Reports for an air source heat pump array on a block
One heat pump on a house is permitted development. A dozen on a block is not. Here is the document set, and why passing the MCS calculation answers very little.
Heat pump planning reports for a single unit on a house are usually unnecessary, because Class G of the General Permitted Development Order covers it. An array of units on a block of flats is a different matter. The class permits a limited number of heat pumps on a building, and once you are installing one plant item per flat, or a bank of units serving a communal system, you are outside it and applying for planning permission. The document that decides the application is a noise assessment, and the thing that decides the noise assessment is the cumulative effect of several fans running together at night.
Class G has been amended more than once, most recently in 2024, and the volume limit, the boundary distance and the number of units permitted have all changed. Read the current text of the Order rather than a summary, including an older article on this site or anyone else's.
Where permitted development stops
The class attaches conditions and limitations, and any one of them failing takes the whole installation out of permitted development. The recurring ones are the number of heat pumps on the building, the volume of the outdoor unit including its housing, the position relative to a roof edge or a pitched roof, restrictions where the wall or roof fronts a highway in a conservation area or world heritage site, exclusion on and within the curtilage of a listed building, a requirement that the installation be used solely for heating, and a requirement that it be removed when no longer needed.
The noise condition is the one people forget. Class G applies only where the installation complies with the microgeneration certification scheme planning standards, which set a numerical noise limit rather than a general requirement of reasonableness.
For a block, run the test properly before you conclude anything. Count the units you actually intend to install, including any future phase, and check the position of each against the class. A scheme that is permitted development for the first six flats and not for the next twelve is not a permitted development scheme.
Where the block is a converted building rather than purpose-built flats, check whether it is a dwellinghouse or a block of flats for the purposes of the class, because the limitations differ and the answer is not always obvious from the tenure.
The MCS noise standard and what it actually tests
The microgeneration planning standard sets a limit of 42 dB assessed at a point 1 metre from the nearest habitable room window of a neighbouring property, using a prescribed calculation rather than a measurement. It takes the sound power level from the unit's published data, applies distance attenuation and a barrier correction where a solid obstruction interrupts the path, and compares the result with the limit.
That is a screening test, not an assessment. It uses a single fixed limit regardless of the background sound level, it takes no account of tonality or of the defrost cycle, it assumes the manufacturer's data is achieved in the installed condition, and it considers one unit at a time.
So a proposal that passes the calculation for each unit individually can still generate a rating level above the background once all the units are running, and passing the standard is not an answer to an objection in a full application. Say so in the submission rather than leaving the officer to notice it.
Assessing several units together
For a full application, the assessment method is BS 4142:2014+A1:2019, and there are four things that a heat pump array does which a single unit does not.
Levels add. Doubling the number of identical sources adds around 3 dB, so twelve units produce roughly 11 dB more than one, before any reflection effects.
Facades reflect. Units mounted in a courtyard or a rear lightwell sit in a partially enclosed space, and reflections from the surrounding walls raise the level at receptors above what a free-field calculation predicts.
Defrost cycles are the worst case, not the average. In cold damp weather the unit reverses to clear ice from the coil, the fan behaviour changes and the sound becomes more tonal. Different units defrost at different times, which means the worst case is intermittent and unpredictable rather than steady.
Low frequency content is poorly handled by the standard method. Fan blade passing frequency and compressor tones in the lower bands travel further, penetrate windows more readily and are the usual subject of complaints, so a Noise Impact Assessment should report octave or third-octave data rather than only the A-weighted figure, and should address the lower bands explicitly.
Assess the night period on measured night background. Heat pumps run when the heating demand exists, and for domestic hot water that includes the small hours.
Siting, screening and roof-level plant
The ranking of effective measures is fairly stable across schemes.
| Measure | What it achieves |
|---|---|
| Siting units away from bedroom windows and out of enclosed courtyards | The largest single reduction, and it costs nothing at design stage |
| Consolidating to fewer, larger units where the system allows | Fewer sources, fewer defrost events, easier acoustic treatment |
| Acoustic louvres or an enclosure with attenuated intake and discharge | Effective if designed for the airflow, damaging to efficiency if not |
| A solid screen breaking line of sight to the receptor | Moderate, and only where it does not restrict airflow |
| Anti-vibration mounts and flexible connections | Removes structure-borne transmission into the building itself |
| Selecting a quieter unit at the specification stage | Often overlooked and frequently the simplest fix |
Roof-mounted plant brings two extra issues. Structural loading and vibration isolation through the roof deck, which a Structural Survey or a structural engineer's appraisal should confirm before the layout is fixed. And visual effect, since a bank of units and their screening changes the roofline, which matters most on a block that already has a parapet line worth respecting. The same considerations arise on rooftop development generally, as set out in our note on reports for a rooftop extension to a block of flats.
Refrigerant choice has a siting consequence too. Units using a flammable refrigerant have manufacturer requirements on clearances, ventilation and proximity to openings and escape routes, and those requirements can conflict with the acoustically preferred position. Resolve that early.
Listed buildings and conservation areas
Class G does not extend to a listed building or to a building within the curtilage of one, so any installation on a listed block needs planning permission and listed building consent. A Heritage Statement should address the visual effect, the fixings into historic fabric, the pipe and cable routes through the building, and the reversibility of the intervention.
In a conservation area, the class restricts installation on a wall or roof fronting a highway, and the practical answer is usually a rear or courtyard position, which then runs into the reflection problem described above. That tension is the central design problem on a period block, and there is no general solution to it.
Conditions and commissioning evidence
Expect a condition requiring a post-installation noise verification. The usual form requires measurement after commissioning, at agreed positions, demonstrating compliance with a stated rating level, with remedial work if the level is exceeded.
Negotiate the wording. A condition requiring compliance to be demonstrated in all operating modes including defrost, in weather conditions that occur for a few weeks a year, can be impossible to satisfy on demand. A condition requiring measurement within a stated window after commissioning, with a defined method, is workable.
Other conditions to expect are the plant layout as approved, acoustic treatment installed before first use, a maintenance regime, and sometimes a restriction on the operating mode at night.
The document list for a multi-unit installation
A Planning Statement setting out the proposal, the number and type of units and the policy case. A Noise Impact Assessment covering cumulative operation, defrost, low frequency content and the night period. Elevations and roof plans showing every unit, its screening and its acoustic treatment. A structural appraisal for roof-mounted plant. An Energy Statement where the scheme forms part of a wider retrofit and the authority's policy requires one. A Heritage Statement and listed building consent application where the building is listed. And a commissioning and verification method statement, offered rather than waited for.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. 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.
Enter your postcode on PlanScope to see which reports a plant installation on your building is likely to need.
Frequently asked questions
Is a heat pump on a block of flats permitted development?
A limited number may be, under Class G of the General Permitted Development Order, subject to conditions on volume, position, listed buildings, conservation areas and noise. An array with one unit per flat, or a bank serving a communal system, will normally exceed the number the class permits. Class G has been amended more than once, so read the current text.
What is the 42 dB heat pump noise limit?
It comes from the microgeneration certification scheme planning standard that Class G relies on. The limit is assessed at 1 metre from the nearest habitable room window of a neighbouring property, by calculation from published sound power data with distance and barrier corrections, rather than by measurement. It is a screening test for one unit at a time.
Why does passing the MCS calculation not settle a full application?
Because it applies one fixed limit regardless of the local background sound level, takes no account of tonality or the defrost cycle, assumes manufacturer data is achieved as installed, and considers a single unit. Several units passing individually can still produce a rating level well above the background once they run together.
How much does adding units increase the noise?
Doubling the number of identical sources adds roughly 3 dB, so twelve units produce about 11 dB more than one before reflection effects. Units in a courtyard or lightwell add more, because reflections from surrounding facades raise levels at receptors above what a free-field calculation predicts.
What conditions should I expect on approval?
A post-installation noise verification at agreed positions demonstrating a stated rating level, the plant layout as approved, acoustic treatment installed before first use, a maintenance regime and sometimes a night-time operating restriction. Negotiate the verification wording, since a requirement to demonstrate compliance during defrost on demand may be unachievable.
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.