The external (outdoor) unit
By Airva Editorial Team · Checked against published UK guidance · Updated
Where the outdoor unit goes decides more about your project than which brand you buy. It sets the price, because access drives labour. It sets the noise, because sound is assessed at your neighbour's window rather than yours. It sets whether you need planning permission. And it sets what every future service visit will cost, for the next fifteen years.
Get this decided properly at survey stage and everything downstream is routine.
What the unit actually needs
An outdoor unit is a heat exchanger with a fan. To work, it needs to breathe.
- Clear air in and clear air out. The unit draws air through its coil and discharges it, usually from the front. Boxing it in, or putting it in a tight recess, causes it to re-ingest its own discharge air — which in cooling mode means it is trying to reject heat into air it has already heated. Performance falls and the compressor works harder. Clearance requirements are model-specific and published in the installation manual; they are not advisory.
- Somewhere for water to go. In heating mode the outdoor unit produces condensate, and during a defrost cycle it sheds meltwater. On a path or a doorstep in January that becomes ice. Plan the drainage rather than letting it discharge onto a walking surface.
- A solid, level mounting. Either wall brackets into structural masonry, or a ground stand on a firm base. Anti-vibration mounts in both cases.
- Access for a person with tools. Discussed below, and consistently underrated.
What the 2025 rule changes allow
The permitted development rules for England changed on 29 May 2025, and two of the changes are directly about siting [2]:
| Rule | Before | Now |
|---|---|---|
| Distance from the boundary | At least 1m from the property boundary | No boundary distance rule — position is governed by the sound calculation |
| Maximum volume | 0.6 m³ | 1.5 m³ on a house, including housing |
| Number of units | One | Two on a detached house |
The removal of the boundary rule is the significant one for siting. A unit may now sit against a fence or a boundary wall, provided the MCS 020(a) sound calculation supports it [2]. That opens up narrow side returns and small courtyards which were previously ruled out on geometry alone.
What has replaced it is a calculation rather than a rule of thumb. MCS 020(a) establishes the sound level at an assessment position — broadly, a neighbouring habitable room — and permitted development depends on staying within 42 dB(A). The 2025 revision clarified what counts as a solid barrier, how the assessment position is determined, and what counts as a reflective surface [2]. That last one matters in exactly the tight spaces the boundary change now permits: a unit between two hard walls is a different acoustic problem from the same unit in an open garden.
Where planning still constrains the position
Beyond the sound test, the Class G conditions restrict position directly [1]:
- Conservation areas and World Heritage Sites: not on a wall or roof fronting a highway, and not nearer to a bounding highway than any part of the building.
- Everywhere else: not on a wall above ground-floor storey level if that wall fronts a highway.
- Roofs: pitched roofs are excluded entirely; on a flat roof, every part of the unit must be at least one metre from the external edge.
- Listed buildings: no permitted development rights within the curtilage at all — see period homes.
Our planning permission guide covers the full checklist, including the condition that the system must not be used solely for cooling.
The main tests, all of which must pass. The full Class G checklist adds further conditions — existing wind turbines, removal when no longer needed, and siting to minimise impact. England only: Scotland, Wales and Northern Ireland set their own thresholds. The 1 metre boundary rule was removed in 2025; the sound calculation now does that work instead.
Ranking the common positions
| Position | Verdict | Why |
|---|---|---|
| Ground level, rear or side, on a stand | Best | Cheap to install, cheap to service forever, easy drainage, no working at height |
| Flat roof of a rear addition | Very good | Short pipe runs, hidden from the street — but check the 1m edge rule [1] and use anti-vibration mounts |
| Side return or passage | Good since 2025 | Now viable without the 1m boundary rule, if the sound calculation works in a reflective space |
| Wall bracket at first-floor level | Workable | Adds access cost at install and at every service |
| Wall bracket at second floor above a drop | Avoid where possible | Scaffold or tower needed, repeatedly, for the system's whole life |
| Balcony of a flat | Depends entirely on consent | Freeholder permission and usually planning permission — see our flat guide |
| Boxed into a tight recess | Avoid | Recirculation kills performance regardless of what the rules allow |
The access cost nobody prices twice
A condenser two storeys up above a drop is not a one-off cost. It is a cost at installation, and again at every service visit, and again at every repair, for fifteen years. Scaffolding or a tower for what would otherwise be an hour's filter and coil clean transforms routine maintenance into a project.
When two quotes differ significantly, access assumptions are one of the most common reasons. Ask each installer how they intend to reach the unit — and then ask how a service engineer will reach it in five years.
On a larger system the sums get worse, because some of those visits are not optional. Once a system holds refrigerant equivalent to 5 tonnes of CO₂ or more, leak checks become a legal duty on the equipment operator — at least every 12 months, rising to every 6 months from 50 tonnes [3]. Most single splits sit well below that threshold; a substantial multi-split can reach it. If it does, an awkward condenser position adds a scaffold charge to a statutory obligation you cannot skip.
Where a high position is unavoidable, a longer pipe run to a reachable ground-level position is frequently the cheaper answer over the life of the system, even though it costs a little more on day one.
Winter, noise and neighbours
Defrost and drainage. In cold weather the unit periodically melts ice off its coil, producing water and visible steam. Both are normal. Site so that meltwater does not run across a path.
Vibration. The most common noise complaint is not the fan but a low hum transmitted through the structure. A solid fixing point and anti-vibration mounts prevent it; a lightweight wall or a hollow flat roof above a bedroom does the opposite.
Neighbours. The sound assessment is taken at their position. Beyond compliance, siting the unit away from a neighbour's bedroom window and garden seating is straightforward goodwill that costs nothing at design stage and is expensive to fix afterwards.
Frequently asked questions
How close to the boundary can the outdoor unit be? There is no longer a fixed distance rule in England — the 1m requirement was removed on 29 May 2025 [2]. Position is now governed by the MCS 020(a) sound calculation at the neighbouring assessment position.
How big can the outdoor unit be? Up to 1.5 m³ including housing on a house, or 0.6 m³ for a block of flats [1].
Can it go on the roof? Not on a pitched roof under permitted development. On a flat roof it must be at least one metre from the external edge [1].
Can I put it in a cupboard or box it in? Only with the manufacturer's published clearances maintained. Restricting airflow makes the unit re-ingest its own discharge air, which costs performance and shortens compressor life.
Does it need to be in the shade? It helps slightly in cooling mode but matters far less than free airflow. Never restrict airflow to gain shade.
What if the only position is high up? It will work, but price the lifetime access cost. A longer pipe run to a reachable position is often cheaper over fifteen years.
Get the position decided at survey
The outdoor unit position is the decision worth spending time on. Tell us about the property and we will match you with one vetted installer who will stand outside it, do the sound calculation, and tell you what the position costs to service later: get an installer match.