Overheating and Part O: why UK homes get too hot, and what the rules now say

For decades the entire British housing conversation was about staying warm. That has quietly flipped. Overheating - a home that stays uncomfortably hot for long stretches of summer - is now a recognised health and design problem, serious enough that the Building Regulations were changed to address it directly. This guide explains why homes overheat, what the rules measure, and what you can actually do if yours is already too hot.

Overheating is about the home, not just the heat

A hot spell is the trigger, but whether your home overheats is decided by its fabric and its shape. The same July week leaves one flat pleasant and the one above it unbearable. The drivers are physical and specific:

• Roof and top-floor exposure. A top-floor flat or a loft room takes the roof's solar gain with little or nothing above to absorb it.
• Thermal mass. A solid-brick period home soaks up heat all day and releases it back overnight, so it stays hot long after the outside air has cooled.
• Airtightness and single aspect. A modern, well-sealed home traps the heat that gets in; a flat with windows on only one side cannot cross-ventilate to purge it.

Our overheating model turns these into a single score. A typical top-floor flat, for instance, scores around 70 out of 100 - "high" - and the model shows you which drivers pushed it there. We explain each home type in its own guide, with real per-borough data - see, for example, cooling a new-build flat in Croydon, where airtightness rather than age is the culprit.

What Part O and CIBSE TM59 actually measure

Building Regulations Part O came into force to limit overheating in new homes. It leans on the industry standard CIBSE TM59, a modelling method that checks whether living spaces spend too many hours above roughly 28°C, and whether bedrooms stay above about 26°C at night when people are trying to sleep. In plain terms, the rules ask a simple question: across a realistic warm year, does this home stay comfortable, or does it cook?

Two things are worth knowing. First, Part O applies to new homes - if you own an older property it sets no requirement on you, but the same physics still governs whether you overheat. Second, better insulation is not the answer to summer heat: the fabric that keeps winter warmth in is exactly what keeps summer heat from leaving. That is why so many new, efficient flats overheat despite excellent EPC ratings - and a big reason well over a third of homes in some inner-London boroughs sit in dense, hard-to-ventilate stock.

What you can do about a home that is already too hot

Start with the measures that stop heat getting in: external shading on sun-facing windows, lighter-coloured surfaces, and genuine night ventilation where noise and security allow. These are cheaper and always worth trying first. Where they are not enough - and for a top-floor or loft room they usually are not - active cooling removes heat that is already inside. Our guide to how to cool a UK home walks through the systems, the costs and the planning rules, and our air-to-air versus air-to-water comparison explains which low-carbon option actually cools as well as heats.

How we work this out

Our overheating scoring follows the CIBSE TM59 methodology that underpins Part O, applied to your property's real characteristics from public data. We show the drivers behind every score rather than hand you a number to trust. You can read the full methodology and sources here.

Check how hot your own home runs

Every home overheats differently - your floor, roof, fabric, aspect and heating all move the answer. Our free cooling checker pre-fills your home from public property data (EPC and UPRN records) and gives you a property-specific verdict on screen, straightaway: your overheating score, the drivers behind it, the right response and what it would cost. No phone call, no waiting.

Check your home in 60 seconds ▸

You will see your full cooling plan instantly, and nothing happens next unless you ask - we only get in touch if you request it.