New extensions are usually the point at which underfloor heating gets seriously considered, because the floor is being dug up and built anyway and the extra depth and cost are far lower than retrofitting it into an existing room.
The decision isn't only about comfort. It also affects floor build-up, wall layout and how the whole system behaves day to day.
Comfort and heat distribution
Underfloor heating warms a room from the floor up, giving an even temperature across the whole space with no cold spots away from a radiator. For a room with a lot of glazing, such as a bifold or sliding door installation, that even background heat counters the cold air that pools near large glass areas.
Radiators heat the air near them fastest, which creates a slight temperature gradient across the room, but modern radiators sized correctly still heat a well-insulated extension perfectly well.
Floor build-up and wall space
Wet underfloor heating needs a screed depth to house the pipework, which has to be accounted for in the floor level design from the outset — get this wrong and you end up with a step between the extension and the existing house.
Radiators need no additional floor depth at all, but they do need a run of wall free of fitted furniture, which matters in a small extension where every wall is wanted for storage or seating.
Response time and running pattern
A screeded underfloor system has significant thermal mass, so it takes hours to heat up and hours to cool down. It suits a background heating pattern left on a steady low setting rather than switched on and off around a household's daily routine.
Radiators respond within minutes, which suits households that want to heat a room quickly for a specific period rather than keep it constantly warm.
Cost and compatibility with the existing system
Underfloor heating in a new slab is comparatively straightforward to install because the pipework goes down before the floor finish. Retrofitting it into an existing floor at a later date is far more disruptive and rarely worth it unless the floor is being dug up for another reason anyway.
Mixing systems — underfloor in the new extension, radiators in the existing house — is common and works fine provided the heating system's zoning and controls are designed to run both circuits correctly from the same boiler or heat source.
Common questions
Can underfloor heating go under any floor finish?+
Most finishes work, including tile, engineered wood and stone, but the manufacturer's guidance on thermal resistance has to be followed — thick carpet with a dense underlay reduces the system's output and should be avoided or accounted for at design stage.
Does underfloor heating cost more to run than radiators?+
Running cost depends more on the heat source, insulation and how the system is controlled than on whether it's underfloor or radiators. A well-insulated extension with underfloor heating run at a low, steady temperature is often efficient, but the comparison isn't a simple one either way.
Is electric underfloor heating an option instead of wet systems?+
Electric mats are simpler to retrofit and need far less floor build-up, which suits a small extension or single room, but they're generally more expensive to run than a wet system connected to a boiler or heat pump for whole-room heating.
Get heating advice for your extension
We'll work through floor build-up, insulation and your household's habits before recommending underfloor heating, radiators, or a mix of both.