Air Conditioning & Heat Pumps

Air Conditioning & Heat Pump Noise Rules UK: The MCS Planning Standard Explained

Installing an outdoor unit without planning permission depends on passing a noise calculation most homeowners have never heard of — and one dB over the limit can mean ripping out a unit that's already fitted. Here's what the rule actually says, and why it doesn't apply the same way to every system.

Last reviewed: 21 September 2026

37.0 dB(A)

Current MCS Planning Standard limit

Measured outside neighbour's window

Class G

GPDO category the rule applies to

2025

Year the standard was last revised

⚡ Quick answer 🏠 Aircon or heat pump? 📏 The 37dB rule ⚠️ What happens if you're over ✅ Before you install

Quick answer: If your outdoor unit is installed under Class G of the GPDO as an air source heat pump, it must pass a noise calculation set by the MCS Planning Standard — currently 37.0 dB(A), measured 1 metre outside the centre of any habitable-room window at your nearest neighbouring property. This isn't a suggestion; it's a legal condition of permitted development. If your installer's calculation shows you're over the limit, you don't have permitted development rights and need full planning permission — or the unit has to move. Plain air conditioning units installed as cooling plant (Class E) follow different, less strict physical rules and don't require this specific MCS calculation, though general noise-nuisance law still applies.

Why the Same Unit Can Follow Two Different Rulebooks

This is the single biggest source of confusion, and it's why two homeowners with near-identical outdoor units can be given completely different advice. Under the Town and Country Planning (General Permitted Development) (England) Order 2015, domestic outdoor units fall under one of two classes depending on what the system is installed and used for — not just what the hardware technically is.

Class G — Air Source Heat Pumps

Covers systems installed as renewable heating technology — including reversible air-to-air systems marketed and installed primarily to heat the home.

Must pass the MCS Planning Standard noise calculation
Outdoor unit volume limit: 0.6m³
One metre from the boundary of the property

Class E — Plant or Equipment

Covers domestic air conditioning installed principally for cooling — the more common route for a straightforward split AC install.

No MCS noise calculation required
Outdoor unit volume limit: 1.5m³ (house), 0.6m³ (flat)
Cannot sit on a pitched roof or a front elevation above ground floor

⚠️ Why this matters: A lot of modern split systems are physically capable of both heating and cooling using heat pump technology — so the same box could plausibly fall under either class depending on how it's presented and installed. Don't assume which rules apply; ask your installer directly which class your installation is being assessed under, and get it in writing. This is the same pattern we've seen with other home energy rules recently — see our breakdown of the battery storage location rules under BS 7671 Amendment 4, where physical placement, not the technology itself, is what decides compliance.

The MCS Planning Standard, Explained Simply

If your installation falls under Class G, this is the calculation that decides whether you have permitted development rights at all.

The rule, set out in MCS 020(a), works out the sound level your outdoor unit will produce at your nearest neighbour's window — not in your garden, not at your boundary, but 1 metre outside the centre of any door or window belonging to a habitable room (a kitchen, bedroom or living room — not a bathroom, hallway or utility room) of a neighbouring property.

The calculation combines four things: the unit's certified sound power level (from the manufacturer's data — not a "quiet mode" figure), the distance to that window, how many reflective surfaces are nearby (a corner or narrow gap between walls bounces sound back and makes it worse), and any solid barrier between the unit and the window that would reduce what actually reaches it.

The result has to come out at or below 37.0 dB(A) under the current (2025-revised) version of the standard. Older guides online quote 42 dB(A) — that was the figure under the previous version of the same standard, so if you're reading advice that predates the 2025 update, the number it's working from is out of date.

What Happens If the Calculation Comes Out Over the Limit

This isn't hypothetical. In an online installer discussion, one UK engineer described fitting a heat pump that was technically within the permitted requirements at the time of installation — but after a neighbour complained and the council re-ran the calculation, it came out roughly 1 dB over the limit. The council never attended to measure the actual noise; the decision was made entirely on the paper calculation. The outcome: the unit had to be removed and relocated to a different position on the property, entirely at the installer's cost, months after it had already been fitted and working normally.

The lesson isn't that the rule is unfair — it's that the calculation is what gets checked, not the real-world noise the unit actually makes. Councils generally don't send someone out with a sound meter; they ask for the paperwork. A perfectly quiet unit can still lose its permitted development status if the numbers on the form are wrong or marginal, and a borderline calculation is a real risk worth taking seriously before installation, not after.

Before You Install: What to Ask Your Installer

✅ Which GPDO class is this installation being assessed under — G or E?
✅ If Class G, ask to see the actual MCS 020(a) noise calculation for your specific proposed position — not a generic figure
✅ How close is the result to the 37dB limit? A calculation that only just passes carries real risk if a neighbour complains and the council re-checks the figures
✅ Has every habitable-room window at neighbouring properties been assessed, not just the nearest one?
✅ If the calculation is borderline, ask about repositioning before install, not after