Walkthrough: rooftop plant on an office retrofit

Rooftop plant on an office retrofit is almost always development needing permission, because the plant and the screen around it materially affect the external appearance of the building. The illustrative project below is a 1980s office block in a central London conservation area, within a protected vista and close to a grade II listed terrace, where the retrofit needed air source heat pumps on the roof and the planning case turned on two pieces of evidence: a verified view and a noise assessment at the nearest residential facade.

The building, the retrofit and the plant it needed

Seven storeys, concrete frame, a flat roof with an existing lift overrun and two redundant cooling towers. Around 4,200 m² of net internal area, single let, due for a full refurbishment at lease expiry.

The brief was to remove gas heating, improve the fabric, and reach a target energy rating that the landlord's funder required. That meant air source heat pumps, new air handling units, and a larger electrical switchroom, almost all of it on the roof because there was no plant space at ground level and no basement capacity.

The proposal was eight heat pump units, two air handling units, associated pipework, and an acoustic and visual screen about 2.4 m high enclosing the whole arrangement. The screen was the item that drove the planning case.

Whether permission was needed at all

Under section 55 of the Town and Country Planning Act 1990, works that affect only the interior of a building, or that do not materially affect its external appearance, are not development.

The architect took the view that a 2.4 m screen running most of the roof perimeter, visible from the street and from the protected view, materially affected external appearance. The building was also in a conservation area, which removes or restricts a number of permitted development rights, and permitted development rights for plant on non-domestic buildings are narrower than people assume and differ by building type, so an office is not in the same position as an industrial building or a warehouse.

So the conclusion was straightforward: a full application for the plant and the screen, with the fabric works mostly falling outside planning control.

One point worth stating plainly, because it recurs. Replacing like for like plant in the same position at the same size is frequently not development, and a retrofit that increases the volume, raises the height or adds a screen almost always is. The honest answer in a borderline case is a certificate of lawfulness of proposed use or development, which is the only formal way to confirm that works are lawful.

The visual case from the protected view

The site sat within a protected vista identified in the London Plan's view management policies, which meant the assessment could not be a set of sketch montages.

What was prepared was a Townscape and Visual Impact Assessment containing accurate visual representations from the assessment points the view framework identifies, produced from a surveyed model of the building with the screen at its proposed height, and a wireline overlay showing the relationship between the screen and the landmark that the vista protects.

Three findings came out of it, and the second changed the design.

The screen was visible in the view but did not break the skyline above the protected landmark. The screen read as a continuous solid band, which was more obtrusive than the plant behind it would have been, because a solid parapet extension changes the perceived height of the whole building. And setting the screen back from the parapet by 1.2 m removed it from two of the three assessment points entirely.

The design changed to a set back screen with an open louvre upper section in a recessive finish. That is not a cost neutral change, because setting the screen back reduces the usable plant area and the louvre specification has to be acoustically rated.

Heritage was handled alongside. The setting of the listed terrace opposite was assessed as part of the same exercise, with the conclusion that the set back screen caused no harm to the significance of those buildings, and the statutory duty to have special regard to preserving the setting of a listed building was addressed on its own terms rather than folded into the townscape conclusion.

The acoustic case at the nearest residential facade

The nearest noise sensitive receptor was a converted flat on the fourth floor of a building 18 m away, with a bedroom window facing the roof.

The assessment followed the British Standard method for rating industrial and commercial sound, which compares a rating level for the plant against the measured background sound level at the receptor. Background monitoring ran over a full week to capture night time levels, because heat pumps run at night and night is when the background is lowest and the bedroom window is open.

Four technical points mattered.

• The rating level includes corrections for tonality, impulsivity and intermittency, and heat pump compressors and fans are often tonal, so an uncorrected sound power figure from a manufacturer's datasheet understates the result
• The relevant comparison was at the fourth floor facade, not at ground level, and the propagation path from a roof to a facade at similar height has less screening than a ground level calculation suggests
• Defrost cycles produce a different and more noticeable sound character than steady running, and the assessment had to address them
• The plant schedule had to specify sound power levels per unit, in octave bands, because a single overall figure cannot be used to design attenuation

The outcome was an acoustic screen with a specified mass per unit area, attenuators on two of the air handling unit discharges, and a night time operating limit on two of the heat pumps. A Noise Impact Assessment sets out that chain of reasoning, and councils expect it from a suitably qualified acoustician with professional review before submission.

Extract systems raise a related but separate set of issues, and our note on a commercial extraction system and the odour objection covers the odour side where a retrofit includes catering.

Structural capacity of the existing roof

Eight heat pumps, two air handling units and a screen add load to a roof designed in the early 1980s for little more than its own weight and maintenance access.

A structural appraisal with opening up established the slab construction, the beam layout and the column positions, and confirmed that the plant could be supported on steelwork spanning between column lines rather than bearing on the slab. Two local areas needed strengthening. The screen needed its own wind loading check, because a 2.4 m solid screen on a seven storey roof is a sail.

Anti vibration mounts and inertia bases were specified, and that detail belongs in the planning submission as well as the construction information, because structure borne vibration transmitted through the frame is a nuisance complaint the acoustic screen does nothing about. A Structural Survey carried the opening up findings, and councils expect a suitably qualified author for this work.

Access and maintenance

Plant that cannot be maintained will not be maintained, and an officer who sees no access strategy will ask for one.

The scheme provided a new roof access hatch within the existing core, a fixed ladder with edge protection, a permanent walkway between units, and a lifting point for unit replacement. The replacement route mattered: the units were sized to pass through the goods lift, which avoided a crane lift over a public highway later in the building's life.

The pack submitted and the conditions imposed

The application comprised a planning statement, the townscape and visual impact assessment with accurate visual representations, a heritage assessment addressing the setting of the listed terrace, the noise assessment with monitoring data and the plant schedule, roof plans, elevations and sections at a scale that showed the screen set back, a structural note, and an access and maintenance strategy.

Permission was granted with conditions covering the plant noise limit expressed relative to the measured background at the identified receptor, installation of the acoustic screen before the plant is brought into use, the screen material and finish to be agreed, no additional plant on the roof without a further permission, and submission of a maintenance regime.

The last condition is the one to negotiate. A condition prohibiting any additional rooftop plant without permission is reasonable for a protected vista and inconvenient for a building that will be re-fitted again in fifteen years, so ask for it to be framed around a volumetric envelope rather than an absolute bar.

If you are scoping a retrofit and want to see which supporting documents the building is likely to need, enter the postcode on PlanScope and work through the constraints first.

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