MCS 020 heat pump noise calculation

MCS 020 a) is the sound calculation used for domestic air source heat pumps installed under permitted development in England. It predicts sound outside neighbouring habitable rooms before the unit is installed.

Passing the calculation is only one permitted development condition. It is not planning permission for a proposal that fails another limit, and it is not a guarantee that the installed unit can never cause a noise problem.

Current standard in England

As at 22 July 2026, England’s planning legislation names MCS 020 a) Issue 1.1, dated 14 April 2025, as the MCS Planning Standard for air source heat pumps. The older combined MCS 020 document also covered wind turbines; the split standard now uses MCS 020 a) for heat pumps and MCS 020 b) for wind turbines.

The legislation previously allowed MCS 020 or an equivalent standard. That wording was removed in May 2025, with a transition for development that had relied on the earlier route ending on 28 May 2026. A new English permitted development assessment should therefore use MCS 020 a), not an installer’s alternative acoustic method in its place.

Planning rules are devolved. The calculation and legal position described below are for England. Scotland, Wales and Northern Ireland have their own permitted development conditions, so an English result should not be assumed to establish rights elsewhere.

The limit in Issue 1.1

MCS 020 a) Issue 1.1 tests whether the predicted level is no more than 37 dB LAeq,5mins at every assessment position, ignoring the sound reflected from the neighbouring facade.

That exact value belongs to the named standard edition. It is not a market statistic that needs annual updating.

Older material commonly refers to 42 dB. The previous method used 42 dB at an assessment position including a facade correction, whereas Issue 1.1 expresses its result as 37 dB without that facade effect. Use the figure and method from the same standard; do not combine the old threshold with the new calculation.

Where sound is assessed

An assessment position is one metre outside the centre of a door or window to a habitable room in a neighbouring property, measured perpendicular to that opening.

Under the standard, a habitable room is used or intended for sleeping or living and is not used solely for cooking. Bathrooms, toilets, service rooms, corridors, laundry rooms, hallways and utility rooms are excluded.

Every position that could be affected needs to be checked. The nearest window is not always the controlling one because barriers, line of sight and reflections can make a more distant position louder.

Inputs to the calculation

For each assessment position, the installer records:

1. the assessment date and a clear description, drawing or photograph of the position
2. the unit’s A-weighted sound power level, not a sound pressure reading
3. the number of nearby reflecting surfaces
4. the straight-line distance from the centre of the heat pump
5. any qualifying solid barrier and the line of sight over it
6. the resulting predicted sound pressure level

The sound power figure comes from the product data identified by the standard. A figure for a selectable “low noise mode” must not be used. That prevents a proposal being assessed on a quiet setting that the unit will not necessarily maintain.

Reflections and barriers

Issue 1.1 assigns a directivity factor for reflecting surfaces within one metre of the unit:

Directivity factor	Reflecting surfaces	Typical arrangement
Q2	One	Ground-mounted in open space, with the ground as the reflector
Q4	Two	Ground-mounted beside one wall
Q8	Three	Ground-mounted in a corner between two walls

A reflecting surface has to meet the size test in the standard. Vegetation does not count. A unit with more than three qualifying reflecting surfaces, such as one inside a small lightwell, does not comply with the MCS 020 a) method.

The standard’s barrier corrections are:

Barrier	Assessment position fully hidden	Partial view	Full view
Type 1	-10 dB	-5 dB	0 dB
Type 2	-5 dB	-2.5 dB	0 dB
No qualifying barrier	0 dB	0 dB	0 dB

A Type 1 barrier is solid brick or masonry, or a solid fence at least 18 mm thick, without gaps and with the required extension beyond the heat pump. A Type 2 barrier is a thinner solid fence meeting the other conditions. Slatted or damaged fences and vegetation provide no allowance in the calculation.

These values are prescribed by Issue 1.1. They are not a promise of the attenuation that every wall or fence will deliver in every acoustic situation.

The formula

For one heat pump, the method uses:

Lp = Lw + 10 log10(Q / (4πr²)) − AB

where:

• Lp is the predicted A-weighted sound pressure level at the assessment position
• Lw is the heat pump’s A-weighted sound power level
• Q is the directivity factor
• r is the distance in metres
• AB is the permitted barrier attenuation

Two heat pumps affecting the same position are calculated separately and their levels are combined logarithmically. Their decibel values are not simply added together.

The installer rounds the result to 0.1 dB. Every position must be at or below 37.0 dB for the proposal to pass this sound condition.

Options after a failed calculation

The most useful changes are normally made at design stage:

• choose a quieter unit using the correct sound power figure
• increase the distance to the controlling windows
• move the unit away from corners and other reflecting surfaces
• use a qualifying barrier without restricting airflow or service access
• avoid a position that directs discharge air or sound towards a neighbour
• calculate every relevant elevation before fixing the final location

MCS has separate mitigation guidance for a proposal that fails, an installed unit that becomes noisy and a complaint. An acoustic enclosure should not be treated as a generic cure: it must preserve manufacturer clearances, airflow, performance and safe maintenance access.

Other English permitted development conditions

As at 22 July 2026, England’s Class G also restricts unit numbers, size and location. Among other conditions:

• a detached house may have no more than two air source heat pumps under Class G; another house or a block of flats may have no more than one
• the outdoor unit may not exceed 1.5 m³ for a house or 0.6 m³ for a block of flats
• an air source heat pump cannot share the property with a wind turbine under this right
• it cannot be installed on a pitched roof
• on a flat roof it must be at least one metre from the external edge
• the right does not cover a scheduled monument or a listed building or its curtilage
• extra siting limits apply in conservation areas, World Heritage Sites and on some highway-facing walls
• a unit capable of heating and cooling may qualify, but it cannot be used solely for cooling

The legislation contains the complete conditions and should be checked for the actual property. Lease terms, landlord or freeholder consent and an Article 4 direction can create separate restrictions even where the national Class G limits would otherwise be met.

If the calculation fails

A failed MCS 020 a) result means the proposal cannot use Class G in that form. The practical options are to change the model or location, add a suitable design measure and recalculate, or apply for planning permission with the acoustic evidence requested by the local planning authority.

A planning application is not assessed by changing the MCS input until it passes. The authority may require a fuller site-specific assessment, often using BS 4142, and can consider background sound and acoustic character in a way the simplified MCS calculation does not.

Noise-assessment record

Ask for the completed table for every assessment position, together with:

• the drawing or photographs identifying each position
• the sound power source for the installed model
• measured distances and the recorded directivity factor
• barrier type and line-of-sight evidence
• the calculation for one or two units as applicable
• confirmation that the installed model, position and surroundings match the assessment

MCS 020 a) requires the contractor to keep one copy and give another to the customer. If the model, location, barrier or nearby building changes before installation, the calculation should be checked again.

Noise after installation

Passing the pre-installation calculation does not remove the need to install, commission and maintain the heat pump correctly. Loose panels, vibration transmission, obstructed airflow, poor defrost behaviour or a changed operating condition can produce a different outcome from the design evidence.

Permitted development status also does not disapply the statutory nuisance provisions in Part III of the Environmental Protection Act 1990. A persistent problem should first be checked against the design, installation and manufacturer requirements; the local authority can investigate where a statutory nuisance may exist.

Related entries

• Acoustic assessment for ASHP
• Planning constraints for ASHP
• Sound power level
• Noise assessment
• MIS 3005 heat pump standard
• Air source heat pumps