Plant noise from a rooftop condenser next to flats

Condenser unit noise and planning permission come together for most commercial occupiers at the point a landlord's agent asks where the plant is going. External refrigeration and air conditioning plant on a shop, cafe or restaurant almost always needs planning permission, because it alters the external appearance of the building and the permitted development rights for plant are narrow and aimed mostly at industrial land. Add flats above or behind, and the application turns on one number: the rating level of the plant against the background sound level at the nearest bedroom window at three in the morning.

Whether the plant itself needs permission

Start with the building, not the plant. Permitted development rights for the installation of plant and machinery sit mainly in Part 7 of Schedule 2 to the General Permitted Development Order 2015 and relate to industrial land, with limits on height and position. A retail unit or a restaurant in a parade does not have the benefit of them.

Four other checks before you assume anything:

• A condition on the existing permission may require plant details to be approved, which overrides any general right
• A listed building needs listed building consent for external plant, and a condenser on a principal elevation will be resisted
• In a conservation area, plant visible from a public place attracts more scrutiny and an Article 4 direction may remove rights entirely
• Where the unit is a flat conversion or has residential above, the roof is often demised to another party, which is a lease question before it is a planning one

Where the plant is small, entirely concealed and has no external effect, there may be no development. That is a narrow argument and it is worth asking the authority rather than guessing.

Finding the nearest noise sensitive receptor

The nearest receptor is rarely the one people assume. Plant at roof level is closest to the upper floor windows of the same building and of the building behind, and those windows have no screening from the parapet that hides the unit from the street.

Identify every habitable room window within range, on a plan, with distances and heights. Include the flats directly above the unit, which are often in the same ownership as the shop and are still noise sensitive receptors. Include windows across a rear yard, where the enclosed space reflects sound and raises the level at the facade above what a free-field calculation predicts. And include balconies and roof terraces used for relaxation.

Then choose the assessment position honestly. A measurement point at the front of the building on a busy street will show a high background and a comfortable result; the same plant assessed at the rear bedroom window of the flat above will not. Environmental health officers know which one to ask for.

Manufacturer data and what it leaves out

Selection data is the weakest part of most submissions. Four common problems:

1. A sound pressure level quoted at 1 m in free field is not a sound power level, and using one where the other is required produces an error of several decibels
2. Published figures are often for a part-load or reduced fan speed setting, while the unit will run at full speed on the hottest night of the year
3. Defrost cycles, compressor start-up and fan speed changes are not in the steady-state figure and are exactly the events that wake people
4. Tonality from the fan or compressor is rarely in the data sheet at all, and under the assessment method for industrial and commercial sound a tonal correction is added to the measured level

Resonance through the support frame is the other omission. A unit bolted directly to a timber deck or to a lightweight roof transmits structure-borne sound into the rooms below, which no facade measurement will capture and which anti-vibration mounts are there to prevent.

State the data source, the operating condition assumed, and whether corrections for acoustic character have been applied. Our note on unattended noise monitoring duration, position and validity covers the background survey that the whole calculation rests on.

Attenuation options in order of cost

Work down the list, because the low cost options are also the effective ones and the expensive ones are what you are left with after ignoring them.

Two cautions. Attenuation restricts airflow, and a condenser starved of air trips out or runs inefficiently, so any enclosure has to be designed by someone who understands both. And a barrier only works if it breaks the line of sight to the receptor, which for a window two storeys above the plant means a very tall screen.

Night-time operation and the background problem

This is where cases fail. The assessment method for commercial and industrial sound uses a fifteen minute reference period at night, from 23:00 to 07:00, and compares the rating level with the background sound level measured in that period. In a residential street behind a parade, that background can be very low in the small hours, and the rating level then has almost no headroom.

The consequence for a shop with chilled and frozen cabinets is uncomfortable. The plant has to run continuously to protect stock, so a condition restricting plant hours is not an option, and the attenuation has to achieve the target on its own.

Three approaches that sometimes work:

• Oversize the plant and run it slowly at night, which trades capital cost for acoustic headroom
• Split the load, so that only the cabinets requiring continuous cooling run overnight
• Locate the plant remotely, in a basement or an internal plant room with attenuated ducting, rather than on the roof

Where none of them gets you there, the honest answer is that the unit cannot support the intended fit-out at that plant location, and it is better to know before the lease is signed.

What the environmental health officer will ask for

Officers see a great many of these assessments and they read them the same way. Give them:

• The background survey, with the measurement position, dates, weather, instrumentation and calibration, and the adopted night-time background value with its justification
• The plant schedule, with the source of the sound data and the operating condition assumed
• The calculation from source to receptor, with distances, reflections, barrier effects and directivity stated
• Any acoustic feature correction applied, with the reasoning
• The mitigation proposed, with the insertion loss claimed for each element and its source
• The resulting rating level at each assessed receptor, against the background

A Noise Impact Assessment that does this can be checked in half an hour. One that gives a conclusion and an appendix of manufacturer leaflets will come back with a request for the working. Where kitchen extraction is part of the same installation, deal with it in the Ventilation and Extraction Statement rather than mixing the two assessments.

Illustrative example: a cafe fit-out where two condensers are moved from the rear parapet to the front roof slope behind an existing plant screen, mounted on rubber-in-shear isolators, with a night speed limit set in the controls and verification testing offered by condition. Details here are illustrative only.

Conditions you should expect

Four conditions recur, and offering the first three yourself strengthens the application:

The first sets a rating level limit at the nearest noise sensitive facade, expressed against the measured background, commonly at background or a stated number of decibels below it. The second requires verification testing within a set period after commissioning, with remedial work if the limit is exceeded. The third requires the attenuation and anti-vibration measures to be maintained for the life of the plant. The fourth prohibits additional external plant without a further approval, which matters if the business expands later.

Read the wording carefully before agreeing it. A condition requiring a rating level 10 dB below background at a rear window may be impossible to achieve, and a condition that cannot be complied with is a refusal in slow motion. Where the noise also has a licensing dimension, our note on planning control and licensing control of entertainment noise explains how the two regimes divide.

Enter your postcode on PlanScope to see which reports a commercial fit-out is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.