The guides disagree. Here's what the law actually says.
Published 17 Jul 2026 · every claim cited to legislation.gov.uk · corrections welcome and will be published
Search "can I install air conditioning in my flat" and the top results flatly contradict each other. One London installer's 2026 guide says installation in flats "usually" needs no planning permission because it falls under . Another guide says flats "do not enjoy the same Permitted Development freedoms" and you'll "almost always" need a formal application. A third says most domestic AC needs nothing as long as the box is small and discreet.
They can't all be right, and mostly they aren't — because the law changed on 29 May 2025 and much of the internet hasn't caught up. Here is what the legislation actually says, with the receipts.
The five rules that decide your case
1 · A cooling-only unit is not permitted development. A heat-and-cool one can be. The permitted development right (GPDO Schedule 2, Part 14, Class G) is for air source heat pumps. The May 2025 amendment deleted the old condition that the unit be used "solely for heating" and replaced it with a new one: the unit must not be "solely used for the purpose of cooling." Translation: a unit that heats and cools — which describes most modern air-to-air systems — can qualify. A cooling-only condenser cannot, and needs a planning application.
SI 2025/560, art.5(4): omits condition G.3(a), inserts G.3(aa) "the air source heat pump is not solely used for the purpose of cooling" · in force 29 May 2025
2 · Flats are IN — which half the guides deny. But one unit per block. Class G has covered blocks of flats for years, and since May 2025 that includes units that heat and cool. The catch nobody mentions: permitted development is not allowed if it would result in more than one heat pump on, or within the curtilage of, a block of flats. If a neighbour anywhere in your building already has one, the block's slot is taken and your route is a full planning application. (Houses: one unit if semi-detached or terraced; two if detached.)
GPDO Class G para G.2(a), as substituted by SI 2025/560 art.5(3)(a)
3 · The size limit is stricter for flats than houses — most guides quote the wrong number. You'll see "0.6 cubic metres" everywhere, quoted from the pre-2025 rules. Since May 2025 the outdoor unit limit is 1.5m³ for a house — but it stays at 0.6m³ for a block of flats. Quoting one number for both is wrong in both directions.
GPDO Class G para G.2(d), as substituted by SI 2025/560 art.5(3)(b)
4 · The old 1-metre boundary rule is gone; the noise calculation is now mandatory, one scheme only. The requirement to keep the unit 1m from the property boundary was deleted. In its place, the sound calculation does the work of protecting neighbours — and since 28 May 2026, after the transitional year ended, "equivalent standards" are no longer accepted: it's MCS 020(a) or a planning application.
SI 2025/560 art.5(3)(c) omits G.2(e); art.5(2) omits "or equivalent standards"; art.6 substitutes MCS 020(a); art.7 transitional period ended 28 May 2026
5 · Planning permission and your lease are different locks on the same door. Even a textbook Class G installation can be blocked by your freeholder, because permitted development is planning law and your lease is private law. The reverse is also true: freeholder consent doesn't grant planning permission. Flat owners need both keys — which is why our checker treats the freeholder conversation as step one, and generates the letter for it.
Quick reference
| Situation | Position since 29 May 2025 |
|---|---|
| House, heat-and-cool unit | Normally permitted development: ≤1.5m³, one unit (two if detached), not on a pitched roof, ≥1m from flat-roof edge, MCS 020(a) calc, siting rules near highways |
| Flat, heat-and-cool unit, wholly residential block, no existing unit | Can be permitted development: ≤0.6m³ + all conditions above · freeholder consent separately required |
| Flat, building already has one unit | PD not available — full planning application |
| Cooling-only condenser, anywhere | Not PD — planning application |
| Listed building / scheduled monument | Class G doesn't apply; listed building consent likely needed |
| Conservation area | PD possible but no unit on a wall/roof fronting a highway; s can remove rights — check the borough |
Why so many guides get it wrong
Three honest reasons. The rules changed mid-2025, and pages written before that are confidently out of date. The change is subtle — it lives in amendment language ("omit sub-paragraph (e)…") that you only get right by reading the statutory instrument itself. And most guides are written by installers, whose job is fitting units, not construing the GPDO. We've done the construing, we cite every line, and if we've got something wrong we'll correct it in public — tell us.