The short answer
An 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.
The outdoor unit's position is usually settled at the survey, and the reasoning behind it is worth understanding. The tidiest-looking spot isn't always the one that works: a unit squeezed into a corner can struggle for air and be louder at a neighbour's window than one on an open wall.
What the unit needs
- Free airflow. Most units draw air in through the back and sides and blow it out of the front. Manufacturers set minimum clearances, and a unit boxed in by walls, fences or dense planting can end up drawing in its own exhaust air, which reduces performance.
- A firm, level fixing. A sound wall for brackets, or a stable base for a stand. The options are compared in wall brackets and ground stands.
- Access for servicing. Engineers need to reach the panels and valves safely. A unit high on a wall may need access equipment every time it's serviced.
- Somewhere for water to go. In heating mode, especially when defrosting, water runs from the base. Above a path, step or doorway it can freeze into a slip hazard. More on this in where the water goes.
- A sensible pipe run. Every model has limits on pipe length and on the height difference between the units, and shorter runs are simpler.
- Some shelter from the extremes. Some manufacturers advise against facing the fan into strong prevailing winds. Spots where snow drifts or leaves collect are best avoided, and near the coast a sheltered side can reduce corrosion from salt air.
- Distance from flue outlets. Exhaust from a boiler flue shouldn't be drawn through the unit.
Common positions
| Position | Good for | Watch for |
|---|---|---|
| Ground stand by a back wall | Easy servicing | Space, knocks, leaves and snow |
| Wall brackets, low | Keeping the ground clear | Vibration through the wall |
| Wall brackets, high | Out of the way | Harder servicing |
| Flat roof | Out of sight | Roof access, distance from the edge |
| Side passage | Hidden from view | Restricted airflow, reflected sound |
The front of a house is possible in some cases, but planning rules restrict road-facing walls, as explained below.
Noise and the neighbours
In England, a unit fitted without planning permission must meet the MCS 020(a) noise standard. Noise from the unit must be no more than 37 dB(A) at 1 metre from the nearest window or door of a habitable room in a neighbouring home.
Habitable rooms are living rooms, bedrooms, dining rooms, studies and kitchen-diners. Bathrooms, toilets, halls, landings, utility rooms and kitchens used only for cooking don't count.
The calculation starts from the unit's sound power level and takes into account:
- Reflecting surfaces. Each hard surface close to the unit, such as a wall or the ground, adds to the level. A unit on the ground against a wall has two; in a corner, three.
- Distance from the unit to the neighbour's window or door.
- Barriers. A solid barrier that completely hides the unit from the window takes 10 dB off; one that partly hides it takes 5 dB off.
That's why a tucked-away corner can come out louder than an open stretch of wall further along. The installer keeps the calculation, and you should get a copy. Noise rules and your neighbours goes into more detail.
Planning rules in England
Air source heat pumps, including air-to-air systems, can usually be fitted to a house without planning permission under permitted development, as long as the conditions are met. An air-to-air system must not be used solely for cooling. The main conditions are:
- One heat pump on a semi-detached or terraced house, and two on a detached house. An air source heat pump already on the property is likely to count towards the limit.
- The outdoor unit, including any housing, must be no more than 1.5 cubic metres.
- Not on a pitched roof. On a flat roof, at least 1 metre from the edge.
- Not on a wall facing a road above the ground floor.
- The installation must meet the MCS 020(a) noise standard.
The old rule that the unit had to be at least 1 metre from the boundary was removed in 2025.
Conservation areas and other protected areas, such as World Heritage Sites, have stricter rules, including on walls and roofs facing a road. Listed buildings need listed building consent, and permitted development doesn't cover them. Flats, and homes whose permitted development rights have been removed, follow different rules, so check with your local council.
If the conditions aren't met, planning permission is needed. A Lawful Development Certificate from the council gives certainty that it isn't. Scotland, Wales and Northern Ireland have their own rules. The full picture is in do you need planning permission?
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.
- Noise rules and your neighboursIn 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.
- Wall brackets and ground standsWall brackets fix the outdoor unit to a sound wall and keep the ground clear, while ground stands hold it on a firm, level base. Both should raise it clear of water, snow and debris and sit it on anti-vibration mounts.
- Looking after the outdoor unitKeep the space around it clear so air flows freely, brush off leaves and snow with the system switched off, and leave cleaning the coil and anything inside the casing to an engineer. Otherwise it needs little beyond its regular service.