Measuring Quiet: Air Source Heat Pumps, Permitted Development and the 42 dB Boundary in 2026

Air source heat pumps have moved from the margins of retrofit to the centre of domestic heating policy. Grants, rising gas prices and the gradual replacement of fossil-fuel boilers have made the technology familiar to homeowners, installers and planning authorities alike. Yet familiarity has not made the planning position simple. For many houses in England, an air source heat pump can be installed under permitted development. For others, the same unit in a slightly different position will require a full application. The difference is often measured in decibels, metres and the precise location of a neighbour's window.

The permitted development route, and where it stops

The relevant provision in England is Class G of Part 14 of the General Permitted Development Order. It allows the installation, alteration or replacement of an air source heat pump on domestic premises, subject to conditions. The most important condition is that the equipment must comply with the current MCS 020 planning standard, or an equivalent methodology accepted by the local planning authority. The right is also unavailable in certain locations, most notably listed buildings and their curtilage, and it can be withdrawn by an Article 4 direction.

That last point matters because permitted development is not a universal freedom. It is a narrow permission granted by national legislation, and local circumstances can remove it. A homeowner who assumes that any heat pump on any house is automatically permitted is taking an unnecessary risk. A short check of the local planning authority's Article 4 directions, conservation area designation and listed building records is the sensible first step.

What the 42-decibel figure actually measures

The MCS 020 standard is the practical gatekeeper for most domestic installations. It requires the installer to calculate the sound pressure level produced by the heat pump at a defined receptor: normally one metre from the facade of the nearest noise-sensitive room in a neighbouring property. The calculated level must not exceed 42 dB(A) as an equivalent continuous sound level. That is the figure behind the familiar phrase about the quiet neighbour.

The number is not a measurement of how loud the pump sounds in the owner's garden. It is a planning proxy for the risk of disturbance at the place where it matters most: the neighbour's window or wall. The calculation takes the manufacturer's declared sound power level, applies distance and directivity corrections, and accounts for the position of the unit relative to the receptor. In practice, installers use a standard calculator and the manufacturer's datasheet. A marginal result should prompt a redesign, not a confident shrug.

Siting is not a cosmetic question

Distance does much of the work. Sound pressure falls with distance, and a unit moved a few metres further from a neighbouring window can transform a failing calculation into a compliant one. Corners, narrow side returns, rear extensions and boundary walls all change how sound travels. A heat pump tucked into a recess may be visually discreet but acoustically poor, particularly where hard surfaces reflect sound towards a neighbour.

Homeowners should also think about servicing access, defrost drainage and the visual relationship with the house. An air source heat pump is a permanent addition to the elevation, not a temporary appliance. In a terraced house, the nearest neighbour may be only a few metres away, and the calculation must use the closest relevant window rather than a convenient average. In a semi-detached property, the party wall and the position of the neighbour's kitchen or bedroom window will often determine the outcome.

Anti-vibration mounts, acoustic enclosures and quieter models can all help, but they should not be treated as a substitute for a proper siting assessment. Some enclosures reduce airflow and invalidate manufacturer performance data. The better approach is to design the installation around the noise limit from the outset.

Conservation areas, listed buildings and flats

Conservation area designation does not automatically mean that a heat pump needs planning permission, but it does change the context in which the installation is assessed. Local planning authorities may have Article 4 directions removing permitted development rights, and visual impact carries more weight where the unit would be visible from the street or within a sensitive townscape. Early pre-application advice is usually worthwhile.

Listed buildings are a clearer case. Permitted development is not available for installations on a listed building or within its curtilage, and listed building consent will often be required in addition to planning permission. The test is not only whether the unit is visible but whether the works affect the special architectural or historic interest of the building.

Flats present a narrower route still. Class G is framed around domestic premises, but the practical position for flats is more restrictive than for houses. A flat may lack the same permitted development rights, particularly where the unit would be fixed to a shared wall, roof or communal area. Leaseholder and freeholder consent is a separate legal matter, and it should be resolved before any planning question is answered.

When a full application is the better route

If the installation cannot meet the permitted development conditions, a full planning application is usually the answer. The submission should include a site plan showing the unit, the boundaries and the nearest neighbouring windows; elevation drawings; the manufacturer's noise data; and the MCS 020 calculation or an equivalent acoustic assessment. A covering statement explaining why the chosen position is the least sensitive available option is helpful, particularly in a conservation area or a dense terrace.

Local planning authorities may grant permission subject to conditions controlling hours of operation, acoustic performance or screening. In some cases, an application is preferable to relying on a marginal permitted development position, because it provides certainty and a decision that neighbours can see and comment on.

The evidence pack to assemble first

Before installation, homeowners should gather a small but decisive set of documents. These include the MCS 020 calculation, the manufacturer's sound power data, a scaled site plan with boundary distances, photographs of the proposed position, confirmation of the installer's certification, and any pre-application advice from the local authority. Where permitted development is being relied upon, a lawful development certificate application can provide a useful record, particularly if the property is later sold.

The planning question is ultimately about evidence, not assertion. A heat pump that meets the 42 dB(A) test at the right receptor, sits within a lawful permitted development route and is documented properly is unlikely to trouble a buyer's solicitor or a planning enforcement officer. One that is installed first and justified afterwards is a different proposition.

Thornbury Plan's planning applications coverage treats these questions as part of the wider business of reading a site before committing to it. With heat pumps, the quietest installation is usually the one that was measured before it was ordered.