Why Class MA conversions struggle with overheating

Class MA gives you a prior approval route from Class E commercial use to residential, and it gives you almost no ability to change the outside of the building. That combination is why office conversions overheat. The glazing ratio, the orientation, the depth of the floorplate and the openability of the windows are fixed by a building designed for daytime occupation with mechanical ventilation, and the permitted development right does not let you alter any of them.

So the overheating question has to be answered before you buy, not after the prior approval is granted. A 1990s curtain-walled office with sealed glazing on a south-west elevation and a 15 metre floorplate can be a perfectly good conversion on paper and an unlettable one in July.

What a material change of use triggers

Class MA, in Part 3 of Schedule 2 to the General Permitted Development Order 2015, requires a prior approval application rather than full planning permission. The prior approval matters are limited, and they include transport and highways, contamination, flood risk, noise from commercial premises in the vicinity, the provision of adequate natural light in all habitable rooms, and in conservation areas the effect of losing the ground floor commercial use.

The right has been amended since it was introduced, including changes in 2024 to the vacancy and floorspace conditions, so check the current wording of the class and its conditions before relying on any summary of it.

Two things are absent from that list and both matter here. Overheating is not a prior approval matter under Class MA. And external alterations are not permitted by the class at all, so new openings, opening lights, external shading, brise soleil and rooftop plant each need their own planning permission.

Under the Building Regulations the position is different again, and more complicated than most buyers assume. Whether the overheating requirement in Part O applies to a material change of use, and in what form, depends on the scope of that requirement and on the conversion provisions in the Regulations. Settle it with your building control body in writing at the outset rather than assuming either way, because the answer changes the design brief.

Deep plan floorplates and single aspect units

Office floorplates are designed around a core, with workstations served by artificial light and mechanical ventilation. Residential layouts need daylight and ventilation at the perimeter, which means the usable depth is limited.

The natural light prior approval condition forces that issue. Every habitable room needs adequate natural light, so deep plans get carved into units with long internal corridors, borrowed light arrangements that do not qualify, or rooms pushed to the perimeter with awkward proportions. The result is usually a scheme of single-aspect flats.

Single aspect is the overheating problem in one phrase. Cross-ventilation is impossible, so the only route for purging accumulated heat is the windows on one elevation, and on a still summer night that route barely works. Orientation then decides severity: south and west elevations take the afternoon and evening gain that drives night-time bedroom temperatures.

An illustrative example. A three-storey office with a central core and an 18 metre depth yields perimeter flats of 7 metres depth and a dead zone in the middle. The dead zone becomes circulation and storage, the flats are single aspect, and the south-west corner units are the ones that will generate complaints.

Glazing that cannot be altered

Commercial glazing ratios bear no relation to residential ones. A curtain-walled elevation can be most of the facade, with solar gain to match, and the existing glass may have a high solar transmittance by modern standards.

Your options without planning permission are narrow.

Replacing glazing units within existing frames is the measure that does the most work. It is also the one most often missed at appraisal stage, because the cost sits in the facade line rather than in the mechanical line of the budget.

Where the facade alterations are needed, a planning application follows, and at that point the scheme loses the speed advantage that made Class MA attractive.

Ventilation strategies in a sealed facade

If the windows do not open, the ventilation strategy is mechanical, and mechanical ventilation with heat recovery supplies fresh air without removing meaningful heat. Cooling and ventilation are different jobs and the distinction gets blurred in appraisals.

The hierarchy to work through is familiar from new build and harder to apply in a conversion: reduce the gain at the facade, then reduce internal gains from lighting, hot water distribution and appliances, then use passive ventilation where the facade allows it, then mechanical ventilation, and only then active cooling.

Internal gains are worth attention because they are the one area where a conversion has freedom. Communal heating distribution running through single-aspect corridors and risers is a known source of unwanted heat, so insulate and specify the pipework properly and locate plant away from habitable rooms.

An Overheating Risk Assessment using the recognised dynamic modelling methodology tells you where you stand. Councils usually expect a suitably qualified author for that document and professional review is strongly advised, particularly where the conclusion supports a departure from a local plan policy.

Noise and the restricted opening problem

Here is the trap that sinks the most schemes. Noise from commercial premises in the vicinity is a prior approval matter, so where the building sits next to a plant yard, a service road or a late-night use, the acoustic assessment will recommend a closed-window ventilation strategy to achieve internal noise levels.

Overheating modelling then has to assume the windows stay shut, or opened only to a restricted free area, for the periods when the noise source is active. In a single-aspect flat on a warm night that assumption usually produces a fail.

The two assessments therefore have to be run together rather than sequentially. A Noise Impact Assessment that recommends closed windows without reference to the thermal consequences simply shifts the problem into someone else's report, and the acoustician and the thermal modeller need to agree the openable areas, the periods and the attenuation approach before either finalises.

Where restricted opening is unavoidable, acoustically attenuated ventilators sized for purge rates are the usual answer, and they need space in the facade zone and a maintenance regime.

Mechanical cooling and its consequences

Active cooling solves the temperature problem and creates four others.

Plant has to go somewhere, and roof-mounted condensers are external alterations requiring planning permission, with their own noise assessment for neighbouring occupiers. Energy use rises, which affects the energy performance rating and the compliance calculation. Running and maintenance costs land on the service charge, which affects values and lettability. And local plan policies, particularly in London, place active cooling last in the hierarchy and expect evidence that the earlier steps were exhausted.

None of that makes cooling wrong. It makes it a decision with consequences that belong in the appraisal rather than in a later variation application. The energy compliance baseline is a related source of confusion, and the note on the notional building and why baselines confuse officers explains what the comparison involves.

Testing feasibility before buying

Do this at offer stage, not after exchange.

1. Measure the floorplate depth and identify how many units will be single aspect
2. Record the orientation of each elevation and the glazing ratio
3. Establish whether the windows open, and whether frames can take replacement units
4. Commission a thermal model of the worst-case unit, typically the top floor south-west corner
5. Commission a noise survey if commercial uses or busy roads adjoin, and brief the acoustician on the thermal interaction
6. Price replacement glazing units, attenuated ventilators and any cooling in the facade and mechanical lines
7. Decide whether a full planning application for facade alterations is needed, and reprice the programme if so

The fabric measures that help here are the same ones that matter in new build, and the values involved are set out in fabric first: the U-values officers look for. Where an energy strategy has to be submitted alongside, the Energy Statement should carry the overheating conclusions rather than leaving them in a separate document nobody cross-reads.

This is general guidance, not 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.