Siting an air source heat pump on a terraced house

Most air source heat pump installations on a terraced house in England are permitted development, so no planning application is needed, but the conditions attached to that right are where terraced houses run into trouble. Air source heat pump siting on a terraced house has to satisfy a noise requirement measured at your neighbour's window, fit the size limits, sit in an acceptable position on the building or in the garden, and avoid the restrictions that apply in conservation areas and to listed buildings. Work through those four points before you accept a quote.

The permitted development right and its conditions

The relevant right sits in Part 14 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, under the class dealing with air source heat pumps on domestic premises. It allows the installation of a unit on a house or within its curtilage, subject to limits and conditions.

That class has been amended more than once, including changes to the permitted volume of the outdoor unit and to the distance it must sit from a boundary, so read the current text on legislation.gov.uk rather than an article or a supplier's leaflet. The conditions that have stayed constant are the ones that matter most in a terrace.

• The installation must be used solely for heating the dwelling, and must be removed as soon as reasonably practicable when no longer needed for that purpose.
• It must be sited, so far as practicable, to minimise its effect on the external appearance of the building and on the amenity of the area.
• The unit must meet the Microgeneration Certification Scheme planning standards, or equivalent standards, which is the noise test.
• A unit on a pitched roof is not permitted, and on a flat roof it must sit back from the edge.
• Tighter limits apply on land within a conservation area or a World Heritage Site, and the right is not available within the curtilage of a listed building.

An Article 4 direction can remove the right altogether in a designated area, which is common in terraced streets within conservation areas. Check the council's Article 4 register before anything else.

The noise condition and the distance to your neighbour's window

This is the condition that decides siting on a terrace, and it is widely misunderstood as a general noise limit. It is not. The Microgeneration Certification Scheme planning standard sets out a defined calculation: the sound pressure level from the unit, assessed at a position one metre from the nearest habitable room window of a neighbouring property, must not exceed the stated limit of 42 decibels.

The calculation starts from the manufacturer's sound power level and applies corrections for distance and for the installation position, since a unit in a corner between two walls reflects more sound than one standing clear. A competent installer completes the assessment form as part of the job, and you should ask for the completed sheet rather than a verbal assurance.

In a typical Victorian terrace the back addition creates an outside corner that is the easiest place to stand a unit and often the worst place acoustically. The nearest habitable room window may be the neighbour's kitchen or ground floor bedroom, two or three metres away. Where the assessment fails, the usual fixes are a quieter unit, a different position further from the window, or a modest acoustic enclosure that does not restrict airflow.

Two practical points go beyond the planning condition. Do not fix the unit to a party wall, because vibration transmits through masonry and generates complaints that no planning rule addresses. And ask what the unit does in defrost mode at four in the morning, since the duty cycle rather than the headline figure is what neighbours hear.

Options where the rear yard is too tight

Terraced yards are small, and the heat pump is not the only thing competing for space.

Where the obvious position fails, consider mounting the unit on a frame against the rear boundary wall facing away from windows, standing it in the side return clear of the corner, using a smaller unit with a buffer vessel to manage cycling, or using two smaller units if the right allows and the acoustic case works. Where nothing in the rear will work, the realistic alternatives are a different technology or an application.

A ground loop is rarely practical on a terraced plot, since the yard is too small for either trenches or a borehole rig, though in some mid-terrace plots with long gardens it is worth testing. The borehole route brings its own permitting questions, set out in our note on ground source heat pumps and borehole permitting.

Front elevation siting and designated areas

Putting the unit in the front garden solves the acoustic problem and creates a different one. On land within a conservation area, the permitted development right does not extend to a unit on a wall or roof fronting a highway, and it does not cover a unit sited on land in front of the house where that land fronts a highway. In a street of listed terraces, the right is unavailable and listed building consent will be needed for the fixings and any penetrations through historic fabric.

That does not make front siting impossible. It makes it an application, and a planning application for a front-garden heat pump in a conservation area succeeds or fails on the visual case: position behind a wall or hedge, a muted finish, pipework run internally rather than across the facade, and an honest set of photographs of the existing frontage.

Where the street has a uniform rhythm of bay windows and front walls, officers resist anything that breaks it. That is a design judgement, not an objection to the technology.

Pipework, cylinder space and the internal work

The planning question is the small part. Inside, a terraced house needs room for a hot water cylinder, which many gas-heated terraces no longer have, and that usually means losing part of an airing cupboard, a bedroom corner or loft space with the attendant pipe runs.

Flow temperatures are lower than a gas boiler, so radiator sizes often increase and pipe runs may need upsizing. Microbore pipework is the common constraint in houses rebuilt in the 1980s. Condensate from the outdoor unit needs a route to a drain, and in freezing weather it needs to not discharge across a shared path.

Building Regulations apply to the work regardless of the planning position, including Part L for the heating system and Part G for an unvented cylinder. If you intend to claim grant funding under the Boiler Upgrade Scheme, the installation has to be carried out by a certified installer, so confirm the installer's certification before signing.

When an application is the better route

Sometimes the application is simply easier than forcing the permitted development case. Consider applying where the noise assessment fails at the only workable position, where the house is listed or in a tightly controlled conservation area, where you want the unit on the front and the street is sensitive, or where the neighbour relationship means you would rather have a formal decision than an argument about whether the right applied.

An application also produces a decision notice, which is a useful document on a future sale. A buyer's solicitor cannot easily verify a permitted development claim from photographs.

Noise is the issue most likely to attract an objection, and on a constrained site it is worth commissioning a proper acoustic assessment rather than relying on the standard form, which is the territory of a Noise Impact Assessment.

Keep the evidence, whichever route you take

Build a small file and keep it with the house papers: the manufacturer's data sheet showing sound power level, the completed noise assessment sheet, dated photographs of the installed position with a tape measure in shot, the installer's certification, the Building Regulations compliance certificate, and the Article 4 and conservation area check you carried out at the start.

A Lawful Development Certificate is the only formal way to confirm that works are lawful, and for a borderline case, particularly in a conservation area, it is worth the fee. The same logic applies to other permitted development classes with numerical limits, as in our note on farm wind turbines and permitted development limits.

Enter your postcode on PlanScope to check whether your street carries designations that affect the installation, and which documents an application would need.

This is general guidance, not legal advice. Permitted development rights depend on the precise facts of your property and have been amended several times, so check the current text of the Order and the position with your local planning authority or a qualified professional before relying on it.