Air conditioning noise: how loud units are and what the 2025 planning rules allow

Air conditioning noise comes mostly from the outdoor unit, which holds the compressor. Indoors, the Daikin Perfera and Bosch Climate 3200i wall units we fit, in the 2.5 to 3.5 kW sizes, are rated at 19 to 21 dB(A) on their quietest setting; outside, their makers list 46 to 56 dB(A). In England, since 29 May 2025, a unit that also heats can go in without a planning application if, among other limits, the calculated level 1 m outside a neighbour’s window is no more than 37 dB(A). We’re based in Christchurch, and the rules below are England’s.

How loud is air conditioning noise, indoors and out?

Here are the figures Daikin and Bosch publish for the 2.5 to 3.5 kW wall units we fit, in dB(A); their larger sizes are louder:

Unit (model)	Indoor, quietest	Indoor, top speed	Outdoor, sound pressure	Outdoor, sound power
Daikin Perfera 2.5 kW (FTXM25A + RXM25A9)	19 (Silent)	41 (High)	46 cooling, 47 heating	Not in Daikin’s UK tables
Daikin Perfera 3.5 kW (FTXM35A + RXM35A9)	19 (Silent)	45 (High)	47 cooling, 49 heating	Not in Daikin’s UK tables
Bosch Climate 3200i 2.6 kW (CL3200i-Set 26 WE)	20 (Silent)	37 (High)	56	62
Bosch Climate 3200i 3.5 kW (CL3200i-Set 35 WE)	21 (Silent)	37 (High)	55	63

Daikin’s indoor figures are for cooling. The tables we read don’t say how far away the outdoor sound pressure was measured, so compare brands with care; sound power, the energy-label figure, is what the planning check uses.

What those decibels compare with

Decibels are logarithmic, so a few of them go a long way:

Level	What it is
18 to 20 dB(A)	Existing sound in very rural areas, councils told a 2023 government review
About 30 dB(A)	Existing sound in many suburban areas, the same councils said
19 to 30 dB	What Daikin likens to a whisper
37 dB(A)	The planning limit, 1 m outside a neighbour’s window
40 dB	Night-time bedroom average above which Approved Document O (in effect since 15 June 2022) expects windows to stay shut; a quiet library, says Daikin
+3 dB	Two equal sources together, a doubling of sound energy

The same review found units caused the most disturbance at night, when background noise is lowest.

What changed for air-to-air units on 29 May 2025

Class G of England’s General Permitted Development Order lets you fit an air source heat pump at home without a planning application. Until 29 May 2025 it covered heating only. The amendment order (SI 2025/560), in force from that date:

• permits units that “can also, but not solely, provide cooling”, which the government said would let air-to-air heat pumps benefit;
• dropped the rule keeping every part of the unit 1 m from the boundary;
• raised the outdoor unit limit, housing included, from 0.6 to 1.5 cubic metres on a house (blocks of flats stay at 0.6);
• allows two units on a detached house, and one on other houses and blocks of flats;
• names MCS 020 a), the Air Source Heat Pump Sound Calculation, as the noise standard.

Each unit still has to be positioned so that its effect on the building’s appearance and the area’s amenity is as small as practicable.

Good to know Since 29 May 2025, an air-to-air unit in England can be permitted development, as long as it isn’t used solely for cooling.

The 37 dB(A) noise check, step by step

MCS 020 a) (Issue 1.1, April 2025) sets 37 dB(A) at each assessment position: 1 m outside the centre of any door or window to a neighbouring property’s habitable room, meaning one used for sleeping or living.

1. Take the outdoor unit’s sound power from the maker’s data, such as the energy label, never its “low noise mode” figure.
2. Count reflecting surfaces within 1 m: the ground and walls count, plants don’t. One surface is Q2, two Q4, three Q8; more than three, as in a small lightwell, fails.
3. Measure the straight line from the centre of the unit to that point.
4. Deduct up to 10 dB for a solid wall, or a gap-free fence at least 18 mm thick, extending 1 m past each edge of the unit and hiding the window.
5. Repeat for every neighbouring window that could be affected, on every floor.

Each extra reflecting surface doubles Q, adding 3 dB. Under MCS 020 a), the installer keeps a copy of the calculation and gives you one.

A worked example with a Bosch outdoor unit

We’ve run the MCS formula for Bosch’s 2.6 kW Climate 3200i outdoor unit (sound power 62 dB) to find the shortest distance to the assessment position, 1 m outside a neighbour’s window, that still passes 37.0 dB(A), rounded up to the next 0.1 m:

Where the outdoor unit sits	No barrier	Wall or 18 mm+ fence, partial view	Wall or 18 mm+ fence, no view
On a wall bracket more than 1 m above the ground (Q2)	7.1 m	4.0 m	2.3 m
On the ground against a wall (Q4)	10.1 m	5.7 m	3.2 m
On the ground in a corner (Q8)	14.2 m	8.0 m	4.5 m

In MCS terms, partial view means the window is hidden from the unit’s top edge but in sight within 0.25 m of it, and no view means it’s fully hidden. Put the other way, that unit more than 1 m up a side wall, 3 m from the point outside a neighbour’s bedroom window and in full view, works out at about 44.5 dB(A). It’s arithmetic, not a survey: a real check uses your actual unit, distances and barriers.

When air conditioning still needs planning permission

Passing the noise check doesn’t grant permitted development on its own. You’ll need to apply if the unit would be used solely for cooling, fail the check, or:

• have an outdoor unit over 1.5 cubic metres on a house, or 0.6 on a block of flats (Bosch’s 2.6 kW one is under 0.1);
• be a second unit on a house that isn’t detached or on a block of flats, or a third on a detached house;
• go on a pitched roof, within 1 m of a flat roof’s edge, or on a road-facing wall above ground-floor level;
• in a conservation area, go on a road-facing wall or roof, or nearer the road than the house;
• be at a listed building or a scheduled monument.

Flats matter here: at the 2021 Census, 36.9% of households in Bournemouth, Christchurch and Poole lived in a flat, maisonette or apartment, against 22.2% across England. MCS counts the other flats in the same block as neighbouring properties, so the window above yours can decide it, which is worth knowing before you plan air conditioning for a Poole flat or house. For rooms in the roof, see our guide to air conditioning for a loft conversion.

Good to know The 37 dB(A) check is one condition among several: the size, number and siting limits apply as well.

Why a fitted unit can be louder than its data sheet

Research cited in the government’s review monitored ten air source heat pumps at UK homes for a week each: they averaged 4 dB above their data sheets, and up to 8 dB, because of operating conditions and installation location. Daikin’s outdoor-unit guide agrees that a level measured on site “might be higher” than its data book, because of environmental noise and sound reflections. The usual culprits:

• Reflections. MCS advises keeping units out of corners, enclosed areas and hollow spaces, which “can amplify noise”. Councils called the reflected noise from a unit on the side of a house, 2 or 3 m from the neighbour, a “canyoning effect”.
• Vibration. Daikin says the outdoor unit mustn’t touch a ceiling or a wall, which may cause vibrations, and calls for vibration-proof rubber where vibration could reach the building. MCS adds rubber grommets on wall brackets and anti-vibration feet or pads.
• Base and direction. The review traced one complaint to an uneven base, and others to fans facing the neighbour rather than the owner’s garden.
• Hard work and age. Cold spells, defrosts, missed maintenance and loose parts all make a unit more noticeable, MCS says.

That’s why, in any air conditioning installation, where the outdoor unit goes matters as much as which unit you choose.

Good to know Reflections, vibration and the direction the fan faces can all make a fitted unit louder than its data sheet.

Quiet and night modes on Daikin and Bosch units

• Daikin Silent. Lowers the Perfera indoor unit’s sound by 3 dB(A), Daikin says.
• Daikin Outdoor unit quiet operation. Lowers the outdoor unit’s sound by 3 dB(A) by limiting the compressor’s speed; it can’t run alongside Powerful mode.
• Daikin Night quiet. On multi-split systems, set up at installation, it quietens the outdoor unit at night at some cost to cooling.
• Bosch Silent mode. Cuts noise from both units, “particularly advantageous at night, if you need to show consideration for your neighbours”, though the unit may then fall short of the cooling or heating capacity you need. It isn’t available on a multi-split.

Quiet modes may reduce output, MCS warns, and the planning check never uses them. Compare the Daikin Perfera and Comfora range with Bosch’s Climate 3200i system for the full figures.

Sounds the makers say are normal, and ones to get checked

What you hear	What the maker says causes it
Flowing water or a quiet hiss	Refrigerant moving through the system, or water draining in cooling (Daikin, Bosch)
Ticking, creaks or squeaks	Parts expanding and contracting with temperature (Daikin, Bosch)
Whistling or flowing on a cold day	A defrost cycle; Daikin says to wait 4 to 12 minutes
Clicking	Refrigerant control valves or electrical parts operating (Daikin)
The outdoor fan stopping and starting	Variable fan control (Bosch)

Rattling, buzzing or vibration is different: MCS lists loose parts and debris in the fan among the first checks, and Daikin’s fix for a vibrating indoor unit is to secure it. Don’t take covers off yourself: Bosch says never to interfere with the refrigerant circuit or dismantle the product, and to “always ask a professional”. A short wait before restarting is normal too: here’s why air conditioners pause for three minutes.

If a neighbour’s unit is keeping you awake

Noise “emitted from premises so as to be prejudicial to health or a nuisance” is a statutory nuisance under section 79 of the Environmental Protection Act 1990. The route in England:

Step	What happens
Talk first	GOV.UK advises talking to your neighbour, or mediation, before contacting the council
Report it	BCP Council, the council for Christchurch, asks what the noise is, where it comes from, how long, how often and when it happens, and how it affects you
Investigation	The council must take reasonably practicable steps to investigate
Abatement notice	If it finds a statutory nuisance, it must serve one, which can stop the noise or restrict it to certain times
Ignoring it	Prosecution and fines set by the court

BCP says a noise must be “unreasonable and substantial and not just an annoyance”, and lists everyday household noise among what it can’t investigate. Permitted development is no shield: the complaints councils investigated for the government’s review all involved permitted development units, mostly over disturbed sleep within three months of installation, and councils tried first to fix them by repairing or moving the unit or adding a noise barrier.

So, how much air conditioning noise should you expect?

Very little indoors: 19 to 21 dB(A) on the quietest setting of the 2.5 to 3.5 kW wall units we fit. Outside it’s louder, and siting decides what reaches the neighbours: a barrier alone can shift the calculation by 10 dB. Since 29 May 2025, a unit that also heats, passes the 37 dB(A) check and meets the other limits needs no planning application in England. We weigh both halves together when fitting air conditioning around Christchurch.

Book a free survey and our team will look at where the outdoor unit can go and which neighbouring windows it faces, so air conditioning noise is planned for before anything goes on the wall.