Is air conditioning noisy? Noise levels, neighbours and the UK rules

The short answer

A properly sited wall-mounted split system is quiet indoors and, in England, its outdoor unit has to be quiet enough at a neighbour's window to meet a legal limit of 37 dB(A) if you want to install it without planning permission.

The loud part of any air conditioner is the compressor. In a split system it sits outside in the condenser unit, so what you hear in the room is a fan pushing air across a cold coil. Portable units put the compressor in the room with you, which is why people who have only used one tend to expect the worst.

Noise problems with split systems almost always come from where the outdoor unit goes, how it is fixed to the wall, and whether it is looked after. All three are under your control before the installer drills a single hole.

How decibels work, in plain numbers

Manufacturers quote two different figures and mixing them up causes most of the confusion. Sound power level (Lw) describes how much noise the unit itself produces. Sound pressure level (Lp) describes what you would measure at a particular distance. Spec sheets for outdoor units often give both. Planning rules in England use sound power as the starting point and then work out the sound pressure at your neighbour's window.

The decibel scale is logarithmic. A rise of 10 dB(A) is generally heard as roughly twice as loud, and a rise of 3 dB(A) is about the smallest change most people notice outside a lab. So a unit listed at 46 dB(A) is not slightly louder than one at 40 dB(A). On paper it is putting out four times the sound energy, and most listeners would describe it as clearly louder.

Two identical units side by side add 3 dB(A), not double the number. That matters on terraces where a neighbour already has a condenser on the shared yard wall.

What you will hear inside the room

Indoor units are rated at several fan speeds. Compare the lowest speed figure, which is the one that applies when the room is already at temperature or the unit is in night mode. Many current 2.5kW wall units list a lowest-speed figure in the low 20s dB(A) or below, but check the data sheet for the exact model you are quoted, because figures vary between ranges from the same brand.

For context, the World Health Organization's night noise guidelines for Europe treat 30 dB(A) inside a bedroom as a level that should not disturb sleep for most people. A wall unit on its lowest fan speed is usually below that. On full fan, when you have walked into a hot room at 6pm and set it to 20C, it will sound more like a desk fan for ten or fifteen minutes.

Normal sounds you may notice: a click when the louvres move, light creaking as the plastic casing warms or cools, a brief gurgle as refrigerant settles after start-up, and a soft drip in the condensate pipe. A rhythmic rattle, a high whine or a loud hum is not normal and is covered further down.

The outdoor unit and your neighbours

The condenser holds the compressor and a larger fan, so it is the part people outside hear. Inverter models, which are now the norm, slow the compressor once the room is at temperature instead of switching on and off at full power. Most of the evening, the unit is running well below its rated maximum noise.

Two situations are louder. The first is the hottest afternoons, when the unit is working hardest. The second is heating mode in cold, damp weather, which is common in Leeds from November to March. The outdoor coil frosts up and the unit runs a defrost cycle every so often. You may hear a hiss as the valve changes over, then see steam rising from the unit for a minute or two. That is normal, but it is also why a condenser under a neighbour's bedroom window can draw complaints in winter even if nobody noticed it in July.

The 37 dB(A) rule: what permitted development actually requires

Since 29 May 2025, air-to-air heat pumps (split air conditioners that also heat) can go up in England under permitted development, the same route as air source heat pumps. Before that change, adding a condenser to a house in England normally needed a planning application. The rules sit in Class G, Part 14, Schedule 2 of the General Permitted Development Order 2015, and the noise test is set out in the MCS 020 a) standard.

The headline test: the calculated sound pressure from the outdoor unit must be 37.0 dB(A) or lower at the assessment position. That is normally one metre out from the centre of the nearest habitable room window or glazed door of a neighbouring home (a bedroom, living room or kitchen-diner, for example). If it is over 37.0 dB(A), the installation is not permitted development and needs planning permission.

Other limits apply at the same time. The outdoor unit can be up to 1.5 cubic metres in volume on a house and 0.6 cubic metres on a block of flats. Houses that are not detached can have one unit under these rights. Detached houses can have two. Units on pitched roofs are excluded, units on flat roofs must be at least one metre from the roof edge, and listed buildings are excluded. In a conservation area the unit cannot go on a wall or roof facing a highway. A lot of inner Leeds, including parts of Headingley, Chapel Allerton, Roundhay and Far Headingley, sits inside conservation areas, so check the Leeds City Council conservation area map before choosing a front wall.

The noise figure is a calculation from the manufacturer's sound power data, not a measurement after installation. A competent installer should be able to show you the working. Our planning permission guide covers the non-noise conditions in more detail.

Worked example: a semi in Horsforth

Here is the MCS 020 a) arithmetic for a typical job, so you can check a quote yourself. The formula is: sound pressure at the window = sound power of the unit + 10 x log10(Q / (4 x pi x distance squared)) + barrier correction.

Q is the directivity factor. It reflects how many hard surfaces bounce sound towards the neighbour. A unit on a ground bracket against the house wall has two reflecting surfaces (ground and wall), so Q is 4. Tuck it into a corner between the house and a side return wall and Q becomes 8.

Say the quoted outdoor unit has a sound power of 60 dB(A) at its cooling rating, and the neighbour's kitchen window is 5 metres away with nothing in between. Then 4 x pi x 25 = 314.2. Q divided by that is 4 / 314.2 = 0.0127. Log10 of 0.0127 is about -1.90, times 10 is -19.0. So 60 - 19.0 = 41.0 dB(A). That fails the 37.0 limit by 4 dB(A).

Move the same unit round to the side of the house so it sits 8 metres from that window. Now 4 x pi x 64 = 804.2, and 4 / 804.2 = 0.0050. Ten times log10 of that is -23.0, giving 60 - 23.0 = 37.0 dB(A). That passes, just. A solid garden wall or close-boarded fence that fully blocks the line of sight can take off up to 10 dB(A) under the standard, and partial screening up to 5 dB(A), which is often the cheaper fix on tight plots.

The 60 dB(A) figure above is an illustration only. Use the sound power from the data sheet of the model on your quote. A smaller 2.5kW condenser is often quieter than a 5kW one, which is one more reason not to oversize.

Leeds housing: where the outdoor unit usually ends up

Back-to-back terraces, common in Harehills, Armley, Beeston and Holbeck, have no rear wall. The realistic options are the front elevation, a gable end on end-terrace houses, or a flat roof on a rear extension where there is one. Front walls in conservation areas are ruled out under permitted development, and neighbours' windows on a narrow street can be under 10 metres away across the road as well as next door, so the 37 dB(A) calculation needs doing for more than one window.

Through terraces in areas like Hyde Park, Kirkstall and Bramley usually have a small yard. The yard looks like the obvious spot, but yards are boxed in by walls on three sides, which pushes Q up and the next-door kitchen window is often within 3 or 4 metres. A wall bracket high on the rear elevation, angled away from the shared wall, can work better than the yard floor.

Interwar and post-war semis in Roundhay, Moortown, Cross Gates and Horsforth tend to have a side passage. A unit on the side wall often passes easily, but a passage between two houses acts like a channel and bounces sound towards both side windows. The worked example above shows how a few metres either way changes the result.

Stone-built houses in Otley, Ilkley, Pudsey and much of Calderdale have thick solid walls. They take a wall bracket well and transmit less vibration than a thin brick or block cavity wall. Timber cladding, render over insulation board, and garden-room walls are the worst choice for mounting a condenser.

Installation choices that cut noise

• Fix the condenser to solid masonry or a ground plinth with rubber anti-vibration mounts, never to a stud wall or timber frame.
• Keep the manufacturer's minimum clearances. A unit that cannot breathe runs its fan faster and its compressor harder.
• Avoid corners and enclosed yards where you can. Each extra reflecting surface raises the level at the neighbour's window.
• Use pipe clips with rubber inserts where the pipework runs along the wall, so vibration does not travel into the building.
• Pick the size the room needs. An oversized unit cycles more and a larger condenser usually has a higher sound power.
• Ask for the model's night or low-noise outdoor mode to be enabled during setup. Many inverter models cap the compressor speed at night.

If a neighbour complains, or you want to complain

Talk first. Showing next door where the unit will go before it is fitted, and the calculation that says it passes, heads off most problems. If a unit is already up and annoying someone, moving it a few metres or adding a screen usually costs far less than any formal process.

If it goes further, noise from equipment at a home can be a statutory nuisance under section 79 of the Environmental Protection Act 1990. Leeds City Council takes complaints through its noise nuisance service. The council says it aims to respond within 10 working days, may ask the complainant to keep a noise diary, and gives case updates every two weeks. Normal household noise is not usually treated as a nuisance unless it is excessive or prolonged.

If an officer decides there is a statutory nuisance, the council must serve an abatement notice, which can require the noise to stop or be limited to certain hours. The person served has 21 days to appeal. Leeds publishes its section 80 notices as open data on data.gov.uk. Passing the 37 dB(A) planning test does not make a unit immune from nuisance law, but a unit that passes and is maintained is very unlikely to reach that point.

Outside Leeds, the process is the same law run by your own district: Bradford, Calderdale, Kirklees and Wakefield councils each handle complaints for their area.

When a system gets louder than it used to

A system that was quiet for two summers and is now noisy is telling you something. Start with the indoor filters. In heavy use they clog within a few weeks, the fan speeds up to compensate, and the unit sounds busier. Rinse them in lukewarm water, dry them and refit.

Outside, look for leaves or litter in the fan guard and check the mounting feet. A unit that has shifted on its bracket, or has perished rubber mounts, will rattle at certain compressor speeds and the sound can carry through the wall into the house.

Grinding, squealing, a metallic knock on start-up or a loud steady buzz point to a fan motor, bearing or compressor problem. Those are jobs for a qualified engineer. Anything involving the refrigerant circuit legally needs someone holding an F-Gas certificate. An annual service is when these faults usually get caught early.

Sources

• MCS 020 a) Air source heat pump sound calculation, issue 1.1: https://mcscertified.com/wp-content/uploads/2025/07/MCS-020-a-Issue-1.1-Final.pdf
• MCS, New permitted development rules in England (29 May 2025): https://mcscertified.com/new-permitted-development-rules-in-england-air-source-heat-pump-installations-must-now-comply-with-mcs-020-a/
• General Permitted Development Order 2015, Schedule 2 Part 14 Class G: https://www.legislation.gov.uk/uksi/2015/596/schedule/2/part/14/crossheading/class-g-installation-or-alteration-etc-of-air-source-heat-pumps-on-domestic-premises
• Leeds City Council, Making a noise complaint: https://www.leeds.gov.uk/antisocial-behaviour-and-crime/making-a-noise-complaint
• GOV.UK, Noise nuisances: how councils deal with complaints: https://www.gov.uk/guidance/noise-nuisances-how-councils-deal-with-complaints
• Environmental Protection Act 1990, Part III: https://www.legislation.gov.uk/ukpga/1990/43/part/III
• Leeds City Council noise abatement notices (open data): https://www.data.gov.uk/dataset/c717c8e9-99d0-4881-b005-ddc80b30dd61/noise-abatement-notices
• WHO Night Noise Guidelines for Europe: https://www.who.int/europe/publications/i/item/9789289041737