The short answer
In England, a heat pump fitted without planning permission must be no louder than 37 dB(A) at 1 metre from a neighbour's nearest habitable-room window or door, worked out with the MCS 020(a) calculation. Councils can also investigate noise complaints separately.
On this page
The outdoor unit is the part neighbours are most likely to hear: a steady hum from its fan and compressor, most noticeable when everything else is quiet. The planning rules deal with this through a calculation made before the unit goes in, rather than a measurement taken afterwards. The limit described here is a condition of permitted development in England; the rest of the UK sets its own rules.
The limit and where it applies
The standard is called MCS 020(a). It limits noise from the outdoor unit to no more than 37 dB(A) at 1 metre from the nearest window or door of a habitable room in a neighbouring home. Meeting it is one of the conditions for fitting a heat pump without planning permission, covered in Do you need planning permission?.
The limit applies at neighbours' homes, not your own, and only to the windows and doors of habitable rooms.
| Counts as habitable | Doesn't count |
|---|---|
| Living rooms | Bathrooms and toilets |
| Bedrooms | Halls and landings |
| Dining rooms | Utility rooms |
| Studies | Kitchens used only for cooking |
| Kitchen-diners |
Every neighbouring window or door that counts has to pass, so the nearest or least sheltered one usually decides whether a position works.
How the calculation works
The calculation starts with the unit's sound power level, which the manufacturer publishes. It's easy to confuse with sound pressure, which specification sheets often quote too: outdoor units are typically 45–50 dB(A) in sound pressure at 1 metre, and their sound power figures are higher.
Because the calculation uses sound power and then allows for the unit's surroundings, the 37 dB(A) limit can't be compared directly with either figure on its own. Decibels explained covers the difference.
From there, the result depends on where the unit sits.
| Factor | Effect on the result |
|---|---|
| Sound power level | A quieter unit starts lower |
| Hard surfaces close to the unit | Each one adds to the level |
| Distance to the window | Further away gives a lower level |
| Solid barrier hiding the unit completely | Takes 10 dB off |
| Solid barrier hiding it partly | Takes 5 dB off |
Reflecting surfaces are hard surfaces near the unit, such as walls and the ground, which bounce sound back. A unit on the ground against a wall is close to two; tucked into a corner, it's close to three.
A barrier has to be solid to count: a wall, a building, or a fence with no gaps, though a thin fence counts for less than a wall. Planting doesn't count. The barrier value is judged from each window's point of view, so a fence can hide the unit from one neighbour's window and not from another.
The installer keeps the calculation, and you should be given a copy. It sets out the figures behind the result, so it's also a useful record if a question about noise comes up later.
Positioning and fitting
The same things that help a position pass the calculation also tend to make a unit less noticeable day to day.
- Distance from windows. The further the unit is from neighbours' bedroom and living room windows, and from your own, the less it's heard indoors.
- Open space. Corners, narrow passages and alcoves add reflecting surfaces and can concentrate sound.
- Solid mounting. Anti-vibration mounts on the bracket or stand reduce the vibration that travels into the wall or ground. Wall brackets and ground stands explains the options.
- Quiet settings. Some outdoor units have a night or quiet mode that limits fan and compressor speed, usually with some loss of output. What's available varies by model.
Sound can change as a unit ages. Loose panels, debris in the fan and worn mounts can all cause rattles or hums that weren't there at first, and regular servicing can pick these up. Some sounds are normal, though: when heating in cold weather, the unit defrosts from time to time, with a whoosh and a cloud of vapour. Noises: what's normal and what isn't goes through them.
Neighbours and complaints
Permitted development doesn't involve a planning application, so neighbours aren't formally consulted. A conversation before the work, showing where the unit will go, gives them the chance to mention a concern while the position can still change.
Where a unit needs planning permission instead, the council considers the effect on neighbours as part of the application and can attach its own conditions.
Separately from planning, councils can investigate complaints about noise under nuisance rules, usually through their environmental health team. Meeting the planning noise standard doesn't rule out that kind of complaint. If a neighbour raises a concern, the practical first steps are usually checking that the unit is working normally, using any quiet mode, and looking at whether a barrier would help.
Related guides
- Do you need planning permission?Usually not in England, where air-to-air heat pumps that aren't used only for cooling count as permitted development if conditions on number, size, position and noise are met. Listed buildings, flats and protected areas follow different rules.
- How loud is air conditioning?Quiet indoor units are often around 19–26 dB(A) on their lowest fan speed and louder at higher speeds, while outdoor units are typically 45–50 dB(A) at 1 metre. How loud it seems also depends on where the units are and how hard they're working.
- Decibels explainedA decibel (dB) is a unit on a logarithmic scale, so adding 10 dB means ten times the sound energy and sounds roughly twice as loud. Air conditioner figures are usually in dB(A), which reflects how sensitive hearing is to different pitches.
- Where the outdoor unit can goAn outdoor unit needs a firm fixing, free airflow, service access and somewhere for defrost water to drain. On a house in England it can usually go on a wall, the ground or a flat roof without planning permission if set conditions are met.