How loud is an air conditioning outdoor unit? Noise and neighbours

What you actually hear

An outdoor unit has two moving parts that make noise: a fan that pulls air through the coil, and a compressor that pumps the refrigerant. Most modern units use an inverter compressor, which speeds up and slows down to match demand instead of switching fully on and off. That means the unit is often running quietly at part load and only gets louder when the house is working hard: a 30°C afternoon, or a cold January morning when it is heating.

The sound is a steady hum with a fan whoosh on top. Problems usually come from three things: a unit placed close to a bedroom window, a unit boxed into a corner that bounces sound around, or vibration travelling through a wall bracket into the brickwork. None of those are about the brand.

The neighbour test in the planning rules

In England, a heat pump outdoor unit that also heats can be installed under permitted development only if it complies with the MCS 020(a) planning standard. As explained in Planning Geek’s Class G guide, that standard is a noise calculation done on paper before installation. The installer takes:

Input	What it means
Sound power level	the manufacturer’s figure for the outdoor unit at source
Directivity	how many hard surfaces are next to it: ground only, ground and a wall, or a corner
Distance	straight line from the unit to the assessment point
Barrier	any solid wall or close boarded fence between the unit and the window

The assessment point is 1 metre outside the centre of a door or window to a habitable room of a neighbouring property, such as a bedroom or living room. Bathrooms and hallways do not count. Every neighbouring window on every floor has to be checked, and the result at each one must be 37.0 dB(A) or lower. Planning Geek also notes that a unit enclosed by more than three reflecting surfaces, such as in a small lightwell, cannot meet the standard at all.

If the unit is used only for cooling, it falls outside permitted development regardless of noise and needs planning permission. The council will then judge noise on its own policies. The Planning Portal lists the full set of conditions.

Why Fulham and Hammersmith gardens are tricky

A typical Fulham terrace has a narrow garden, the width of the house, a side return extension and neighbours on both sides whose back bedrooms look straight down onto it. The closest habitable room window might be a couple of metres from the obvious spot for an outdoor unit. Upstairs windows matter too: a first floor bedroom next door can have a clear line of sight to a unit on your extension roof, even if a fence screens it from the ground floor.

Hammersmith and Fulham Council has 44 conservation areas covering about half the borough, so in many streets the front of the house is off limits as well, which pushes every unit to the back. Residents search specifically for the borough’s air conditioning planning rules, and the answer usually turns on this noise figure. Our Hammersmith and Fulham page covers the local position.

Positions that usually work

These are the spots that most often pass the calculation in a London terrace:

• low on the rear wall at the end of the garden, facing your own house
• behind a solid brick garden wall that hides the unit from the neighbour’s windows
• on a flat roof set well back from the edge and from the party wall, with a solid parapet
• on the side return wall facing your own windows rather than the neighbour’s

Positions that often fail are tight corners between an extension and the main house, small enclosed lightwells, and high brackets on a rear wall directly under a neighbour’s bedroom.

Simple things that cut the noise

A few practical steps make a noticeable difference:

1. Anti vibration mounts under the feet or on the bracket stop hum travelling through brick into your house or the one next door.
2. A solid screen, not slatted, between the unit and the window. Gappy fencing gives almost no reduction.
3. Clear airflow: a unit starved of air works harder and louder, so do not box it in tightly.
4. Night mode: many units have a setting that caps fan and compressor speed at night.
5. Correct sizing: a unit that fits the load runs steadily at low speed instead of cycling.

Talk to your neighbours

There is no legal duty to consult neighbours for permitted development, but it is good practice on a terrace. Show them where the unit will go and share the calculated level. If you need a planning application, they will be notified anyway, and an early conversation often avoids an objection. If you are in a flat, the freeholder and other leaseholders will want the same information.

If there is a complaint later

Councils investigate noise that may amount to a statutory nuisance. A unit that was installed without the MCS 020 calculation, or that does not meet it, may also not be lawful as permitted development. Keep a copy of the calculation, the installer’s details and the commissioning record. Regular servicing helps too: a damaged fan blade, a loose panel or a clogged coil makes a unit louder over time. Our home air conditioning page explains how we plan the outdoor unit position at the survey, and you can compare models in our guide to the quietest air conditioning units.