How permitted development works
Some building work in England is granted permission in advance by a national order, so long as it stays within the order's limits. Heat pumps are covered in Part 14 of that order: Class G for air source heat pumps and Class C for ground source. Within the limits, nothing needs to be sent to Wirral, Sefton, St Helens, Knowsley or Halton councils. Outside them, the normal planning application applies. The rules are the same in all five boroughs because they are national.
Class G, condition by condition
- Number. One air source unit on a semi-detached or terraced house, or on a block of flats; up to two on a detached house.
- Size. No more than 1.5 cubic metres including any housing on a house, and 0.6 cubic metres on a block of flats.
- Roofs. Never on a pitched roof. On a flat roof, at least a metre in from the edge.
- Walls facing a road. Never above the ground floor on a wall that faces a road. In a conservation area, not on any wall or roof that fronts a road, and not closer to the road than the house itself.
- Sound. The installation meets the MCS 020 a) standard.
- Protected sites. Not on a listed building or within its curtilage, and not on a scheduled monument.
- Siting and removal. Placed so far as practicable to limit its effect on the building's look and on neighbours, and taken away when no longer needed.
Changes in May 2025 loosened the rule: the requirement to keep a unit a metre from the property boundary was dropped, the size limit on houses went up to 1.5 cubic metres, and detached houses may now have two units.
What it means for the boroughs' homes
With 43.1% of households in semi-detached houses and 25.3% in terraces, most homes in the five boroughs can have one unit. The side passage of a semi, or the rear of a terrace, is usually where it goes, since a road-facing upper wall is ruled out. 16.9% of households live in detached houses, highest in Halton at 21.0%, where two units are allowed. 50,855 households live in purpose-built flats, where the smaller size limit applies and the building's owner must agree.
The sound calculation in practice
MCS 020 a) is a calculation, not a measurement on site. The installer starts from the unit's published sound power, then accounts for the distance to the nearest habitable-room window of a neighbouring home, how many hard surfaces reflect the sound back, and whether a solid barrier blocks the line between the two. The result must fall within the standard's limit. On a semi, the attached neighbour's rear window is often the one that counts. If the first position fails, a different model, a different spot or a barrier may pass; if nothing does, the unit needs planning permission.
Ground source: Class C
A ground source heat pump within the grounds of a house or block of flats is permitted development under Class C, which carries none of Class G's size, number or position limits because the collector is buried. Horizontal trenches need a large garden, which most semis and terraces do not have; a borehole needs less ground but costs more. Check with the council if the house is listed or the land holds anything protected.
Conservation areas
Councils designate their own conservation areas, and Class G still applies within them, with the stricter rule that nothing may face a road. A council can also withdraw permitted development rights in a particular area with an Article 4 direction. If your home is in a conservation area, look at the council's map and any directions before settling on a position.
Listed buildings
Class G never applies to a listed building or anything within its curtilage, so a heat pump there always needs a planning application, and listed building consent where the work affects the building's special interest. The five boroughs hold 1,689 listed-building records between them, 736 in Wirral and 568 in Sefton. See listed buildings.
When an application is the right route
- The building is listed, or you cannot rule it out.
- The only practical position faces a road in a conservation area, or sits on a pitched roof.
- The sound calculation will not pass at any workable position.
- You need a second unit on a semi or terrace, or a larger unit than Class G allows.
- You want written proof that the plan is permitted development: apply for a lawful development certificate.