be used at all, and the exempt appliance list is maintained centrally rather than by the council.

Listed buildings and internal routing

Class G still exists on a listed dwelling. Listed building consent is a separate consent and is not displaced by it.

Section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 makes it an offence to carry out works affecting the character of a listed building as a building of special architectural or historic interest without consent, and that includes internal works. Cutting a flue liner through a historic stack, forming a new penetration in an early roof structure or removing lath and plaster to run a pipe all engage it. A Heritage Statement setting out the significance of the affected fabric, the alternatives tested and the reason the chosen route causes least harm is what the conservation officer will read first.

Reusing an existing redundant flue, relined rather than rebuilt, is the route that most often gets agreed.

Applying where the position cannot change

A householder application is the normal vehicle for a house, and a full application for a flat. The drawing set is small: existing and proposed elevations at 1:50 or 1:100, a roof plan showing the terminal position, and a section if the flue height is the point in dispute. Manufacturer data for the terminal and a note of the Approved Document J requirement that fixes the height are both worth submitting, since they turn a design preference into a technical constraint the officer can weigh.

A short Planning Statement covering the appliance need, the alternatives considered and the visual effect on the roofscape does more for a designated-land case than another elevation. Application fees are set nationally and indexed annually, so check the current fee schedule before you submit.

If the flue is already in place and you believe it was permitted development, the route is a certificate of lawfulness for existing use or development under section 191, supported by dated photographs and invoices. A certificate is the only formal way to confirm that works are lawful. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which supporting reports a flue or roof alteration on designated land is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.