It’s very common for an extension to feel colder than the original part of the house, even when the heating settings are the same. Radiators may be hot and the boiler may be running normally, yet the space never quite reaches the same level of comfort. In most cases this isn’t a fault with the heating system. It’s the result of how extensions lose heat differently to the rest of the building.
When an extension struggles alongside otherwise warm rooms, it usually isn’t down to one single cause. Extensions often combine several heat-loss factors at once, which is why it helps to start with the house cold diagnostic to understand the full picture before focusing on individual fixes.
The biggest difference is exposure. Extensions are usually wrapped by external surfaces on more sides than the original house. Where an older room might share walls with heated spaces, an extension often has multiple external walls, a roof, and sometimes large areas of glazing. Each of these surfaces allows heat to escape, and together they raise the background heat loss of the room.
Construction differences also matter. Many extensions are built to different standards or with different materials to the original house. Even where insulation meets regulations on paper, cold surfaces can still dominate how the room feels. Floors, walls and ceilings in extensions often cool more deeply, so a large share of the heating energy is absorbed by the structure before the air temperature rises.
Glazing plays a role too. Extensions frequently rely on big windows, patio doors or roof lights to bring in light. These features lose heat faster than solid walls, especially at night or during cold spells. The room can feel permanently behind the rest of the house because the heating is constantly replacing lost warmth rather than building comfort.
Air movement is another quiet contributor. Junctions between the original house and the extension are common places for small gaps to form. Even minor draughts allow cold air to seep in continuously, which undermines the heating output. This effect mirrors what happens in rooms with high exposure, as described in why rooms near external walls feel colder.
A common failed fix is simply turning the thermostat up or running the heating longer when using the extension. This can improve comfort while the system is active, but it doesn’t address how quickly the space sheds heat. As soon as the heating pauses, temperatures drop back again, making the room feel difficult to keep warm.
The least disruptive improvements focus on slowing heat loss rather than forcing more heat in. Preventing the extension from cooling too far between heating cycles helps surfaces stay closer to room temperature, which makes the heating feel more effective without increasing run time.
If the extension cools noticeably within minutes of the heating switching off, that rapid drop is a strong sign that heat loss is the limiting factor rather than radiator output. In those cases, control changes rarely solve the problem on their own.
There are situations where extension cold does point to a system issue. If radiators in the extension never heat properly while others do, or if the problem appeared suddenly after system changes, balancing or flow problems may be involved. Those patterns behave differently from long-standing extension cold.
In most homes, though, extensions feel colder because they lose heat faster and store less warmth than the original building. Understanding that mechanism makes it easier to target the real constraint instead of endlessly adjusting settings. For a broader view of keeping all parts of a UK home warm without driving up costs, the guide on how to keep a UK home warm for cheap helps put extension issues into context.
