MCS 020 a): the sound calculation that keeps a job out of planning

A sound calculation is the cheapest thing on a heat pump job and the most expensive thing to get wrong. Get it right at survey and the install is permitted development: no application, no eight-week wait, no planning fee, no conversation with a neighbour who has read something online. Get it wrong and you are either back on site moving a unit you have already fixed down, or explaining to a homeowner why the job they signed for now needs planning permission.

Since 29 May 2025 the rules in England have been tighter than most trade content admits, and the fallback route that used to save awkward gardens has gone. Here is what MCS 020 a) actually asks for, and where jobs come unstuck.

The short answer

MCS 020 a) is the sound calculation an installer must complete for a domestic air source heat pump to be permitted development. It predicts the sound pressure level from the unit at every window and door of a habitable room in every neighbouring property that could be affected. If the result at all of those positions is 37.0 dB(A) or lower, the noise condition is met. If any single position comes back higher, the installation is not permitted development, and the job needs planning permission instead.

Three things worth knowing before you take the next survey. The calculation applies whether or not the installation is MCS certified — it is planning law, not a scheme rule. Passing it does not on its own make a job permitted development, because the size, siting and property limitations still apply separately. And in England there is no longer an alternative method: an acoustic consultant's report is not a route to permitted development.

What MCS 020 a) is, and what it is not

MCS 020 a) is a standards document, currently Issue 1.1 dated 14 April 2025, titled Air Source Heat Pump Sound Calculation (For Permitted Development Installations). It was prepared by the MCS Heat Pump Working Group and BSRIA. It replaced the old combined MCS 020, which covered air source heat pumps and small wind turbines together; wind turbines now sit in MCS 020 b).

It is not a noise survey and it is not a measurement. Nobody stands in the garden with a meter. It is a desk prediction built from the manufacturer's declared A-weighted sound power level, the number of hard surfaces around the unit, the straight-line distance to each assessment position, and any solid barrier in between. That is the whole model.

What matters commercially is that the document is named in law. The Town and Country Planning (General Permitted Development) (England) Order 2015 defines the MCS Planning Standards by reference to MCS 020 a) Issue 1.1, and Class G of Part 14 says that development is not permitted unless the heat pump complies with them. MCS states plainly that compliance under permitted development rights applies irrespective of whether the installation is MCS certified. So an uncertified installer fitting an uncertified unit in England still needs this calculation done, and done properly.

MCS is also clear about the consequences of skipping it. MCS can impose penalties or sanctions on a contractor who fails to ensure compliance before undertaking an installation, and an installation that does not meet the standard may be subject to enforcement action by the local planning authority. Both of those land on the installer, not the homeowner.

The route that closed in England

Until 29 May 2025, Class G said the heat pump had to comply with the MCS Planning Standards "or equivalent standards". That phrase was the escape hatch. It is the reason a third-party acoustic assessment, or manufacturer data used directly, could carry a difficult garden.

The 2025 amendment order deleted those four words. It also included a transitional provision: development that was permitted immediately before 29 May 2025 and was no longer permitted because of that deletion could still be carried out until the end of 28 May 2026. That window has now passed. In England there is one method, and it is MCS 020 a).

This is the single change most likely to catch out a business that has not read the Order. If your process still has a line in it about commissioning an acoustic report when the numbers look marginal, that line is out of date. The options now are: change the siting, change the unit, add a barrier, or apply for planning permission.

How the calculation works, step by step

The standard sets out eight steps. Filling in the table is straightforward. The judgement is in steps 2, 4 and 6, which is also where the arguments with a planning officer happen.

Step 1 is the date. Not filler: the standard warns that if time passes between assessment and installation you may need to check nothing has changed, such as a new extension next door or a fence that has come down.

Step 2 is the assessment position, described precisely enough to identify it: property address, which opening, which floor. An assessment position is a point 1 m outside the centre of any door or window to a habitable room of a neighbouring property, measured perpendicular to the plane of that opening. Habitable means used or intended for sleeping or living. Kitchens count unless they are used solely for cooking. Bathrooms, toilets, service rooms, corridors, laundry rooms, hallways and utility rooms do not.

Step 3 is the A-weighted sound power level from the manufacturer's data, taken from the product fiche, the energy label or the MCS product database. Two traps here. Sound power is not sound pressure, and if a datasheet says only "noise level" you are supposed to go back to the manufacturer for clarity rather than guess. And the low noise mode figure must not be used, however tempting it looks.

Step 4 is directivity, Q. A reflective surface is any surface within 1 m of the unit, including the ground but not vegetation, and it has to extend past the edge of the unit by 1 m or more in two or more directions — so a low wall beside the unit does not count. One reflecting surface is Q2, two is Q4, three is Q8. More than three, and the standard is explicit: it will not meet MCS 020 a). A unit in a small lightwell is the worked example, and no amount of recalculating changes it.

Step 5 is the distance from the centre of the unit to the assessment position, in a straight line, to the nearest 0.1 m.

Step 6 is barrier attenuation, and it is scored as a matrix of barrier type against line of sight. A Type 1 barrier is solid brick or masonry, or a solid fence at least 18 mm thick, with no cracks or gaps, extending 1 m or more horizontally past either edge of the unit. A Type 2 barrier is the same but under 18 mm thick. Open fences, gappy fences, vegetation and anything in poor repair count as no barrier at all. Line of sight is judged from the top edge of the unit: no view, partial view if the position becomes visible when you move up to 0.25 m in any direction, or full view. Type 1 with no view gives you 10 dB. Type 1 with partial view, or Type 2 with no view, gives 5 dB. Type 2 with partial view gives 2.5 dB. Full view gives nothing, whatever the barrier is made of.

Step 7 is the arithmetic: Lp = Lw + 10 log (Q / 4πr²) − AB. Where a detached house takes two units sharing an assessment position, you calculate each and then add them logarithmically, which is why two 34 dB(A) units are not 68 dB(A).

Step 8 is the test. Round to one decimal place. Every assessment position at or below 37.0 dB(A) passes. One above it does not.

The assessment position most people get wrong

The nearest window is not automatically the worst one. The standard says so directly: the closest assessment position is not always the position with the highest sound pressure level, because a position further away can have a clearer line of sight. A bedroom window 15 m away over a slatted fence can score worse than a sitting room 5 m away behind a solid brick wall. In the standard's own worked example, that is exactly what happens.

So the assessment covers neighbouring properties on all sides, and upper floors as well as ground floor. A first-floor window usually sees over the fence that was protecting the ground floor, which means the barrier correction drops to zero at the position you were relying on. If your survey habit is to check the nearest neighbour and move on, that is the habit to change.

MCS also recommends attaching a map, sketch or photo to the workings. Worth doing. It is the difference between a defensible file and a number somebody has to take on trust two years later.

Check which issue you are working from

MCS published MCS 020 a) Issue 1.0 on 20 March 2025 and Issue 1.1 on 14 April 2025, weeks apart. MCS's own note on the update says the changes were the removal of the mandatory compliance date and a correction to a typo in the barrier attenuation table.

That typo matters. In Issue 1.0 the barrier attenuation cells were written as negative values, while the formula already subtracts the barrier attenuation. Applied literally, that adds the barrier correction instead of removing it. Issue 1.1 states the same cells as positive values, which is consistent with the formula. Issue 1.1 is also the version named in the Order, in both England and Wales.

If there is a saved copy of the standard in your job folder, or a spreadsheet somebody in the office built from one, check the version and check the sign on the barrier figure. MCS offers the calculation as the paper table or as an Excel tool, and the Excel tool handles up to four units for jobs going through full planning permission rather than permitted development.

Passing the sound test is not the same as being permitted development

The standard says this itself: compliance with MCS 020 a) on its own does not bestow permitted development rights. Class G in England carries a set of separate limitations, and several of them changed on 29 May 2025. As things stand:

• Up to two units on or within the curtilage of a detached dwellinghouse. One on anything not detached, and one on a block of flats.

• The outdoor compressor unit, including any housing, must not exceed 1.5 cubic metres on a dwellinghouse, or 0.6 cubic metres on a block of flats.

• The old 1 m boundary restriction is gone. That limitation was removed outright.

• Not on a pitched roof. On a flat roof, not within 1 m of the external edge.

• Nothing on a listed building, on land within its curtilage, or on a scheduled monument site.

• In a conservation area or World Heritage Site, not on a wall or roof fronting a highway, and not nearer a bounding highway than the nearest part of the building.

• Elsewhere, not on a highway-fronting wall above ground-floor storey level.

• No air source heat pump under Class G if a wind turbine is installed on the same building or within the curtilage.

Four conditions also attach: the unit must not be used solely for cooling, it must be sited so far as practicable to minimise the effect on the external appearance of the building, sited so far as practicable to minimise the effect on the amenity of the area, and removed as soon as reasonably practicable when it is no longer needed. The cooling condition is what brought air-to-air units into permitted development in England, and the word doing the work is "solely".

Article 4 directions are the other thing to check locally. Permitted development can be withdrawn street by street, and no calculation gets round that.

England, Wales, Scotland and Northern Ireland

England moved first, on 29 May 2025.

Wales followed on 1 June 2026. The Welsh amendment order, made on 10 March 2026, does broadly the same job on Class G of Part 40: it removes "or equivalent standards", allows up to two units on a detached dwellinghouse, sets the same not-solely-for-cooling condition, and redefines the MCS Planning Standards to mean MCS 020(a) and MCS 020(b). Two differences worth carrying in your head: in Wales the volume limit for a block of flats is 1 cubic metre rather than England's 0.6, and Wales made the same equivalent-standards deletion for stand-alone wind turbines at the same time.

Scotland and Northern Ireland are the ones to be careful with. MCS's guidance when England changed was that installations there continue to comply with the older MCS 020, which uses a different calculation and a higher noise limit. So a garden that fails in England might well pass north of the border, and a spreadsheet built for MCS 020 a) will give you the wrong test. Check the standard that applies where the job is, and if there is any doubt about a specific property, ask the local planning authority. Devolved rules move independently, and this one has moved twice inside fifteen months.

When the number comes back over 37.0 dB(A)

Not a dead job, usually. In order of how little it costs you:

Move the unit. Distance is squared in the formula, so a few metres is worth more than most people expect, and getting the unit off a corner takes it from Q8 to Q4 or Q2. Look at what the reflective surfaces are doing before you look at anything else.

Look at the barrier. A solid fence that already exists but has gaps, or does not extend a metre past the edge of the unit, is scoring you zero. Making it a proper Type 1 barrier with no line of sight is up to 10 dB, which is the largest single correction available anywhere in the calculation.

Change the unit. Sound power levels vary across models at the same output, and it is a design decision, not a compromise on the heating.

Then the standard points you at MCS's own installation guidance on mitigating noise from air source heat pumps for the detail.

And if none of that works, the honest answer is a planning application. Better to price that at survey than discover it after the unit is on the wall.

Paperwork: what you keep, and what the customer gets

One table per assessment position, with your result written into the result column at each step, so it is clear the method was followed. Retain a copy for your records, and give another copy to the customer.

That last requirement is easy to skip and worth doing properly. The completed table is the evidence a local planning authority or MCS would ask for, it is what answers a neighbour complaint two winters later, and it is a straightforward thing to hand over that makes the rest of the job look like it was done to the same standard.

How Ample fits in

Ample is the MCS entity of record for every install done through us, so the MCS certificate, the Insurance-Backed Guarantee and the Boiler Upgrade Scheme paperwork sit with us, and the design and the job stay yours. Our design review is a second pair of eyes on what you have specified, which is exactly the kind of check a marginal garden benefits from before anyone is committed to a position. If you are not certified, the umbrella is the route in: you can work through our accreditation rather than carry your own, which is what an MCS umbrella scheme is. If you already hold your own certification, nothing here changes, and you use us for the parts you want.

We are not a training provider and we do not certify or accredit anyone. What we take off you is the administration and the cash-flow risk around it. The grant applications, the chasing and the redemption sit with us, and you are paid within one working day of each confirmed milestone. There is no membership fee, no subscription, no minimum volume and no tie-in.

To join you need to be a heat pump specialist, hold the appropriate installation qualification, and pass a basic financial stability assessment. Every application is assessed individually. If there are gaps to close first, we can point you towards technical experts and a network of third-party training providers.

Take on more heat pump work without carrying the admin

If the compliance file, the grant paperwork and the wait to get paid are what is holding your heat pump work back rather than the fitting, that is the part Ample takes on. Have a look at how working with us fits around what you already do.

Frequently asked questions

Do I need MCS certification to complete an MCS 020 a) calculation?
No. MCS is explicit that compliance with MCS 020 a) under permitted development rights applies irrespective of whether the installation is MCS certified. The calculation is a planning requirement for any domestic air source heat pump going in as permitted development in England or, since 1 June 2026, Wales.

What is the noise limit for a heat pump under permitted development?
37.0 dB(A) in England and Wales, expressed in the standard as 37 dB LAeq,5mins, at every assessment position, ignoring the effect of the façade. Scotland and Northern Ireland still use the older MCS 020 method and a higher limit.

Where exactly is the assessment position?
1 m outside the centre point of any door or window to a habitable room of a neighbouring property, measured perpendicular to the plane of that opening, at every floor level. Bathrooms, toilets, hallways, corridors, laundry rooms, utility rooms and service rooms are not habitable rooms. A kitchen counts unless it is used solely for cooking.

Can I use an acoustic consultant's report instead?
Not for permitted development in England. The "or equivalent standards" wording was deleted from Class G on 29 May 2025, and the transitional period for development that was already permitted under the old wording ended on 28 May 2026. A consultant may still be worth having if a job is going through a full planning application.

Can I use the low noise mode figure from the datasheet?
No. The standard states that the sound power level in low noise mode shall not be used.

Two units on one house: do I add the decibels?
Not arithmetically. You calculate the sound pressure level from each unit at the shared assessment position, then add them logarithmically using the formula in the standard. Two units are permitted development only on a detached dwellinghouse.

Does passing MCS 020 a) mean the job is permitted development?
No. The standard says so itself. The Class G size, siting and property limitations apply separately, and permitted development can also be withdrawn locally by an Article 4 direction.

What happens if the calculation is not done, or is done wrong?
MCS may impose penalties or sanctions on the contractor, and the local planning authority may take enforcement action against an installation that does not meet the standard. The obligation sits with the contractor.