A flue or chimney for a biomass boiler on an old building

A biomass flue on an old building can need three separate approvals, and the one people miss is the chimney height approval under section 14 of the Clean Air Act 1993. It applies where a furnace burns solid matter above a stated rate or liquid or gaseous fuel above a stated thermal input, and it is granted by the local authority's environmental health function rather than by planning. A flue with planning permission and no section 14 approval cannot lawfully be used.

The other two are familiar. Planning permission for the flue itself, which permitted development may cover on a dwelling, and listed building consent where the building is listed and fabric is being cut.

Permitted development for flues, and what it excludes

For a dwellinghouse there are two relevant routes in Schedule 2 to the General Permitted Development Order 2015. Part 1 Class G covers the installation, alteration or replacement of a chimney, flue or soil and vent pipe. Part 14 Class G covers a flue forming part of a biomass heating system on a dwellinghouse or a block of flats.

Both carry the same two headline limits. The flue must not exceed the highest part of the roof by more than one metre. And on designated land, which means a conservation area, a National Park, a National Landscape or a World Heritage Site, the flue must not be installed on a wall or roof slope that fronts a highway and forms the principal or a side elevation.

Three exclusions catch rural schemes. Permitted development rights under Part 1 do not apply to flats or to buildings that are not dwellinghouses, so a flue on a converted barn used as a holiday let or on an agricultural building is assessed differently. Rights may have been removed by condition when a conversion was permitted. And an Article 4 direction can remove them across a defined area.

A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful without permission. On a building with a complicated consent history it is worth the fee.

The separate chimney height approval under the Clean Air Act

Section 14 of the Clean Air Act 1993 requires the local authority to approve the height of a chimney serving a furnace. The thresholds are set in the Act: burning pulverised fuel, or solid matter at a rate of 45.4 kilogrammes or more per hour, or liquid or gaseous matter at a rate equivalent to 366.4 kilowatts or more.

A domestic log stove falls well below those figures. A biomass boiler heating a farmhouse, a let cottage and a workshop can approach or exceed the thermal threshold, and a chip fired boiler serving a commercial building usually does. Check the appliance's rated input rather than its output, and ask the supplier for the figure in writing.

The authority assesses whether the height is sufficient to prevent smoke, grit, dust, gases and fumes becoming prejudicial to health or a nuisance, taking account of the surroundings, nearby buildings and the local topography. Approval can be refused or granted subject to a greater height.

That greater height is the problem on an old building, because it collides directly with the planning and heritage position. A flue approved at four metres above the ridge for dispersion reasons will not get planning permission in a conservation area. Run both conversations at once rather than in sequence, and if the two cannot be reconciled, the answer is usually a different heat source. Section 4 of the same Act also requires new furnaces to be capable of operating without emitting smoke when burning the fuel for which they are designed.

Smoke control areas and exempt appliances

Much of urban England and parts of many rural districts sit within smoke control areas declared under the Clean Air Act. Inside one, emitting smoke from a chimney is subject to enforcement unless an authorised fuel or an exempt appliance is used.

The practical requirement is an appliance on the published exempt list, operated on the fuel the exemption specifies. Pellet and chip boilers are commonly exempted. A log batch boiler may be exempted only when operated with a specified accumulator volume, and that condition is easy to overlook when the plant room is designed.

Separate rules restrict the sale of wet wood and bituminous coal for domestic use in England, so a farm selling its own logs needs to check the moisture content and labelling requirements. Ask the authority whether the site is in a smoke control area before specifying the appliance. The boundaries are not intuitive and are published on the authority's website.

Listed building consent for a new penetration

Cutting a flue through a listed building affects its character as a building of special architectural or historic interest, so listed building consent is required. That applies to a penetration through a roof slope, through a wall, and often to internal works such as removing a chimney breast or opening a blocked flue.

The argument that works best is the one that uses what is already there. Lining an existing chimney stack with a flexible or rigid liner, terminating within the existing pot, usually causes less harm than a new stainless steel flue on an elevation. Where the existing stack is unsuitable, a new flue in a secondary or later part of the building is easier to justify than one through the principal range.

A Heritage Statement should set out the significance of the affected fabric, the alternatives considered, the degree of harm and the public benefit relied on. Historic England guidance on heritage significance and on setting is the right starting point, and the statutory duties in sections 16 and 66 of the Planning (Listed Buildings and Conservation Areas) Act 1990 apply to the decision.

For curtilage structures, remember that a barn within the curtilage of a listed building that predates July 1948 is generally treated as listed, which surprises owners planning to put the boiler in an outbuilding.

Conservation areas and which elevation the flue sits on

In a conservation area the designated land restriction means the flue cannot go on the principal or side elevation fronting a highway under permitted development. A rear or inner elevation flue may still be permitted, subject to the one metre above ridge limit.

Where permission is needed, the assessment is about visibility and the contribution the roofscape makes to the area's character. A matt black flue on a slate roof behind a parapet reads very differently from a bright stainless flue on a prominent gable.

The design moves that help are consistent: use the existing stack where one exists, terminate as low as dispersion allows, match the finish to the roof covering, keep the flue off principal elevations, and avoid external vertical runs where an internal route is possible. Set them out in the Planning Statement as the alternatives tested rather than leaving the officer to assume none were.

Air quality where the site is in a management area

Biomass combustion emits oxides of nitrogen and particulate matter, and authorities with declared Air Quality Management Areas take that seriously even for small plant. Where the site is inside an AQMA, or close to one, expect to be asked for emissions information.

For a boiler of modest size a short note with the manufacturer's emissions data, the stack height, the nearest sensitive receptors and the dispersion assumptions is often enough. For larger plant, a modelled Air Quality Assessment following the relevant Institute of Air Quality Management guidance is the expectation, prepared by a suitably qualified author, and professional review is strongly advised.

Check the national air quality information service and the authority's latest air quality annual status report for the AQMA boundary and the pollutants of concern. Nitrogen dioxide and particulate matter are the usual ones.

Fuel store, deliveries and access

The boiler is the small part of the planning case. A chip or pellet store sized for a season is a building, and a building in a rural setting raises its own siting and design questions. Silos, hoppers and bunkers all need drawing with dimensions and finishes.

Deliveries need a vehicle that can reach the store and leave in forward gear. Blower deliveries need the lorry within hose reach of the fill point, which is often 20 metres or less. Show the tracking on a plan if access is tight, because a highways objection on an otherwise acceptable scheme is avoidable.

Ash handling, fuel moisture and storage ventilation are operational matters rather than planning ones, though a condition on hours of delivery is common where the nearest dwelling is close.

Drawings and specification to submit

A workable pack is existing and proposed elevations showing the flue and its height relative to the ridge, a roof plan, a cross section through the flue route, a site plan showing the fuel store and delivery access, the appliance specification including rated input and exempt appliance status, the heritage statement where the building is listed or in a conservation area, and the air quality information where an AQMA applies. Where the scheme forms part of a wider retrofit, an Energy Statement ties the heat source into the building's overall strategy.

Enter your postcode on PlanScope to see which reports a listed or rural site 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. For specialist technical reports, qualified professional input may be required.

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