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. The two things that most often change that answer are a cooling-only system, and a listed building.
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. Four things moved:
There was a transitional period for installations permitted under the old rules, and it ended on 28 May 2026. Everything below is the position as it now stands.
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. A cooling-only unit is not, and needs a planning application.
In practice this rarely bites, because reversible units dominate the market and cost roughly the same. But it is a good reason to specify heating and cooling even if you only ever intend to cool, and a good question to ask any installer who quotes you a cooling-only system.
| Condition | What it means for your house |
|---|---|
| Size | Outdoor unit no more than 1.5 cubic metres. Domestic units are far below this. |
| Number | Up to two on a detached house. Fewer on other property types. |
| 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 | Must meet the noise condition, calculated using the MCS 020 methodology and assessed at your neighbour's nearest habitable window. |
| Removal | The unit must be taken down when it is no longer needed. |
The noise limit tightened in May 2026, and from 28 May 2026 the MCS 020 methodology became the only accepted route — the previous allowance for equivalent standards was removed. In practice this is the installer's job, not yours, but it is worth confirming that whoever quotes you works to it.
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 face a highway. Outside a conservation area, the limit is on installing above ground floor level on a highway-facing wall. Inside one, the restriction is stricter, and siting on the front of the building is the thing most likely to lose you permitted development.
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. Permitted development is not the relevant question for a listed building — listed building consent is required for works affecting the character of the building, and that applies to the outdoor unit, the pipework penetration through the wall, and often the internal routing as well.
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 outdoor units does not extend to flats and maisonettes in the way it does to houses. 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. 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 July 2026, under Class G of Part 14 of Schedule 2 to the General Permitted Development Order 2015, as amended by SI 2025/560. 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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