Putting plant on the flat roof of a leased commercial unit

Roof plant planning permission commercial questions usually get the wrong answer from a contractor, and the wrong answer is that condenser units are permitted development. The plant right in Part 7 of Schedule 2 to the GPDO 2015 applies to land used for an industrial process or as a warehouse. A shop, a restaurant, a gym or an office has no equivalent right, so an air conditioning condenser or an extract fan on that roof is an alteration that materially affects the external appearance of the building and needs permission.

Then there is the lease, the structure and the environmental health officer, and only one of those three is a planning matter.

Whether roof plant is permitted development

Three questions settle it.

What is the land used for. The Part 7 plant right covers industrial and warehouse land. If the unit trades as a shop, a food and drink outlet, an office, a clinic or an indoor sports use, the right does not apply whatever the building looks like. A unit on an industrial estate that has been converted to a trade counter with a retail element may have moved out of the right without anyone noticing.

Is the site on article 2(3) land. Conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads are excluded from the plant right altogether.

Does the plant exceed the height limit. Where the right does apply, the Order caps the height of the plant, measured against both an absolute figure and the height of any plant being replaced. Read the current text of Class H before relying on it, because a replacement of like for like is treated differently from a new installation.

Where the right does not apply, you are making a full application, and the fee is based on floorspace or on a flat rate for alterations. It is usually determined in eight weeks, and the determining issue is almost always noise rather than appearance.

Height, position and the setback from the edge

Plant position is driven by four constraints that pull against each other.

Noise wants the plant far from the nearest residential facade and low behind a parapet. Airflow wants it clear of obstructions, with adequate separation between units to avoid recirculation of warm discharge air, which the manufacturer specifies. Structure wants it over a beam or a wall rather than mid-span. And maintenance wants it reachable, with a safe working platform and a route for replacing a compressor.

Two rules of thumb that hold up. Set plant back from the roof edge so the parapet does a useful acoustic and visual job, and so edge protection is not needed. And keep it away from the elevation facing housing even where that costs pipe run, because moving a unit six metres can be worth several decibels at the receptor.

The temptation is to put plant where the risers already are. Resist it long enough to model the noise, because relocating plant is far less expensive than attenuating it.

Plant noise and the nearest receptor

Plant noise is assessed under BS 4142:2014+A1:2019, which compares a rating level from the plant against the background sound level at the nearest noise-sensitive receptor. The method is specific and councils apply it closely.

The sequence an acoustician follows:

1. Measure the existing background sound level, as the LA90, over a representative period including the quietest time the plant will run
2. Establish the specific sound level from the plant at the receptor, from manufacturers' sound power data and a propagation calculation
3. Add character corrections for tonality, impulsivity and intermittency, which can add several decibels
4. Compare the resulting rating level with the background and assess the difference in context

The headline figures are these. A rating level around 10 dB or more above background indicates a likely adverse impact. Around 5 dB above is of marginal concern. At or below background, the impact is low. Context then modifies the conclusion, so a plant installation in a quiet residential back garden setting is judged more harshly than the same plant on a busy high street.

Councils commonly condition the rating level at 5 dB below background at the nearest receptor, which is a tighter standard than the standard itself implies. If that condition is offered, check it is achievable before accepting it, because a condition you cannot meet is a problem that surfaces after the equipment is installed.

Night-time matters most. Refrigeration and ventilation that runs through the night is assessed against the quietest background, usually in the small hours, and that is where schemes fail. Our note on cold stores and refrigeration plant noise covers the continuous-running case in more detail.

Where the plant is kitchen extract rather than cooling, odour joins noise as an issue and the submission needs a Ventilation and Extraction Statement dealing with both.

Screening, enclosures and what the street sees

Acoustic screening works through mass and continuity rather than height alone. A lightweight louvre screen that looks substantial on a drawing may deliver very little attenuation, while a solid barrier of adequate surface mass with no gaps at the base can deliver a useful reduction.

Four measures, roughly in order of effect per pound spent:

• select quieter plant, with a lower sound power level, before doing anything else
• fit attenuators to ducted systems and anti-vibration mounts under every unit
• build a solid screen with sufficient surface mass, continuous to the roof deck, on the sides facing receptors
• enclose fully, with acoustic louvres for airflow, accepting the airflow penalty and the maintenance access problem

Full enclosure is the last resort rather than the default. It restricts airflow, which reduces efficiency and can shorten plant life, and it makes maintenance harder, which means it gets left open.

Visually, the test is what can be seen from the street and other public viewpoints. Draw a sight line from the opposite footway, and from any upper floor window that overlooks the roof, and show the plant and screen in section. Where the roof is visible from a longer view or from higher ground, a plan showing the plant layout is not enough.

Structural capacity and the lease

Flat roofs on commercial units are rarely designed for plant. A roof built to carry a standard imposed load plus a snow allowance may be unable to take a chiller and its steelwork without local strengthening, and the strengthening often lands inside the demise below.

Three structural items to settle before the planning application, not after:

The existing roof structure, from drawings if they exist and from an opening-up survey if they do not. The load path, since plant is best carried on steelwork spanning between existing beams or walls rather than bearing on the deck. And the waterproofing detail at each new penetration, because the commonest failure on these installations is water ingress rather than noise.

The lease is the other half. Almost every commercial lease requires the landlord's consent for alterations, and plant on a roof usually involves the roof itself, which is often retained by the landlord rather than demised to the tenant. A licence to alter is a separate legal process with its own timetable, and landlords frequently require reinstatement at the end of the term.

Apply for planning permission and the licence to alter in parallel. Waiting for one before starting the other adds months for no reason.

Conservation areas and visible roofs

In a conservation area the plant right is withdrawn and the visual test is applied properly. Officers look for plant that is invisible from the street and from any publicly accessible viewpoint, which in practice means set well back behind a parapet, below the parapet line, and screened in a material that reads as part of the building.

Where the building is listed, the fixings and any penetrations need listed building consent, and a roof-mounted unit on a visible slope is likely to be refused. The workable alternatives are plant at ground level in a rear yard, plant inside the building with ducted intake and discharge through an existing opening, or a system that needs less external equipment.

Some plant also needs an environmental permit or registration in its own right, separately from planning, which is the pattern our note on use class and emissions permitting describes for a different sector.

Details to submit and conditions to expect

A submission that gets determined without a request for further information contains:

• a roof plan at a sensible scale showing each unit, its dimensions and its clearances
• elevations and at least one section showing the plant, the screen and the parapet, with the nearest residential window marked
• a plant schedule giving the make, model and sound power level of every unit, with the manufacturer's data attached
• an acoustic assessment to BS 4142 with the background measurement details, the calculation and the proposed mitigation
• a note on structural support and waterproofing
• a maintenance access arrangement

Expect conditions on the rating level at the nearest receptor, on hours of operation if the use allows it, on post-installation verification by measurement, and on no change to the plant without further approval. The verification condition is the one to plan for, because it requires a measured survey after commissioning and the plant may need re-attenuating before it passes.

A Noise Impact Assessment prepared by a qualified acoustician is what councils expect here, and the planning route does not displace the separate statutory nuisance powers under the Environmental Protection Act 1990 if neighbours complain later.

Browse the report types to see which documents a commercial fit-out application usually needs.

This is general guidance, not legal advice. Acoustic assessment should be prepared by a suitably qualified acoustician and councils normally expect a qualified author. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.