Air conditioning units and external plant on a house

There is no permitted development class in England for comfort cooling, so the starting point for air conditioning unit planning permission on a house is that you have no automatic right to fit one. The renewable energy classes in Part 14 of the General Permitted Development Order cover air source heat pumps, solar equipment and biomass flues. A split-system air conditioner is none of those. Whether you need an application then depends on a narrower question: does bolting a condenser to the wall materially affect the external appearance of the building.

Why cooling plant sits outside the microgeneration classes

Part 14 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 is headed installation of domestic microgeneration equipment. Class G deals with air source heat pumps on domestic premises, subject to conditions on volume, siting, noise and compliance with the microgeneration planning standards. Those limits have been amended more than once, most recently in 2024, so read the current text rather than an older summary.

An air source heat pump and an air conditioner look similar on the drive and are not the same thing in law. Class G is drafted around equipment that generates heat for the building. A unit installed solely to reject heat from the building falls outside it. Reversible air-to-air heat pumps sit in an awkward middle: they can heat, and a manufacturer will say so, but if the installation is not certified against the microgeneration planning standards it cannot rely on Class G in any event.

The practical effect is that homeowners who were told "heat pumps do not need permission" arrive at the council with a condenser already on the wall.

Whether the installation is an alteration to the exterior

Section 55(2)(a) of the Town and Country Planning Act 1990 says that works for the maintenance, improvement or other alteration of a building which affect only the interior, or which do not materially affect its external appearance, are not development at all. No development means no permission.

A single small condenser tucked behind a rear extension, invisible from any public vantage point and no larger than a domestic wheelie bin, has a good argument under that provision. Three units racked on a side elevation facing the street do not. Between those extremes, authorities take different views, and some treat wall-mounted plant as an alteration falling within Part 1 Class A, which brings its own limits on siting forward of the principal elevation and on article 2(3) land.

The way to settle it is a certificate of lawfulness for a proposed use or development under section 192, which costs half the householder fee and gives a binding answer on the legal position. Where a heat pump could lawfully be installed under Class G in the same position, the existence of that alternative is a genuine material consideration in favour of the air conditioner, along the lines set out in our note on the permitted development fall-back as a material consideration.

Check the deeds and the original permission too. Estates built in the last twenty years frequently carry conditions removing Part 1 rights, and lifting one is a separate exercise explained in our piece on removing a permitted development condition under section 73.

Noise as the substantive objection

Whatever the route, noise decides the case. A domestic condenser runs at night in a heatwave, when background sound levels are at their lowest and bedroom windows are open, which is the worst possible combination for the person next door.

Planning and environmental health assess it against BS 4142:2014+A1:2019, which compares the rating level of the specific sound against the representative background sound level at the nearest noise-sensitive facade. The rating level is the measured or predicted specific sound level plus corrections for acoustic features: tonality from a compressor, intermittency from cycling, impulsivity from a defrost cycle. A difference of around +10 dB is an indication of a likely adverse impact, around +5 dB an indication of an adverse impact, and a rating level below the background sound level is an indication of a low impact. Those are indications rather than pass marks, and context matters.

Separately, section 79 of the Environmental Protection Act 1990 lets the council serve an abatement notice on noise amounting to a statutory nuisance. That power applies whether or not planning permission was needed, which is why "I did not need permission" is not the end of an argument with a neighbour.

Siting away from neighbouring windows

Distance helps, and geometry helps more. Sound pressure falls with distance from the source, but a condenser in a narrow passage between two semi-detached houses is surrounded by hard reflective surfaces and the passage behaves like a duct pointing at both back gardens.

Better positions, roughly in order:

1. On a flat roof set back from the parapet, with the discharge facing away from all habitable windows.
2. At ground level in an open rear garden, at least the manufacturer's stated clearance from any wall, with the fan discharge facing into the plot.
3. Against a rear elevation on anti-vibration mounts, with a screen that blocks the line of sight to the nearest window without choking the airflow.

Avoid mounting on a party wall or on a lightweight timber-framed wall shared with a bedroom, because structure-borne vibration transmits straight through and is far harder to fix afterwards. Most modern units have a night mode that reduces fan speed and compressor output; set it, and say in the application that it will be set.

Conservation areas and front elevations

In a conservation area, on a World Heritage Site or in a National Landscape, the permitted development limits in Part 1 tighten and any argument that plant on a side or front elevation is immaterial becomes much weaker. Councils commonly resist visible plant on a principal elevation or a roof slope facing a street, and an Article 4 direction may have removed the relevant rights altogether.

Listed buildings are stricter again. Fixing plant to a listed building, or within its curtilage in a way that affects the setting, engages listed building consent under the Planning (Listed Buildings and Conservation Areas) Act 1990, and carrying out the work without it is a criminal offence. Curtilage structures are covered in our note on permitted development rights in a listed building curtilage.

Flats and why the position differs

Part 1 of Schedule 2 applies to dwellinghouses, and a dwellinghouse does not include a building containing one or more flats or a flat within such a building. A leaseholder therefore has no Part 1 rights at all, and external plant on a flat almost always needs planning permission.

Two other consents run alongside. The lease will usually prohibit alterations to the exterior or to common parts without the freeholder's written licence, and fixing a bracket into a structural wall may need the building owner's separate agreement. Planning permission does not override either of them, and a condenser installed with permission but without a licence to alter can still be ordered off the wall.

Assessment evidence a council will expect

For anything beyond a single small unit on a detached house, provide a short acoustic note prepared by someone competent, containing:

• The manufacturer's sound power level in dB(A) at the proposed operating mode, taken from the published data sheet rather than the brochure headline figure.
• A background sound survey at the nearest noise-sensitive facade, covering the night-time period when the unit will run.
• The predicted specific sound level at that facade, with the calculation shown.
• Any acoustic feature corrections applied, and the reasoning for each.
• The resulting rating level and the comparison against background.
• Mitigation: enclosure, barrier, mounts, night setback, and the residual level after mitigation.

Where the unit serves a business use in a house, or several units are proposed, the council may ask for a full Noise Impact Assessment rather than a note.

Applying for permission with an acoustic note

A householder application needs the location plan, a block plan showing the unit and the nearest habitable windows in both directions, elevations at scale showing the unit in position, the equipment data sheet and the acoustic note. Add a short Planning Statement dealing with the visual effect on the elevation, the amenity case and the alternatives you rejected.

Offer conditions. A condition fixing the rating level at the nearest facade, requiring night mode operation and requiring the unit to be maintained gives the officer something to recommend rather than a judgement call to make. Applications that arrive with that already drafted get decided faster.

To check which supporting documents your property is likely to need before you commit to a system, enter your postcode on PlanScope.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and note that a Lawful Development Certificate is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.