Siting an air source heat pump in a conservation area
Three tests decide where an air source heat pump goes in a conservation area: whether Class G applies, visibility from the highway, and the 42 dB(A) sound calculation.
An air source heat pump in a conservation area is usually possible, and the position it ends up in is decided by three things in this order: whether Class G of Part 14 of the GPDO 2015 is available at all, where the unit can go without being seen from the street, and whether the calculated sound level at the neighbour's window comes in at or below 42 dB(A). Get the third one wrong and even a planning permission will not save you from an environmental health complaint.
If the house is listed, Class G does not apply at all, and you need listed building consent as well as, in most cases, planning permission. Start there, because it changes the sequence.
What Class G allows on designated land
Class G of Part 14 permits the installation, alteration or replacement of an air source heat pump on a dwellinghouse or block of flats, or within its curtilage, subject to limits and conditions.
The restrictions that matter in a conservation area are about position. On designated land, which for these purposes includes conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites, the right is withheld where the unit would go on a wall or roof in the more visible positions, broadly those fronting a highway or forming part of the principal elevation. A unit in a rear garden, behind the building line and not visible from the highway, is the straightforward case.
Two conditions apply to anything installed under the class. The unit must be sited, so far as practicable, to minimise its effect on the external appearance of the building and on the amenity of the area. And it must be removed as soon as reasonably practicable when no longer needed.
One point to check rather than assume. The numerical limits in Class G, including the permitted volume of the outdoor unit and the rules about proximity to a boundary, have been amended more than once in recent years, and guidance written before those amendments is still circulating. Read the current text of Part 14 Class G on legislation.gov.uk before you rely on a dimension, and check whether your conservation area is covered by an Article 4 direction that removes the right entirely.
The 42 dB(A) sound test
Class G requires the installation to comply with the applicable planning standards for permitted development installations, which set a sound limit assessed at the nearest neighbouring habitable room window.
The test is a calculation, not a measurement. The installer takes the unit's declared sound power level, applies corrections for distance and for the acoustic effect of the position, including barriers and reflecting surfaces, and demonstrates a resulting sound pressure level no greater than 42 dB(A) at the assessment position. Corner positions, narrow passages between terraced houses and units bounced off a neighbouring flank wall all push the figure up.
The practical consequences for a conservation area terrace are worth spelling out:
- A rear garden position tight to a party fence can fail on distance alone, even where the unit is modest.
- A side passage between two terraces is usually the worst acoustic position available, because reflections from both walls add to the level.
- Moving a unit 2 m further from a boundary does more for the calculation than any acoustic housing bolted on afterwards.
- Acoustic enclosures add volume, which can take the installation outside the permitted dimensions, and they restrict airflow, which reduces output.
Where the calculation fails, the installation is not permitted development and a planning application is needed. In that case the authority's environmental health officer will want a proper assessment rather than the standard calculation, and a Noise Impact Assessment prepared by a suitably qualified acoustician is what gets the application through. Councils expect a qualified author for this type of assessment and professional review is strongly advised.
Listed buildings: a different route
Class G is not available for a unit within the curtilage of a listed building. The works need listed building consent under the Planning (Listed Buildings and Conservation Areas) Act 1990 where they affect the special architectural or historic interest, and planning permission in most cases as well.
The decision turns on harm. Section 16 of the 1990 Act requires special regard to the desirability of preserving the building, its setting and features of special interest, and section 72 requires special attention to the character and appearance of a conservation area. In practice an officer will accept a well-sited unit with a clear carbon benefit and resist one bolted to a principal elevation.
What helps the case:
- Show the positions you considered and why each was rejected, with photographs taken from the street.
- Keep the unit off principal and other prominent elevations, and below the eaves line.
- Route pipework internally or on a secondary elevation, and specify the colour and the fixing method into mortar joints rather than through brick or stone.
- Use existing screening, such as an outbuilding, a boundary wall or planting that was already there.
- Commit to reversibility and say what would be left if the unit were removed.
A Heritage Statement setting out significance, the degree of harm and the public benefit is the document that carries this. Where the building is Grade II* or Grade I, there are additional consultation and referral requirements, which we cover in Grade I and II* applications and referral requirements. If you want certainty before starting, a certificate of lawfulness of proposed works to a listed building is available, and our note on that certificate explains when it is worth the application fee.
An illustrative example
This example is illustrative. A Victorian mid-terrace in a conservation area has a 3.5 m wide rear garden and a side return. The installer's first position is in the side return, 0.8 m from the party boundary. The calculation returns a figure above 42 dB(A) at the neighbour's ground floor rear window because of reflections off both flank walls. Moving the unit to the rear garden boundary, 4.2 m from that window and with the fence acting as a barrier, brings the figure under the limit. The unit is not visible from the street. No application is needed, though the owner obtains the installer's written calculation and keeps it with the house papers.
Pipework, condensate and enclosures
The parts nobody draws are the parts that cause the heritage objection.
Flow and return pipework on an external wall, in white plastic trunking, on a stock brick elevation is the detail that officers refuse. Run it internally where the floor construction allows, or on a rear elevation in a colour matched to the brick. Insulate it properly, because an uninsulated external run costs efficiency in exactly the weather when the system is working hardest.
Condensate needs somewhere to go. A heat pump in defrost produces a steady trickle, and in freezing weather that becomes ice on a path if it discharges to the surface. Take it to a soakaway or a trapped gully, and keep it away from a pedestrian route. On a listed building, a new connection through a solid wall is a consentable alteration and should be shown on the drawings rather than discovered on site.
Enclosures are a trade-off. A timber slatted screen helps with appearance and does almost nothing for sound unless it is solid, lined and detailed with adequate airflow clearances. Specify the clearances from the manufacturer's data, because an enclosure that starves the unit of air will cut performance and shorten its life.
Where no position works
Sometimes there is no position that satisfies the sound calculation, the heritage test and the airflow requirement at once. That is worth saying out loud rather than forcing a bad installation.
The realistic options are a smaller unit sized to a fabric-first retrofit, so the output needed falls, a high temperature unit in a different position accepting a lower efficiency, a ground source system where there is garden to take a loop, which falls under different classes in Part 14, or a shared or communal arrangement where the building is in multiple occupation. On a deep terrace with no rear access and a tight side return, the honest answer is sometimes that the building needs insulation and glazing work first.
Record the decision either way. A short note setting out the positions tested, the sound calculations and the heritage reasoning is what you want in five years when a neighbour complains or a buyer's surveyor asks.
Work out the position before you order the unit. Enter your postcode on PlanScope to see which reports a heat pump application on your property is likely to need.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a lawful development certificate is the only formal way to confirm that works fall within permitted development. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Do I need planning permission for a heat pump in a conservation area?
Often not, if the unit sits in a rear garden behind the building line and out of sight of the highway, and the sound calculation passes. Class G of Part 14 withholds the right for the more visible positions on designated land, and an Article 4 direction can remove it entirely. Check the current Class G text and your authority's directions first.
What is the 42 dB(A) heat pump limit?
It is the sound level the applicable planning standards require at the nearest neighbouring habitable room window for a permitted development installation. The installer calculates it from the unit's declared sound power level with corrections for distance, barriers and reflecting surfaces. Side passages between terraces usually fail because reflections from both flank walls add to the level.
Can I put a heat pump on a listed building?
Class G does not apply within the curtilage of a listed building, so the works need listed building consent where they affect the special architectural or historic interest, and planning permission in most cases too. The case rests on siting away from prominent elevations, internal pipe routes, reversible fixings and a clear statement of significance and harm.
Does an acoustic enclosure help?
Only if it is solid and lined, and even then it does less than moving the unit further from the boundary. Enclosures add volume, which can take the installation outside the permitted dimensions, and they restrict airflow, which cuts output. Follow the manufacturer's clearance data rather than building a box to suit the garden.
Where should the condensate go?
To a soakaway or a trapped gully, away from any pedestrian route. A heat pump in defrost produces a steady trickle that becomes ice on a path in freezing weather. On a listed building, a new drainage connection through a solid wall is an alteration that should appear on the consent drawings rather than being dealt with on site.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.