How do I calculate heat loss for an extension room?

Heat loss depends on room size, ceiling height, number of external walls, window area, insulation levels and room type. A well-insulated new extension typically loses 45–60 W/m² of floor area. Bathrooms lose more (higher target temperature), while bedrooms lose less. This calculator uses Part L compliant insulation levels for new extensions.

Will my existing boiler cope with an extension?

A typical extension room adds 2–5 kW to the heating load. If your boiler has spare capacity (most 24–30 kW combi boilers run at 70–80% peak load), it may cope. Enter your boiler kW rating and estimated current load to check. If capacity is tight, consider upgrading the boiler or using UFH which operates at lower flow temperatures.

Should I use radiators or underfloor heating in an extension?

UFH is popular in extensions because the new floor slab or buildup can easily accommodate UFH pipes. It provides even heat, frees up wall space and works well with heat pumps (lower flow temperature). Radiators are simpler and cheaper to install. UFH can deliver up to 100 W/m², which is usually sufficient for a well-insulated extension.

What pipe size do I need to feed an extension radiator?

For loads up to 3 kW, 15mm copper or 16mm plastic pipe is sufficient. Above 3 kW, use 22mm copper or 22mm plastic. The pipe run from the existing system should be as short and direct as possible. Long pipe runs increase heat loss and may require larger pipe sizes to maintain flow rates.

What flow temperature should I use for extension heating?

Radiators typically operate at 55–75°C flow temperature on gas boilers. UFH runs at 35–55°C (controlled by a blending valve). Lower flow temperatures improve boiler efficiency and are essential for heat pump compatibility. New extensions with good insulation can work well with lower flow temperatures.

TradeCalculator

Extension Heating Calculator — Heat Loss & Radiator Sizing

Calculate heat loss for new extension rooms, size radiators or UFH, and check whether your existing boiler has sufficient capacity.

Room type

Room length

Room width

Heating type

Number of external walls

Number of windows

How We Calculate This

This calculator estimates room heat loss based on floor area, room type and exposure, then sizes the heating output accordingly.

Heat loss method

Base heat loss rate (W/m²) for the room type is applied to floor area, with adjustments for external wall exposure (+5 W/m² per wall) and windows (+50W each). The result is the steady-state heat loss in watts.

Heating output

Radiator output is heat loss x 1.1 (10% safety margin). UFH output is limited to 100 W/m² maximum. Boiler capacity is checked by adding the new load to existing system demand.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.