Why sound decides so much here
The 2021 census found 37.9% of Liverpool households in terraced houses and 30.7% in semi-detached houses: 68.5% in all with a neighbour on the other side of a wall. Only 8.2% lived in a detached house. A further 23.2% lived in flats. In streets like these, the nearest neighbouring window can be just across a narrow yard, and the distance to it is one of the biggest numbers in the sound calculation.
How the calculation works
Paragraph G.1 of Class G says development is not permitted unless the heat pump complies with the MCS Planning Standards. Since the 2025 amendments, the "or equivalent standards" wording has gone, and since 29 May 2026 MCS 020 a) is the only version that counts. The installer works it out for the exact position proposed:
- Take the unit's sound power level from the manufacturer's data sheet, at the setting the design will use.
- Identify the assessment position: the nearest door or window of a habitable room in a neighbouring home.
- Allow for the surfaces around the unit. A unit against one wall reflects more sound than one standing clear of walls; a corner reflects more again.
- Allow for the distance and for any solid barrier between the unit and the assessment position.
- Compare the result with the standard's limit. The spot passes or fails.
A fail does not end the project. It ends that position with that unit. The usual fixes are a different spot further from the neighbour's window, a quieter model, a solid barrier or an acoustic enclosure, as long as the enclosure keeps the total within the 1.5 cubic metres size limit.
Positions that tend to help
- Away from corners and recesses, where reflections add up.
- On the ground rather than high on a wall, where sound travels further over fences.
- With a solid garden wall or the house itself between the unit and the neighbour's windows.
- On anti-vibration mounts, so that the unit does not pass vibration into a shared wall.
Not every tip fits every yard, and none of them replaces the calculation. Ask the installer to show the assessment position and the result on the design, and to explain what changes if the unit moves.
The appearance and amenity conditions
Class G also sets conditions that apply whatever the calculation says. Paragraph G.3 requires the unit to be sited, so far as practicable, to minimise its effect on the external appearance of the building and on the amenity of the area, and to be removed as soon as reasonably practicable when no longer needed. Amenity includes the neighbours' enjoyment of their homes, so a spot that passes the calculation but sits right under a bedroom window is worth a second thought.
Talking to the neighbours
A short conversation before the installation saves most problems. Tell the neighbour where the unit will go and when the work is planned, and offer to show the sound figure from the design. If you live in one of the city's 36 conservation areas, the unit will be at the back or side anyway, close to the neighbours' rear windows; the calculation covers that, and explaining it helps.
If a problem comes up after installation, check the basics with the installer first: the unit is level, the mounts are sound, nothing is loose, and the settings match the design. Councils investigate noise complaints under the statutory nuisance rules, which sit apart from planning, so a sensible installation and a good relationship with the neighbour matter even when Class G is met.