Siting an air source heat pump on a small plot

An air source heat pump on a house is usually permitted development under Class G of Part 14 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, and the condition that decides most small plots is noise rather than size. The unit has to be installed in accordance with the Microgeneration Certification Scheme planning standard, which sets a limit of 42 dB(A) measured at the assessment position, normally the nearest habitable room window of a neighbouring property. On a terrace with a 4m back yard, that single number governs where the pump can go, and it is why heat pump planning permission questions almost always turn into acoustics questions.

The dimensional conditions in Class G have been amended more than once, most recently to make installation easier on constrained sites. Read the current consolidated text of Part 14 rather than an older summary, because the volume limit and the boundary conditions have moved.

What Class G covers and what it does not

The right applies to the installation, alteration or replacement of an air source heat pump on a dwellinghouse or block of flats, or within its curtilage. The pump must be used solely for heating purposes, and it must be removed as soon as reasonably practicable when it is no longer needed.

There are absolute exclusions. Class G does not apply on a listed building or within its curtilage, and it does not apply where the site is a scheduled monument. On article 2(3) land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites, the right is restricted, and the restriction generally bites on walls and roof slopes that front a highway and on siting within the curtilage. A pump on the rear elevation of a conservation area house is a different proposition from one on the front.

Class G also carries a design obligation that people skip over. The installer must, so far as practicable, minimise the effect of the development on the external appearance of the building and on the amenity of the area. That is not decorative language. It is the condition an enforcement officer quotes when a unit has been bolted to a front elevation at eye level.

The noise assessment under MCS 020

The planning standard works as a calculation, not a measurement. You take the sound power level of the unit from the manufacturer's data, apply corrections for distance to the assessment position, for the direction the unit faces, and for reflecting surfaces near it, and the result has to come out at or below 42 dB(A).

The corrections are where small plots are won and lost:

• distance from the unit to the neighbour's nearest habitable room window, which is the largest single factor
• whether the unit sits in a corner, against one wall, or in open air, because each reflecting surface adds to the level
• whether there is a barrier such as a solid boundary wall in the direct path
• the manufacturer's declared sound power, which varies between units of the same output by a useful margin

On a narrow plot the practical moves are to choose a quieter unit, to move the pump along the wall away from the boundary, to avoid tucking it into a corner formed by two walls, and to use an existing solid fence or wall as a barrier. Two of those cost nothing.

Where the calculation will not come out under the limit, you are into a planning application, and the council's environmental health officer will want a proper Noise Impact Assessment using the relevant British Standard for rating industrial and commercial sound, assessed against the existing background level rather than against a fixed figure. That is a different and more forgiving test on a noisy road, and a more demanding one on a quiet cul-de-sac.

Distance to a neighbouring window

The assessment position is the neighbour's window, not your boundary. On a mid terrace with back additions this often means the relevant window is diagonally across the gardens rather than directly opposite, and measuring the true line rather than the shortest boundary distance sometimes gains you a couple of metres.

Where two houses both install pumps, each calculation is done on its own, and the combined effect is not assessed under the standard. That is a real gap, and it is the sort of thing a neighbour raises at the objection stage on a full application even though it has no place in the permitted development test.

Record the calculation. Keep the manufacturer's data sheet, the plan showing the measured distance, and the completed assessment form. If a complaint is made two years later, that folder is the difference between a quick explanation and an enforcement investigation.

Listed buildings and conservation areas

On a listed building, the pump needs listed building consent under the Planning (Listed Buildings and Conservation Areas) Act 1990 as well as planning permission, and the case is made on significance. Siting on a secondary elevation, using a low level position behind existing planting, running pipework on the inside rather than chasing it into historic fabric, and choosing a fixing method that can be reversed are the arguments that work. A Heritage Statement that describes the fabric and the setting before it describes the pump is the document that carries it.

In a conservation area without listing, a rear installation is often still permitted development, and the front is not. Officers vary on whether a unit visible through a gate counts as fronting a highway, so ask.

Screening without choking the airflow

Every heat pump needs air, and the volumes are large. Enclosing a unit in a decorative box with a solid back and slatted sides reduces its output, raises its running cost and can cause it to ice up in winter. Manufacturers publish minimum clearances at the inlet, the outlet and above, and those clearances are not advisory.

If screening is needed for visual reasons, use an open slatted screen set at the manufacturer's clearance distance, leave the discharge face clear, and keep the top open. A screen that touches the unit also transmits vibration, which is the commonest cause of a complaint after installation.

Condensate, defrost and the winter problem

An air source heat pump produces condensate whenever it defrosts, and in cold weather that is several times a day. On a small plot the discharge has to go somewhere that does not freeze into a sheet of ice across a shared path or a neighbour's doorstep.

A soakaway with a gravel bed under the unit is the usual answer where ground conditions allow. A connection to a rainwater drain is acceptable in many areas, subject to the water undertaker's position on surface water connections, but it needs a trap and a frost resistant run. Discharging onto a paved area next to a boundary is how neighbour disputes start.

Where a plot is tight enough that the pump has to sit on a shared access or where a rear extension is planned at the same time, resolve the pump position with the rest of the layout, in the same way a side return infill forces the drainage and the bin store to be designed together rather than in sequence.

When an application is the better route

Apply rather than rely on the right where the noise calculation is marginal, where the only workable position is on a front or side elevation on article 2(3) land, where the building is listed, or where the neighbour relationship is already difficult. A planning permission that shows the agreed position is worth more than an argument about whether a condition was met.

Where you are relying on Class G, a Lawful Development Certificate for proposed works under section 192 of the Town and Country Planning Act 1990 records the position formally, and it is the only way to confirm that works are lawful. It is worth having on a plot where the calculation was close.

This is general guidance, not legal advice. The conditions in Class G have been amended repeatedly, so check the current Order and confirm the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which documents your installation is likely to need. Reports should be reviewed before submission, and we do not guarantee planning approval.