Heat Pump Noise and Planning: MCS Permitted Development Rules, Neighbour Distances and Noise Measurement

Summary

Planning and noise are among the most common concerns raised by customers and neighbours when an ASHP is installed. The MCS planning rules provide a clear permitted development (PD) framework for most domestic installations, but the conditions must be understood and met — particularly the noise limit of 42dB(A) from the nearest habitable room window of any neighbouring property.

For heating engineers, understanding how to calculate estimated noise levels from manufacturer sound power data, how to site the unit to comply, and when planning permission is required (rather than PD) is part of the professional installation process.

Key Facts

• Permitted Development (PD) — in England, ASHP outdoor units are Class G PD under the General Permitted Development Order 2015 (GPDO); no planning application required if PD conditions are met
• MCS noise condition — the ASHP must not result in a noise level of more than 42 dB(A) at 1 metre from any window or door of a habitable room in any other building (typically a neighbour's property)
• Boundary condition — the unit must be at least 1 metre from the property boundary (includes the boundary with a neighbour)
• One unit limit — only one ASHP unit per property is permitted under PD; a second unit requires a planning application
• Not on principal elevation — PD does not permit the unit on the principal elevation (the main front wall facing the highway) of the property
• Listed buildings — listed buildings require Listed Building Consent regardless of noise or size; PD rights are removed
• Conservation areas and Article 4 directions — PD rights may be removed in designated areas; always check with the Local Planning Authority (LPA) before installation
• Flats — PD Class G specifically applies to dwellinghouses; flats require full planning permission for ASHP external units in most cases
• Sound power level (LWA) — the total acoustic power radiated by the ASHP unit in all directions; stated in dB(A); used in manufacturer specifications; the starting point for noise calculations
• Sound pressure level (LpA) — the noise level experienced at a specific distance and direction; what an observer or neighbour hears; calculated from sound power level, distance, and propagation model
• Point source inverse square law — sound pressure level drops by approximately 6 dB for every doubling of distance; used to estimate LpA from LWA
• ASHP noise range — typical ASHP outdoor units operate at 40–60 dB(A) sound power level; smaller/quieter units can achieve LWA 55–58 dB; noisier/larger units may reach LWA 65–68 dB; always check the specific model

Quick Reference Table: Approximate Sound Pressure Level at Distance (from LWA 58 dB(A) unit)

These are approximate; actual levels depend on unit directivity, ground reflection, and barriers. Always use manufacturer data and the MCS noise calculation method.

Detailed Guidance

The MCS Noise Calculation Method

MCS provides a noise calculation methodology (in the MCS 020 noise standard or within MCS 007 design guidance) to verify compliance with the 42 dB(A) limit.

Step 1: Obtain the unit's sound power level (LWA) The manufacturer's technical data sheet states LWA in dB(A) at specific operating conditions (typically at full load, A7/W35 or A7/W45). Note that LWA varies with operating conditions — at full load on a cold day, LWA will be higher than at part load on a mild day.

Step 2: Calculate the sound pressure level at the neighbour's window Using the point source approximation: LpA = LWA − 11 − 20 × log10(d) Where d = distance in metres from the unit to the receiver (neighbour's window).

Example: LWA 60 dB(A), distance 5m: LpA = 60 − 11 − 20 × log10(5) = 60 − 11 − 14 = 35 dB(A) — compliant

Step 3: Apply corrections

• Add 3 dB for ground reflection where the unit is close to a hard surface (paving)
• Add corrections for nearby walls/barriers that direct sound
• The MCS noise calculator or a full ISO 9613 assessment can be used for complex cases

Step 4: Compare to 42 dB(A) limit If the calculated LpA at any neighbour's habitable room window exceeds 42 dB(A), the siting must be reconsidered or a quieter unit specified.

Siting to Minimise Noise Impact

Direction of air discharge: ASHP units discharge air from the front (axial fan). Sound levels are highest in the direction of air discharge. Orient the unit so the discharge faces away from the neighbour's habitable rooms — towards the garden, a side boundary, or the rear of the property.

Distance: Additional distance between the unit and the neighbour's window is the simplest noise mitigation. Where a property has a small garden and the neighbour's house is close, a larger separation may not be achievable — check compliance carefully.

Barriers: Acoustic barriers (walls, fences, dense vegetation) can attenuate sound transmission to neighbours. A masonry or concrete barrier of height equal to the unit height, positioned between the unit and the neighbour, can provide 5–10 dB(A) attenuation. Barriers must not restrict airflow to the ASHP evaporator — do not enclose the unit on three or four sides.

Unit selection: Where siting constraints are tight, select a quieter unit. The market now includes "whisper quiet" models specifically marketed for noise-sensitive locations (urban gardens, semi-detached properties with close neighbours). LWA 55–57 dB(A) units are available from several manufacturers.

Anti-vibration mounting: Vibration from the unit transmitted through the concrete slab to the building structure can create low-frequency noise inside both the subject property and the neighbour's. Always mount on anti-vibration pads or a rubber composite mat; fit anti-vibration brackets where a wall-mounted installation is used.

When Planning Permission Is Required

Situations requiring full planning permission:

• Second ASHP unit on the same property
• Unit on the principal (front) elevation
• Listed building (also requires Listed Building Consent)
• Conservation area or Article 4 direction area (check with LPA first)
• Flat or apartment (PD Class G applies only to dwellinghouses)
• Unit that does not comply with the 42 dB(A) noise condition
• Scotland, Wales, Northern Ireland — different PD rules apply; consult the relevant national planning guidance

Planning application process: For a domestic ASHP that requires a planning application (rather than PD), the application is typically a Householder Planning Application. The noise assessment is a required supporting document. Cost: typically £200–£250 application fee plus noise assessment preparation time. Decision time: 8 weeks.

Noise in Scotland, Wales and Northern Ireland

• Scotland: ASHP PD rules are similar to England but subject to Scottish PD Order; check Historic Environment Scotland requirements for listed buildings and conservation areas
• Wales: Technical Advice Note 8 (Renewable Energy) and the Welsh PD Order apply; different boundary distance rules apply in some cases
• Northern Ireland: Planning Policy Statement 18 (Renewable Energy) and the Planning (General Development) Order (Northern Ireland) 2015

Always verify the applicable rules for the specific jurisdiction before installation.

Frequently Asked Questions

The customer's neighbour has complained about the heat pump noise. What should I do?

First, measure or estimate the noise level at the neighbour's window using the MCS methodology. If it is below 42 dB(A), the unit is compliant with PD conditions and the customer has a planning argument in their favour. If it is above 42 dB(A), the installation may not have complied with PD conditions — the customer may be at risk of an enforcement notice. Practical solutions: relocate the unit if possible; install acoustic barriers; replace with a quieter unit; operate the unit only during reasonable hours (7am–10pm) as an interim measure. Engage with the neighbour directly rather than through the council if possible.

Can I install an ASHP on the flat roof of a rear extension?

Possibly — but check that this counts as being on the roof of the principal building. ASHP units on any flat roof that forms part of the principal elevation may lose PD rights. Units on a rear or side flat roof are typically acceptable under PD. The noise calculation must still be completed for the roof-mounted position (which may be at a different height than a ground-mounted unit, changing the propagation distances to neighbours).

Does the 1 metre boundary rule apply to my own boundary (front garden wall, driveway) or only the neighbour's?

The 1 metre requirement applies to the external boundary of the property — any boundary, including boundaries with the highway or with common land. It is not exclusively the boundary with a neighbour. Check the property boundary plans to confirm the unit is at least 1 metre from any boundary line.

Regulations & Standards

• GPDO 2015 (General Permitted Development Order 2015), Class G — permitted development rights for ASHPs in England; the primary planning reference

• MCS 020 — MCS noise assessment standard for heat pump installations

• ISO 9613-2 — acoustics: attenuation of sound during propagation outdoors; used for detailed noise assessment

• BS 8233:2014 — guidance on sound insulation and noise reduction for buildings; indoor noise level targets

• MCS 020 — MCS noise standard — noise calculation methodology for heat pump permitted development

• Planning Portal — ASHP permitted development — summary of Class G PD conditions

• Historic England — listed buildings and heat pumps — guidance on heat pumps in listed buildings

• air source heat pump installation — siting requirements and anti-vibration mounting

• mcs 007 heat pump standard — MCS 007 documentation including noise compliance

• bus grant scheme guide — BUS grant eligibility (requires MCS-certified installation, which includes noise compliance)