Wondering what future British summers will be like, as the climate crisis unfolds? Clue: put away visions of sipping Yorkshire champagne on a Barcelona-style balcony. Think instead of stuffy, overheated homes making sleep impossible, droughts and floods that play havoc with infrastructure, and urgent health warnings for the old and very young.

“What we are facing is climate brutality. That is the reality of the hotter weather coming down the track,” says Simon McWhirter, the chief executive of the UK Green Building Council (UKGBC).

Temperatures have already topped 40C during one summer, in 2022, and that record-breaking heat is likely to be repeated in the next 12 years, according to the Met Office, as global heating drives more weather extremes. A temperature of 28C inside the home is likely to become the norm in London and the south-east in the decades to come, according to McWhirter.

Sweltering summers are more likely even when the sun isn’t shining – muggy weather such as that of last month in many parts of the UK will be more common.

Despite years of warnings, however, the UK is poorly prepared. The Climate Change Committee, the government’s statutory adviser, has repeatedly highlighted a lack of urgent effort to adapt critical infrastructure, from water, transport and communication networks, to the fabric of our buildings.

“The way we are building new housing is not adequate to the climate change we are already seeing, never mind the even hotter weather that’s coming,” says McWhirter. Part of this is owing to the construction of the homes themselves, which need good ventilation to create through airflow that cools rooms, with well-designed windows and external shutters, and reflective paint on roofs or outside surfaces. Homes should also be highly water-efficient, for instance being able to recycle bathwater to flush toilets or water gardens, to cope with the impacts of drought.

Keeping cool also depends on the locality around a building, especially the shade and cooling impact of street trees. The UKGBC advocates a 3:30:300 approach: you should be able to see three trees from your dwelling; you should have 30% tree canopy cover in your neighbourhood; and you should be no more than 300 metres from a biodiverse green space or park.

New rules aimed at overhauling how new homes are built have been promised for two decades. The first attempt was set out by Labour in 2008, only to be scrapped by the Conservatives in 2015, and years later a fresh attempt was mauled and delayed. This autumn, the “future homes standard” (FHS) should finally be published, to come into force from 2027.

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But throughout this needlessly tortuous and expensive process, the focus has been mainly on how to keep draughty homes warm in winter, to stop them leaking heat – and money, and carbon dioxide alongside. The challenge of cooling overheated homes is new – and the current draft of the FHS fails to meet it, McWhirter says.

“The FHS does not address this threat properly,” he says. “The focus has been on energy conservation and reducing bills. Absent has been the other side of the coin, which is climate impacts.”

Fortunately, according to Steve Turner, the executive director of the Home Builders Federation, insulating homes to retain heat in winter also helps to keep heat out in summer. “New-build homes are required to be exceptionally well insulated, which helps keep them stay both warmer in winter and cooler in summer,” he says. “This makes living in a new-build more comfortable and significantly cheaper to run than older, less well insulated homes.”

One obvious answer to overheating may seem to be air conditioning – it’s common in hot countries, so why not here? Ministers are now considering subsidising the installation of air conditioning in the UK for the first time, a position the UKGBC supports.

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But that would be a controversial departure from climate policy. The boiler upgrade scheme offers households a substantial discount on the installation of air-source heat pumps, efficient electrical alternatives to gas boilers, in order to cut fossil fuel use and meet net zero targets. Up to now, only standard heat pumps have been eligible for the grants. Yet a form of technology exists that can heat and cool buildings: “air to air” heat pumps, which rely on taking in air from outside and passing it through a heat exchanger to create hotter or cooler air that is pumped into the home.

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The problem is that air conditioning is extremely energy-intensive. The “air to air” heating and cooling involved is much less efficient than “wet” heating systems that heat water for radiators, so machines that can do both are a poor substitute for the efficient heat pumps needed for net zero, according to the HBF.

Around the world, air conditioning requires such vast quantities of energy that the UN warned in July that the beneficial impacts of a massive surge in renewable power generation were being swamped by the vastly increasing demands of AI, datacentres and air con.

The government intends that if air-to-air heat pumps are allowed, they should mostly be replacing current electric heating systems in small homes, particularly flats, where conventional heat pumps can be harder to install. This, the Department for Energy Security and Net Zero argues, could overall cut bills and energy consumption, as the increase in summer demand could be offset by winter savings. The government has commissioned further research and modelling on the issue.

Even if the government’s target of 1.5m new homes this parliament is achieved, newly built homes will still make up only a small fraction of the UK’s housing stock. Changing existing homes will be far harder.

Ensuring homes do not overheat is governed by Part O of the UK’s building regulations, which came into force only in June 2022 – explaining why so many homes built even in the past few years are overheating. Part O applies only to new homes, though the government is considering whether it should be expanded to cover a change of use, such as offices converted to flats. At the moment, there are no substantial plans to retrofit the bulk of the UK’s existing homes to stop overheating.

A government spokesperson said: “We know the importance of keeping homes cool in hot weather. That’s why building regulations require new homes to be built to reduce the risk of overheating and through the future homes standard consultation we are exploring how to further improve protections. This is alongside considering the use of air-air heat pumps to keep existing homes cool under the boiler upgrade scheme.”

If ministers wanted, they could also ensure that rented accommodation was brought up to decent standards on overheating. Without such enforcement, it will be up to individual householders – in which case, shutters and blinds should be top of the list, way ahead of any type of air con system.