The 42 decibel test for an air source heat pump

An air source heat pump can usually be installed without planning permission, and the heat pump 42 dB test is the condition that decides it. The permitted development right in Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015 requires the installation to comply with the MCS planning standards, and those standards cap the calculated sound level at 42 dB(A) at a point one metre from a window or door of a habitable room of a neighbouring property.

Two details matter immediately. The 42 dB figure is a calculation, not a measurement taken after the unit is running. And the assessment position is your neighbour's window, not yours. A pump that sounds fine from your kitchen can still fail the test if it sits three metres from the bedroom window next door.

Where the condition sits in the Order

Class G permits the installation, alteration or replacement of an air source heat pump on domestic premises, subject to conditions and limitations. Compliance with the MCS planning standards, or equivalent standards, is one of those conditions, and it is the route by which the noise limit becomes a legal requirement rather than guidance.

The rest of the class has been amended more than once, including changes to the permitted size of the outdoor unit and to the restriction on how close it may sit to a boundary. Read the current consolidated text of Part 14 on legislation.gov.uk before relying on any dimension or distance, because the figures in older articles and in installer literature are frequently out of date.

What has not changed is the structure. Fail any single condition and the right is not available, so the installation becomes development requiring a householder planning application. There is no partial compliance.

The 42 dB figure and where it is assessed

The limit is 42 dB(A) at the assessment position, which is one metre from the window or door of a habitable room of any neighbouring residential property. Habitable rooms are the living rooms and bedrooms, so a bathroom or utility window is not the relevant receptor.

The nearest such opening governs. On a terrace that may be the property either side. On a corner plot it may be a house across a side road. Where a neighbouring property has a conservatory or an extension with a habitable room closer to your boundary than the original house, that is the point to assess from.

One honest difficulty: the standard assesses the nearest neighbouring habitable room, so a detached house in a large garden passes easily while a mid-terrace with a shallow back yard can fail with the same unit. The rule produces different outcomes for identical equipment, and there is no discretion in it.

How the calculation works

The method takes the manufacturer's declared sound power level for the unit and converts it to a sound pressure level at the assessment position. Three corrections do the work.

Distance attenuation reduces the level as the sound spreads out from the unit, which is why moving a pump a few metres further from a neighbouring window often resolves a marginal case. Reflections increase it, because a unit against a wall radiates into a half-space and a unit in a corner into a quarter-space, so each nearby hard surface adds to the level at the receptor. A barrier correction can reduce it where a solid obstruction breaks the line of sight between the unit and the window.

The MCS standard provides the calculation sheet, and a competent installer will complete it as part of the quotation. Keep the completed calculation. If a neighbour complains or the council asks whether the right applied, that sheet is the evidence, and reconstructing it two years later with a unit already installed is far harder.

Sound power data and the figure installers get wrong

The input is the declared sound power level from the manufacturer's technical data, tested to the recognised standard for heat pump sound measurement, for the operating condition relevant to the installation.

That last clause is where most arguments start. Many units publish more than one figure: a value at a mild outdoor temperature, a value at a low flow temperature, and a reduced value in a night or quiet mode that throttles the fan. The figure that belongs in the calculation is the one for the condition the pump will actually run in on a cold night at the flow temperature your emitters need, not the quietest number on the data sheet.

Ask the installer for three things in writing: the model and the exact variant, the declared sound power level with the test condition stated, and the completed calculation showing the assessment position used. A quotation that gives a single decibel figure with no context is not evidence of compliance.

Siting, enclosures and the things that make it worse

Position does more for compliance than any accessory. Moving the unit to a flank wall away from the neighbour's elevation, putting it on the far side of a garage, or siting it so the existing boundary wall interrupts the line of sight to the window all reduce the calculated level.

Acoustic enclosures are oversold. A proprietary enclosure only helps if it has enough mass, if it blocks the direct path, and if it still allows the airflow the unit needs. Restrict the airflow and you lose efficiency and may shorten the compressor's life, which trades a noise problem for a heating one. A timber slatted screen with open joints looks tidy and does almost nothing acoustically, though it can still be worth having for appearance.

Avoid mounting the unit on a lightweight timber-framed wall or a hollow structure, because vibration transmitted into the building fabric creates a low-frequency problem inside the house that the external calculation does not capture at all. Anti-vibration mounts on a concrete base, clear of the wall, is the arrangement that causes fewest complaints.

The other conditions in the class

Noise is the condition that fails most often, but it is not the only one. The class restricts installation in the curtilage of a listed building, places limits on designated land including conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites, requires the unit to be used solely for heating, requires siting that minimises the effect on the external appearance of the building and on amenity, and requires removal when no longer needed.

Article 4 directions are a separate trap. Where a direction has removed householder permitted development rights in a conservation area, Class G may be withdrawn along with the rest, and the only way to know is to check the council's own list of directions.

Flats and buildings that are not houses are treated differently from houses, and a scheduled monument is excluded. Where a listed building or a conservation area is involved, a Heritage Statement assessing the effect of the unit on the building and its setting is what the conservation officer will look for.

When you need a planning application instead

If the calculation exceeds 42 dB(A) and resiting cannot bring it down, or any other condition fails, the route is a householder planning application. The assessment then changes character, because the 42 dB figure is a permitted development gateway rather than a planning policy standard.

On a planning application the council's environmental health team will usually assess the unit under BS 4142:2014+A1:2019, comparing a rating level that includes a correction for tonal or impulsive character against the measured background sound level at the nearest receptor. That can be stricter than 42 dB in a quiet rural location and more forgiving on a main road, which is why some pumps that fail the permitted development test are consented without difficulty. A short Noise Impact Assessment from an acoustician is the document that answers it, and councils expect a suitably qualified author with professional review strongly advised.

For context on how background sound varies across an area before you commission measurements, our note on strategic noise mapping for early site appraisal is a useful starting point, and a short Planning Statement setting out the policy case alongside it keeps the application tidy.

Get the calculation before the installer fixes a date. It costs almost nothing and it is the only step that decides whether you need permission at all.

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 reports your project 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.