MCS 020 is the planning standard that decides whether an air source heat pump can be installed under permitted development, and its sound calculation has been mandatory for permitted development installations since 20 September 2025 (MCS, 2025). Pass the calculation and most jobs avoid a planning application; fail it and the installation needs full planning permission.
MCS reissued the standard in March 2025 and split it into MCS 020a for air source heat pumps and MCS 020b for small wind turbines (MCS, 2025). This guide explains what MCS 020 covers, how the noise assessment works, the figures that decide a pass, and the dates every installer needs in the diary.
Key Takeaways
- MCS 020a is the air source heat pump sound calculation used to qualify an installation for permitted development, and it has been mandatory since 20 September 2025.
- The assessment caps noise at 42 dB(A) at the nearest neighbour's habitable room window; a result at or below that lets the job proceed without planning permission.
- Compliance applies whether or not the installation is MCS certified, so the calculation is unavoidable on the permitted development route.
- From 28 May 2026, MCS 020a becomes the only certification scheme accepted for permitted development heat pump installations in England.
- The Excel tool models up to two heat pumps for a permitted development job and up to four where full planning permission is being sought.
What is MCS 020?
MCS 020 is the Microgeneration Certification Scheme planning standard for permitted development installations of air source heat pumps and small wind turbines (Planning Portal, 2026). For heat pumps it is now published as MCS 020a, and it sets out the sound calculation an installer must complete to show the unit meets the noise condition attached to permitted development rights.
In plain terms, permitted development lets you install without a planning application, but only if the heat pump is quiet enough at the neighbouring property. MCS 020a is the method that proves that. It is referenced directly in the planning legislation, so the calculation is not advisory guidance, it is the test the permitted development right depends on. Skip it and the permitted development status simply does not hold, which leaves the homeowner exposed to enforcement and the installer exposed to the complaint.
The standard sits inside a fast-growing market. The government is targeting 600,000 heat pump installations a year by 2028 (Logic4training, 2025), and keeping jobs on the permitted development route rather than in a planning queue is a large part of how the trade reaches that volume. A clean MCS 020a result is what protects that faster route on each job.
What changed in the 2025 MCS 020 update?
The update followed the government's changes to permitted development rights that took effect on 29 May 2025 (legislation.gov.uk, 2025). MCS split the old single standard into MCS 020a for heat pumps and MCS 020b for wind, and rebuilt the sound method around the wider permitted development scope (MCS, 2025).
Those planning changes removed the 1 metre boundary rule, raised the unit size limit to 1.5 cubic metres and allowed two heat pumps on detached homes (pv magazine, 2025). The revised MCS 020a supports that by adding a method for up to two units, and by holding each certified product's A-weighted sound power level on the MCS Product Directory so the figure feeding the calculation is consistent.
MCS also clarified several points that used to cause disputes: what can and cannot count as a solid barrier, how the assessment position is determined, what counts as a reflective surface, and the definition of a habitable room (MCS, 2025). Working with DESNZ, it committed to a separate guidance document on noise mitigation measures.
How does the MCS 020 noise assessment work?
The assessment compares the heat pump's sound at the nearest neighbour's habitable room window against a 42 dB(A) limit (MCS, 2025). It starts from the unit's declared A-weighted sound power level, then accounts for distance, any solid barrier between the unit and the window, and reflective surfaces nearby.
Installers have two options for running it: the paper-based method or the MCS 020a Excel tool, both published in the Standards and Tools Library (MCS, 2025). The Excel tool handles up to two units for a permitted development calculation, and up to four for a full planning application, which is useful on larger or cascade systems.
The result hinges on siting. A unit tucked behind a solid boundary wall reads very differently from one on an open elevation facing the neighbour's window, because the barrier correction and the assessment position both change. Getting the proposed position pinned down before running the sheet is what makes the number reliable rather than optimistic.
The definition of a habitable room is central to where you measure. A kitchen, living room or bedroom window counts; a bathroom, hallway or utility window generally does not, and the 2025 revision tightened this so the assessment position is chosen consistently (MCS, 2025). Picking the wrong window can turn a marginal pass into a fail, or waste a viable position, so it is worth confirming on site rather than from a plan.
MCS 020 noise limits and key figures
These are the figures that decide an MCS 020a assessment and the permitted development status that rests on it (MCS, 2026). Keep them to hand at survey so the calculation is not a surprise back at the office.
- Noise limit: 42 dB(A) at the nearest neighbour's habitable room window is the threshold the calculation must meet.
- Units under permitted development: the Excel tool models up to two air source heat pumps.
- Units under full planning: the tool extends to up to four heat pumps.
- Sound power input: taken from the product's A-weighted figure held on the MCS Product Directory.
- Mandatory from: 20 September 2025 for all permitted development installations, certified or not.
- Sole scheme from: 28 May 2026, when MCS 020a becomes the only accepted certification scheme for permitted development.
When is MCS 020 compliance mandatory?
Compliance with MCS 020a under permitted development has been mandatory since 20 September 2025, and it applies whether or not the installation itself is MCS certified (MCS, 2025). If you are relying on permitted development rather than a planning application, the sound calculation must be completed and kept on file.
A second date matters even more. From 28 May 2026, MCS 020a becomes the only certification scheme accepted for the permitted development route in England (Planning Portal, 2026). After that point there is no alternative method, so building the calculation into your standard survey now avoids a scramble later.
For quotes in the pipeline that straddle these dates, the safe assumption is that any install completing on the permitted development route needs a current MCS 020a calculation on file. Treating the sheet as a standard survey output, rather than something added only when a neighbour raises concerns, keeps the whole book of work on the right side of the rule without case-by-case judgement calls.
MCS 020a and MCS 020b explained
The split is by technology. MCS 020a is the air source heat pump sound calculation, and MCS 020b is the small wind turbine sound calculation (MCS, 2025). For a heat pump job you always use 020a; there were no changes to the requirements for small wind turbines in this revision.
This matters for installers who fit both technologies, because the product data and the assessment inputs differ. If you also handle certification and registration, our guide to becoming MCS certified sets out where the standards sit within the wider scheme, and the heat pump regulations overview shows how MCS 020a fits alongside the other compliance duties on a heat pump install.
How installers pass the MCS 020 assessment first time
Passing first time is mostly about unit choice and siting, decided before you quote. Because the calculation draws on the declared sound power level, specifying a quieter model buys headroom against the 42 dB(A) limit, and on tight plots that headroom is often the difference between permitted development and a planning application.
We run the MCS 020a sheet at the exact proposed position, not a hopeful one, and record the barrier and the distances we used. Where a result sits close to the limit, a solid boundary wall in the right place can bring it down, but it has to meet the standard's definition of a solid barrier to count, which is one of the points the 2025 revision tightened.
Where a position will not pass, mitigation is the next lever rather than an automatic planning application. MCS committed to a dedicated guidance document on noise mitigation measures for air source heat pumps, developed with DESNZ (MCS, 2025), covering the practical steps that reduce a reading. In our experience an acoustic enclosure or a repositioned unit resolves most marginal cases more cheaply than the delay of a full application.
Keep the completed calculation in the handover pack so the permitted development position is evidenced if a neighbour or the council ever queries it. Tools like Reonic can store that sheet next to the DNO and MCS records for each job. Because siting drives both the noise result and the planning outcome, we settle it at the same time as the heat pump planning permission checks and the air-to-air options where cooling is in scope.
Frequently asked questions
What is MCS 020a used for? It is the sound calculation that qualifies an air source heat pump for permitted development in England (MCS, 2025). It proves the unit meets the noise condition at the nearest neighbour's habitable room window, so the installation can proceed without a planning application.
What is the MCS 020 noise limit? The assessment must show no more than 42 dB(A) at the nearest neighbour's habitable room window (MCS, 2025). A result at or below that figure satisfies the permitted development noise condition; a higher result means the job needs full planning permission instead.
Is MCS 020 mandatory? Yes, for permitted development. Since 20 September 2025 the MCS 020a calculation has been mandatory for permitted development heat pump installations, certified or not (MCS, 2025). From 28 May 2026 it becomes the only accepted certification scheme for that route.
Do I use MCS 020a or MCS 020b? Use MCS 020a for air source heat pumps and MCS 020b for small wind turbines (MCS, 2025). The 2025 revision split the standard by technology and made no changes to the small wind turbine requirements, so heat pump jobs always follow 020a.
How many heat pumps can the MCS 020a tool assess? The Excel tool models up to two air source heat pumps for a permitted development calculation and up to four where full planning permission is being sought (MCS, 2025). That covers cascade and larger installations that fall outside permitted development, so one tool serves both the permitted development and the full planning routes.
