MCS 020(a): the only noise route for heat pump permitted development

28 May 2026 was the last day the transitional provision allowed installers to rely on "equivalent standards". From 29 May 2026, an air source heat pump relying on permitted development rights in England must meet MCS 020(a) for its sound assessment. There is no longer an alternative equivalent-standard route.

That doesn't make MCS 020(a) the only condition for permitted development. The installation still has to satisfy the other limits and conditions in Class G, and depending on the property and the work, further permission may be needed - a listed building being the obvious case.

What changed in Class G

The permitted development right for air source heat pumps sits in Class G of Schedule 2 to the planning order. Until last year it allowed a heat pump that met the MCS planning standards "or equivalent standards", and those four words were the fallback. They let a manufacturer's noise figure or a third-party acoustic report stand in for the MCS calculation.

The 2025 amendment deleted them and named MCS 020(a) as the standard instead. It came into force on 29 May 2025 with a year's grace for work already permitted, and that grace ran out at the end of 28 May 2026.

The same amendment made the other changes you've been working with in England: the old 1 metre boundary restriction removed, two heat pumps allowed at a detached house and one elsewhere, and a volume cap of 1.5 cubic metres at a house, still 0.6 at a block of flats. Those figures are England-specific.

Wales made similar changes from 1 June 2026, including removing "or equivalent standards", although Welsh permitted development conditions differ and need checking separately.

Why the number you compare against is now 37.0 dB(A)

Here's the bit worth knowing, because it's widely misread as a 5 dB tightening.

The old MCS 020 worked to a 42 dB(A) LAeq,5min limit, but it built an assumed background noise level of 40 dB(A) into the calculation. You combined the heat pump's calculated level with that assumed background through a decibel correction, then compared the combined figure against 42.0. Work the arithmetic back and the effective ceiling on the heat pump itself was already around 37 dB(A).

MCS 020(a) removes the background-noise step entirely. The calculated sound pressure level of the heat pump is assessed directly against 37.0 dB(A), with no background allowance and no split between quiet rural sites and noisy urban ones.

So the standard didn't quietly make heat pumps 5 dB quieter. It removed a step that obscured what the real limit always was. The practical consequence is the same either way: if your quoting template still compares a result against 42, it's comparing against the wrong figure.

What the Noise Calculation asks of you

Start with the sound power level. For MCS approved heat pumps this is published on the MCS Product Directory, and it's also in the manufacturer's data book. Two things to be careful about. It's a sound power figure, not the sound pressure number off the brochure (different quantities), and only one of them belongs in this calculation. Also, never use the low noise mode figure: it falsifies the calculation, because the unit won't be running in that mode when it matters.

Then work out what the site does to it.

Distance. Every doubling of the distance to the assessment position takes off roughly 6 dB.

Reflecting surfaces. A surface counts where it's within 1 metre of the unit, and each additional qualifying surface adds roughly 3 dB. The ground will normally count as one; add a wall within 1 metre and you're at two; put the unit into a qualifying corner and you're at three. This is why the 1 metre test matters - three surfaces all tight to the unit is a different proposition from two surfaces a metre and a half away plus the ground.

Barriers. A qualifying barrier can reduce the calculated level, but the allowance depends on both its construction and the line of sight.

A masonry wall or solid fence at least 18 mm thick gives 10 dB where it completely obscures the assessment position, or 5 dB for a partial view. A thinner solid fence gives 5 dB or 2.5 dB respectively. Full line of sight gives no reduction at all. Open fences, gaps, damaged barriers and vegetation don't count - and the barrier needs to extend horizontally at least 1 metre beyond either edge of the heat pump.

Then test every position. The assessment positions sit 1 metre out from each door and window of a habitable room at the neighbouring properties. The calculation returns a sound pressure level at each one. All of them must pass, and the worst one decides the job.

A worked example

A heat pump with a sound power level of 58 dB(A), mounted on a rear wall. The ground and the wall both qualify as reflecting surfaces, so that's two. The nearest neighbouring bedroom window is 6 metres away with full line of sight to the unit, so no barrier allowance applies.

That returns a sound pressure level of 37.5 dB(A) at the assessment position. It fails, barely, but 37.0 is 37.0.

Three things change the answer, and only the position is free:

• Move the unit to 8 metres from the same window and the result drops to 35.0 dB(A). It passes.
• Tuck it into a corner at the original 6 metres and you've added a third reflecting surface: 40.5 dB(A). Further from compliance than where you started. Corners are where borderline positions die.
• Break the line of sight with a Type 1 barrier that fully obscures the assessment position and you take 10 dB off: 27.5 dB(A).

Keeping the record with the project

Your completed MCS 020(a) calculation provides a clear record of how the installation meets the permitted development noise requirements. Keeping it with the rest of the project documentation makes it easy to refer back to the design decisions and demonstrate compliance if needed.

Keep the completed calculation and supporting evidence, produced before the installation is undertaken, along with where the sound power figure came from, every assessment position, how you identified the habitable rooms, the reflecting surfaces you counted and the barrier's type, thickness and line of sight. A single number at the bottom of a page isn't a record. The workings are.

In Aireo, the calculation and its supporting information save under the project, so they're always to hand alongside the rest of the design.

One part of a compliant design

MCS 020(a) is one part of a compliant heat pump design, and with Aireo it doesn't need to sit in a separate spreadsheet or process. Aireo brings your heat loss, heat pump selection, emitter design and MCS 020(a) noise assessment together in one project, so you can check compliance as the design develops and keep the supporting information alongside the rest of the job. From survey through to completed design and MCS documentation, everything stays connected, with direct technical support from the Aireo team when you need it.

Book a demo and we'll show you the noise assessment running alongside the rest of the design.