Are permitted development conversion flats prone to overheating?

Official Welsh guidance is direct on the dwelling types most at risk, identifying "a greater overheating risk in certain types of dwellings, such as flats and mid-terraced houses, as well as rooms that face south or west"¹. The Climate Change Committee has gone further, recommending that the overheating requirement in building regulations be expanded "to cover refurbishments of existing buildings and conversions of non-domestic buildings to residential"². That recommendation is not yet the rule, which is precisely why the question matters for anyone considering a converted flat.

The risk is not theoretical even in homes built to current standards. A housing association scheme of 12 homes compliant with Part O and the anticipated Future Homes Standard reported that "the sun-facing sides of the buildings were extremely hot" and that windows and mechanical ventilation were not sufficiently capable of removing temperature gains³.

Why permitted development flats overheat

The overheating problem in converted flats begins with the building they were made from. An office block is designed around a deep floor plate, large glazed façades and mechanical services, not around the cross-ventilation and shading that a dwelling needs. When the use changes to residential, the envelope stays largely as it was.

The Climate Change Committee commissioned Arup to appraise "the current and future risks posed by summertime overheating to the UK housing stock at scale", covering the factors that influence risk and the options and costs for adapting homes⁶. That work treats existing housing stock as a distinct problem from new build, because existing homes were never designed to the overheating standard now applied to new dwellings.

There is a second, quieter driver. Permitted development works by "removing the need to apply for planning permission"⁷, and the householder rights that many people associate with home improvement explicitly "do not apply to houses created through the permitted development rights to change use"⁸. A converted flat therefore sits outside a set of allowances that owner-occupiers of ordinary houses take for granted, including some of the routes used to add shading, external units or ventilation.

For a household, the independence question is blunt. A flat that overheats in summer depends on mechanical cooling, and mechanical cooling depends on electricity, a supplier and, in a leasehold block, consent from a freeholder. The building itself does little of the work.

The conversion route and what it skips

Permitted development is a planning mechanism, not a building standards exemption. Building regulations still apply to the work. What the conversion route skips is the planning application, and with it the design scrutiny that a planning authority would otherwise apply to orientation, glazing ratios and external shading.

The gap is visible in the standards themselves. Approved Document L requires that a new dwelling's "dwelling primary energy rate, dwelling emission rate and dwelling fabric energy efficiency rate must not exceed their respective target rates"⁴. Those targets are set for new dwellings. A change of use from office to residential is not a new dwelling in that sense, so the compliance route differs.

The Climate Change Committee's recommendation to extend the overheating requirement to conversions and refurbishments is the clearest official acknowledgement that the current split leaves a gap². Until that is enacted, a converted flat can be signed off without the summertime overheating assessment that a new build would face.

Scotland operates a different framework. The New Build Heat Standard is "a requirement set out in law"⁹, introduced through building regulations¹⁰, and it prohibits heating and cooling systems within the curtilage of any new building that produce more than a negligible level of greenhouse gas emissions at the point of combustion¹⁰. Amendments to Standard 6.11 apply from 1 January 2025¹¹. The standard is administered by the Heat in Buildings Regulations Unit¹². It governs new buildings, so it does not close the conversion gap either.

Design features that drive heat build-up

Three features recur in converted flats: glazing area, floor level and ventilation type.

Glazing is the first. Office façades are typically more heavily glazed than residential ones, and the solar gain through that glass is what drives afternoon temperatures in south and west facing rooms. Official guidance names rooms facing south or west as a higher-risk category¹.

Floor level is the second. Upper floors take solar gain through the roof as well as the windows, and they lose the shading that neighbouring buildings cast at street level. The standards recognise how much exposed surfaces matter: Approved Document L sets a limit to "limit downwards heat losses to no more than 10W/m2" for ground floors and floors in contact with the outside¹³. That figure concerns heat loss rather than heat gain, but it shows the level of attention the fabric of exposed floors receives in the regulations.

Ventilation is the third, and it is where the housing association evidence is sharpest. In the 12-home scheme built to Part O and the anticipated Future Homes Standard, windows and mechanical ventilation systems were "not sufficiently capable of removing temperature gains" on the sun-facing sides³. If a scheme built to the current standard can fail that way, a conversion with retained office glazing and a legacy ventilation system has less margin still.

Where overheating shows up: top floors and south-facing rooms

The pattern is consistent across the official evidence. Flats and mid-terraced houses carry greater risk, and so do rooms facing south or west¹. In the London call for evidence, reported overheating reached 87 per cent in terraces and 85 per cent in bungalows³. Those figures come from a different dwelling mix, but they establish that reported overheating is widespread rather than marginal in the existing stock.

Top-floor flats combine two exposures. They receive solar gain through the roof and through the windows, and they sit above the shade line. South and west facing rooms receive the strongest afternoon solar gain, which is when internal temperatures peak and when occupants are most likely to be at home.

The permitted development rules for equipment that might mitigate this are tighter for flats than for houses. Permitted development rights for air source heat pumps are "more restricted for flats and shared buildings", and planning permission is more likely to be required¹⁴. On a block of flats the outdoor compressor unit including any housing "must not exceed 0.6 cubic metres"¹⁵, and installation is not permitted if it would result in "more than 1 air source heat pump on the same building or within the curtilage of the building or block of flats"¹⁶. For a non-detached property or a block of flats, more than one air source heat pump requires planning permission¹⁷, and the condition is no more than 1 ASHP within the boundary of a dwelling that is not a detached house¹⁸.

The practical effect is that a top-floor flat in a converted block faces the greatest heat load and the tightest route to adding mechanical cooling. That is the opposite of what a household would want from the tenure.

How new-build standards compare with converted offices

The comparison is stark once the standards are laid side by side. A new dwelling must meet target rates for primary energy, emissions and fabric energy efficiency⁴. A converted office is assessed under a different route, and the overheating requirement that applies to new homes has not been extended to conversions².

Scotland's New Build Heat Standard applies to new buildings and prohibits combustion-based heating and cooling systems within their curtilage¹⁰. It is a requirement in law⁹, introduced through building regulations¹⁰, with amendments to Standard 6.11 from 1 January 2025¹¹. It does not reach a converted office either.

The Future Homes and Buildings Standards consultation shows how the notional building is being tightened. Solar PV is removed from the notional specification for flats over 15 storeys in height⁵, a change that reflects the practical difficulty of fitting generation to tall blocks rather than any relaxation of ambition. The direction of travel is towards more on-site generation and tighter fabric, both of which a conversion typically lacks.

For a household, the table explains why two flats of similar size can behave very differently in a heatwave. The new build has been modelled against a summertime criterion. The conversion has not.

What householders can check before renting or buying

The checks that matter are physical and can be made on a viewing. Orientation comes first: official guidance identifies rooms facing south or west as higher risk¹. Floor level comes second, for the reasons set out above. Glazing area and whether windows open come third.

Ventilation is the check most often missed. The housing association evidence found windows and mechanical ventilation "not sufficiently capable of removing temperature gains" on sun-facing sides even in a scheme built to Part O³. A converted flat with retained office glazing and a legacy ventilation system deserves the same question.

Where a household wants to add cooling, the permitted development position for flats is tighter than for houses. Rights for air source heat pumps are more restricted for flats and shared buildings¹⁴, the outdoor unit on a block of flats is capped at 0.6 cubic metres¹⁵, and more than one unit on a non-detached property or block of flats requires planning permission¹⁷. Rights may also be removed through a planning condition, an Article 4 Direction or other restriction¹⁹. In a listed building or designated area, it is advisable to check with the local planning authority before a flue is fitted, even where permitted development rights apply²⁰.

On the tenancy side, the Warm Homes: Local Grant scheme rules state that landlords should not increase rent as a result of upgrades funded by government through the scheme²². That is a condition on a funding route, not a general right to demand cooling works.

For the wider picture on how converted and top-floor dwellings behave, and the cooling options that suit them, see cooling by home type and air conditioning in flats. The overheating rules that apply to new homes are set out in Part O, and the assessment method behind them in CIBSE TM59.