The rules changed on 29 May 2025. Most of what you will find online — including on installers' websites — still describes the old position, and it is materially different.
The short version. Most air conditioning installed on a Chilterns house today is permitted development and needs no planning application. What most often changes that answer: a listed building, a position that fails the noise test, a front wall in a conservation area, one unit too many, or a system used only to cool.
Air source heat pumps have had permitted development rights for years, under Class G of Part 14 of the General Permitted Development Order. The problem was that those rights were written for heating. Air conditioning sat outside them, which meant a great many domestic installations technically needed planning permission that nobody applied for.
The Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2025 changed that with effect from 29 May 2025. Five things moved:
Everything below is the position as it now stands. A transitional rule that ended on 28 May 2026 covered only heat pumps that met the old rules, never air conditioning.
Permitted development does not apply where the unit is used solely for cooling.
This is the one worth understanding properly, because it is the difference between an application and no application. A reversible unit — one that heats as well as cools — is inside the rules, as long as it is used for heating too. A unit used only to cool is not, and needs a planning application.
In practice this rarely bites, because reversible units dominate the market and cost roughly the same. But the condition is about how the system is used, not what it can do: on the plain wording, a reversible unit only ever used to cool is outside the rules. Ask any installer who quotes you a cooling-only system why.
| Condition | What it means for your house |
|---|---|
| Size | Outdoor unit no more than 1.5 cubic metres, including any box or housing around it; 0.6 on a block of flats. Domestic units are far below this. |
| Number | Up to two on a detached house, meaning one that shares no wall; one on a semi-detached or terraced house, and one for a whole block of flats. A heat pump already there counts. |
| Roofs | Not on a pitched roof at all. On a flat roof, every part must be at least a metre from the edge. |
| Siting | Positioned, so far as practicable, to minimise the effect on the building's appearance and on the amenity of the area. |
| Noise | No more than 37 dB(A) from the unit, calculated with MCS 020 a) at every neighbouring habitable-room window and door: 1 m out, on every side and every floor. |
| Removal | The unit must be taken down when it is no longer needed. |
The noise test changed on 29 May 2025, and MCS 020 a) is now the only accepted route. The calculation is the installer's job, but the rules attach to the installation, which is yours: ask for a copy, as the standard says you should get, and keep it with the house's papers.
Beaconsfield old town, parts of Amersham, Marlow, Chesham Bois and a great many of the surrounding villages sit in conservation areas. This does not remove permitted development for air conditioning, which is the assumption most people make. What it does is tighten where the outdoor unit can go.
The restrictions bite on walls and roofs that front a highway, and a highway includes public footpaths, bridleways and private roads, not just the road outside. Outside a conservation area, the limit is on installing above ground floor level on a wall fronting one. Inside one, the unit can’t go on a wall or roof fronting a highway at all, or anywhere nearer a highway bounding the garden than the house itself.
The practical answer is almost always the same: put the unit at the side or the rear. On most Chilterns houses that is where you would want it anyway, for noise and for looks.
Different rules, and a firmer answer. A listed house has no permitted development for an outdoor unit, on the building or anywhere in its grounds, so the unit needs planning permission. The works need listed building consent where they affect the building's character, which normally covers the outdoor unit on a wall, the hole through it, the indoor units and often the internal routing. The planning fee is £548; consent is free. Works that need consent are a criminal offence without it, so nothing is fitted before both are granted.
This is not a formality and it is not something to discover after the units are ordered. If your house is listed, that conversation happens before anything else does. It is entirely possible to air condition a listed building sympathetically, but it is a different project with a different timeline.
Permitted development for flats is narrower: one outdoor unit for the whole block, no bigger than 0.6 cubic metres, and none at all for a flat in a building that also has shops or offices. Separately — and this catches people regardless of planning — you will almost certainly need the freeholder's or management company's consent under the terms of your lease. That is a private matter rather than a planning one, and it is worth checking early because it can take longer than the council.
Much of our area falls within the Chilterns National Landscape, formerly the Chilterns AONB. This designation raises the bar on visual impact and gives the planning authority more to weigh if an application is needed, but it does not by itself remove permitted development for a domestic air conditioning unit.
A householder application to Buckinghamshire Council is the usual route. The council's fee is £548, and pre-application advice, if you want it first, is £579.40. Budget around eight weeks for a decision, and consider whether the timing works with the rest of your project. This is one of the reasons we ask about planning status at the survey rather than on the day the units arrive.
Where there is genuine doubt, a Certificate of Lawfulness is worth considering. It is an application to confirm that what you are doing is permitted development, and it gives you a formal document rather than an opinion. If you are likely to sell the house within a few years, a buyer's solicitor may well ask.
What we do about it. We check planning status at every survey, before quoting — conservation area, listed status, and where the outdoor unit can sensibly go. It is quicker to establish at the start than to unpick later, and it is the sort of thing that should never be a surprise on installation day.
This guide is general information rather than formal planning advice, and the rules do change. The position described is current as at October 2026, under Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015, as amended by SI 2025/560 (the 2026 amending orders don't change Class G), and MCS 020 a) Issue 1.1. For anything specific to your house, confirm with Buckinghamshire Council or ask us at survey.
Conservation area, listed status, and where the outdoor unit can sensibly go — established before we quote, not on installation day.
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