Key Takeaway
UK summers are getting hotter and demand for home air conditioning has never been higher. But does fitting an external AC unit need planning permission? Here's the definitive 2025 guide — including the rule changes that came into force in May 2025.
The UK had its hottest summer on record in 2022, and temperatures have continued to climb. Demand for home air conditioning has surged across Hampshire — and the South of England broadly — as families look to keep their homes habitable through prolonged heat. But before you call an AC installer, there's an important question to answer: does your external air conditioning unit need planning permission?
The good news is that 2025 brought meaningful changes to the permitted development rules covering external AC units. The bad news is that plenty of properties in Hampshire still fall outside those rules. This guide covers everything you need to know.
Do You Need Planning Permission for an External AC Unit in the UK?
In most cases, no — provided you're in a standard residential house and the installation meets the permitted development criteria. But "most cases" is doing a lot of work in that sentence. The exceptions matter, and getting it wrong means installing a unit you may later be forced to remove.
Under the Town and Country Planning (General Permitted Development) (England) Order, external air conditioning units installed on houses can qualify as permitted development — meaning no planning application is needed. However, the unit and installation must meet a specific set of conditions.
The 2025 Rule Changes: What's New?
On 29 May 2025, the government updated the permitted development rules for air source heat pumps and external AC units under The Town and Country Planning (General Permitted Development) (England) (Amendment) Order 2025. The key changes are:
- Volume limit increased: The maximum permitted size for an outdoor compressor unit on a house rose from 0.6 cubic metres to 1.5 cubic metres. This means most modern split-system AC units now comfortably fall within permitted development.
- Boundary setback rule removed: The previous requirement for a unit to be sited at least 1 metre from the property boundary was removed for houses in England.
- Cooling-only units now covered: Previously, only units providing both heating and cooling were explicitly covered. The 2025 changes clarify that air-to-air models that provide cooling as well as heating are permitted.
- Two units on detached houses: Detached houses can now have up to two permitted development AC units. Non-detached properties (semis, terraces) are still limited to one.
The Permitted Development Criteria: Full Checklist
To qualify as permitted development in England, your external AC unit must meet all of the following:
- Size: The outdoor unit must not exceed 1.5 cubic metres (houses) or 0.6 cubic metres (flats/maisonettes).
- Roof placement: The unit cannot be placed on a pitched roof. If placed on a flat roof, it must be set back at least 1 metre from the roof edge.
- Road-facing walls: The unit cannot be installed on a wall that fronts a public road.
- Visual impact: The unit must be sited to minimise its visual impact as far as practicable.
- Noise compliance: The installation must comply with MCS 020 noise standards. A noise assessment must be completed, and the unit must not create excessive noise at neighbouring properties.
- Not a listed building: Listed Building Consent is required for any external works to a listed building — including AC units.
- Not in a conservation area (wall-facing): In conservation areas, permitted development rights are more restricted. Units on side or front elevations that are visible from the street are likely to need planning permission.
When You WILL Need Planning Permission
Even with the 2025 changes, there are several common scenarios in Hampshire where a planning application will be required:
Listed Buildings
If your home is a listed building, any external works — including an AC unit — require Listed Building Consent in addition to any planning permission. This applies to thousands of properties across Hampshire, particularly in Winchester, Romsey, Stockbridge, and the Test Valley villages.
Conservation Areas
Hampshire has a large number of conservation areas, including significant parts of Winchester city centre, the villages of Wickham, Titchfield, and Bishop's Waltham, and many New Forest settlements. In a conservation area, permitted development rights for external works are restricted. A unit on a rear elevation not visible from the street may still be fine, but anything facing or visible from a public road will almost certainly require planning permission.
Flats and Maisonettes
Flats and maisonettes do not enjoy the same permitted development freedoms as houses. The unit size limit drops to 0.6 cubic metres, and in many cases a planning application will be required regardless of size. If you live in a purpose-built flat, check with your local planning authority before proceeding.
Article 4 Directions
Some areas in Hampshire have Article 4 Directions in place, which remove permitted development rights that would otherwise apply nationally. This is common in parts of Southampton, Portsmouth, and newer residential developments. Check with your Local Planning Authority (LPA) whether an Article 4 Direction affects your property.
Restrictive Planning Conditions
Some newer residential developments — particularly those built in the last 20 years — have planning conditions attached to the original permission that restrict or remove permitted development rights. If your home is on a modern estate in Chandler's Ford, Whiteley, or similar developments, check the original planning conditions.
Noise: The Issue That Catches People Out
Even if your installation is fully within permitted development rights, councils can still take action if an AC unit creates excessive noise. Environmental health officers have powers to require you to move, upgrade, or add acoustic screening to a unit that disturbs neighbours — particularly if it is close to a neighbouring bedroom window or outdoor sitting area.
Compliance with MCS 020 is the standard, but a good installer will also carry out a site-specific noise assessment. If your property is semi-detached or terraced, or your outdoor unit will be close to a boundary or neighbouring window, pay close attention to noise specifications when choosing your unit.
How to Check Before You Install
The safest approach before commissioning an AC installation is to run through this quick checklist:
- Is your property listed? Check the Historic England listed buildings register.
- Is your property in a conservation area? Check your local council's website or the Planning Portal.
- Does an Article 4 Direction apply to your area?
- Does the unit meet the size limit (≤1.5m³ for houses)?
- Is the unit being placed on a side or rear wall — not facing a public road?
- Will your installer carry out an MCS 020 noise assessment?
If you can answer all of these satisfactorily, your installation is likely to be permitted development. If any of the first three apply, you'll need to speak to your LPA before proceeding.
Hampshire Planning Context
Hampshire has multiple planning authorities — Winchester City Council, Test Valley Borough Council, Basingstoke & Deane Borough Council, East Hampshire District Council, Fareham Borough Council, and others — and each has its own approach to conservation areas and Article 4 Directions. The general national rules apply throughout Hampshire, but the local constraints vary significantly between areas.
Winchester city centre and the cathedral close have some of the strictest conservation area controls in the county. By contrast, much of the newer housing stock in Chandler's Ford, Hedge End, and north Basingstoke sits in areas where permitted development rights are relatively unencumbered — though restrictive planning conditions on newer estates remain a common pitfall.
What About Extensions and Air Conditioning?
If you're planning a home extension alongside an AC installation, there's an important interaction to be aware of. Modern building regulations require new habitable rooms to meet thermal performance standards — and a well-designed extension, properly insulated and glazed, will be significantly easier to keep cool than an older room. Overheating risk is now a material consideration in building regulations assessments (Part O of the Building Regulations applies to new extensions in England).
This means your extension designer should be thinking about solar shielding, roof glazing positioning, and ventilation as part of the design — not just insulation. At Hampshire Build, we incorporate overheating risk analysis into all our extension designs, and we can advise on the interaction between your proposed extension and any planned or existing cooling system.
Summary: 2025 Rules at a Glance
| Property Type | Permitted Development? | Max Unit Size |
|---|---|---|
| Detached house | ✅ Yes — if all conditions met (up to 2 units) | 1.5m³ |
| Semi-detached / terrace | ✅ Yes — if all conditions met (1 unit) | 1.5m³ |
| Flat / maisonette | ⚠️ Limited — likely needs planning permission | 0.6m³ |
| Listed building | ❌ Requires Listed Building Consent | N/A |
| Conservation area | ⚠️ Likely needs planning permission | Varies |
Need Advice on Planning for Your Hampshire Home?
While Hampshire Build specialises in home extension design rather than AC installation, understanding your planning position is something we help homeowners with every day. If you're planning an extension alongside an AC installation — or if you simply want clarity on what your property can and can't do under permitted development — we're happy to discuss it during a complimentary consultation.
Our planning expertise and detailed knowledge of Hampshire's various planning authorities means we can give you a clear-eyed view of what's achievable at your property. Book a complimentary consultation or call us on 01722 548911.
