Will my boiler cope with an extension?

Often it will, but the boiler's kW rating alone can't tell you. Approved Document L Volume 1 paragraph 5.8 expects heating to be specified from a heat loss calculation for the dwelling, so ask for one covering the whole house with the extension before any radiator is ordered.

Is underfloor heating or radiators better in an extension?

Wet underfloor heating suits a large, glazed open-plan extension because it frees the walls and runs at a low flow temperature, typically 35-45°C, but it has to be decided before the floor insulation and screed are specified. Radiators cost less and warm up faster, and they can go in later as long as the furniture leaves them a wall.

Does extending central heating need building regulations approval?

Often it's notifiable. GOV.UK lists extensions to central heating systems among the work you must tell building control about unless the installer is registered with a competent person scheme, so ask your heating engineer whether they are registered and will notify the work.

Does an extension's heating have to run at 55°C?

Not automatically. Approved Document L Volume 1 paragraph 5.10 sets that maximum flow temperature for a wet system that is newly installed or fully replaced, so it applies if the whole system is replaced. For an existing system that is only extended, or an extension with its own new boiler or heat pump, ask your building control officer what they expect.

Do I need a separate thermostat for my extension?

The regulation that asks for thermostatic controls in each room (Approved Document L paragraph 5.20) is triggered by a new dwelling or a replaced boiler, not by extending alone. Work on an existing boiler system still needs a boiler interlock (paragraph 5.25), and a separate zone with its own thermostat is good practice for a big open-plan room.

Can you put a rug or carpet over underfloor heating?

A thin one, yes, but every layer on top slows the heat getting into the room. Underfloor heating makers commonly quote a maximum combined thermal resistance for the floor covering of about 0.15 m²K/W, so check your system's figure before choosing carpet, thick underlay or a large rug.

How much does underfloor heating cost in an extension?

Public cost guides price wet underfloor heating per square metre rather than per project. As a general range, a system for a small extension comes to around £1,500 – £3,500 for the manifold, pipe loops and commissioning, with screed and the floor finish priced separately.

Back to Single Storey Rear Extension

Phase 5 · Specification · Task 02 of 03

Heating a Rear Extension: Radiators, Underfloor Heating or Extending Your Existing System

Choose how to heat a rear extension: radiators, wet or electric underfloor heating, whether your boiler copes, and what Part L asks when a system is extended.

10 min read4-8 hours of decisions, questions to the heating engineer and drawing reviewUpdated 2026Premium

Flat infographic of four heating decisions for a rear extension in build order: a heat loss calculation, choosing radiators or underfloor heating, fixing the floor build-up, and sizing and ordering the emitters

Prerequisites

Complete this first

Rear Extension Room Layout

A cold rear extension usually gets blamed on the boiler. Often the boiler isn't the problem. Nobody worked out the heat loss, the radiators were sized by eye, or they were fitted wrong, and it only shows in the first properly cold week.

Heating is also the decision most likely to be made by default: the slab goes in with whatever insulation the drawings show, and a plumber tees a couple of radiators off the nearest pipe. When that turns out wrong, the fix means lifting a floor or living with a room you avoid in January.

The heating engineer sizes and installs the system. The approach is yours to choose, and the builder needs your answer early, because underfloor heating changes the floor build-up and the finished level.

The rules below are for England. Wales has its own Approved Documents and Scotland uses Technical Handbooks.

Do this first

Two things come first. The furnished plan from laying out a rear extension shows which walls stay free for radiators and how each zone gets used. The floor construction comes from ground floor oversite and floor build-up. Settle the heating alongside the floor insulation specification, not after it.

What this guide covers

01Why a rear extension is hard to heat
02Four ways to heat it
03Will the boiler cope?
04What the Building Regulations ask when you extend a system
05Notification, commissioning and who owns the design
06Radiators or underfloor heating in an open-plan room
07Underfloor heating changes the floor
08The decisions and their deadlines

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General guidance for homeowners in England (and Wales where we say so), not professional advice. Every site is different: check anything that affects your build with a qualified professional, your local council or building control. Terms

Frequently asked questions

Reference guides for this task

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