Short answer: An extension is one of the best places for underfloor heating, because the floor is new and can be insulated and screeded with the pipe in it. There are three common ways to heat it. Use a single room mixing pump pack off a nearby radiator circuit for one room. Add a new zone on an existing manifold if the house already has wet UFH. Fit a new manifold with its own pump set for a larger extension. Electric UFH suits small areas. Large glazed areas lose a lot of heat, so plan closer pipe centres or a higher output, and check the floor can meet the heat loss.
Whether it is a kitchen extension, a garden room or a conservatory, a new floor gives you the chance to fit underfloor heating properly from the start, with good insulation under it. This guide covers how to connect the heating to your existing system, when electric makes sense, insulation in the new slab, glazing heat loss and zoning an open-plan space.
If you are still weighing up the costs, see underfloor heating cost in the UK. For floor build-ups, see floor systems explained.
How do I connect extension underfloor heating to my existing heating?
It depends on how big the extension is and what heats the rest of the house.
| Option | Best for | Needs |
|---|---|---|
| Single room mixing pack | One room, one loop | A nearby radiator circuit |
| New zone on existing manifold | Houses that already have wet UFH | Spare ports or room to extend |
| New manifold and pump set | Larger or multi-room extensions | Flow and return from the heat source |
| Electric UFH | Small areas, tiled floors | Electrician, suitable circuit |
Single room mixing pump pack off a radiator circuit
A single room mixing pump pack takes hot water from a nearby radiator circuit, mixes it down to a floor-safe temperature and pumps it round one UFH loop. There is no manifold. Our packs, such as the single room mixing pump pack with Wilo pump and the ESBE version with Grundfos UPM3 Flex, hold a mixed flow of 35 to 60°C.
- It runs one loop only. Our design rules cap a 16mm loop at 100m including tails, so the pack suits one modest room.
- The radiator circuit must be live when the floor calls for heat, or the floor gets nothing.
- Set the mixed temperature to suit the floor build-up, not the top of the range.
The underfloor heating with a boiler guide covers connecting to a radiator system in more detail.
A new zone on an existing manifold
If the house already has wet underfloor heating, the simplest route is often to run new loops from the existing manifold. Each new loop needs a spare port, an actuator and a wiring centre zone.
- Ports: the Dualmix manifold can take a 1 port extension. Our design rules allow a maximum of 12 ports per manifold.
- Tails: the distance from the manifold to the extension counts towards the loop length. A long run of tails through the house can use up much of a 100m loop before it reaches the new room.
- Pump and heat source: check the pump and boiler or heat pump can carry the extra load.
- Controls: add a thermostat for the new zone, wired back to the wiring centre.
If the manifold is far away or full, a second manifold near the extension, fed from the heat source, is usually neater. See how many manifold ports you need and how a manifold works.
Heat pump homes
If the house runs on a heat pump, the extension should be designed for its low flow temperature. That usually means closer pipe centres and a non-mixing pump set. See underfloor heating with a heat pump.
When does electric underfloor heating make sense in an extension?
Electric UFH is quick to fit and doesn't need any plumbing. It makes most sense for small tiled areas, such as a small garden room, a boot room or a utility in the extension, where running a pipe from the heating system would cost more than it is worth.
- Mats: 100, 150 or 200W/m², laid in the tile adhesive. Our 200W/m² mat is aimed at conservatories and rooms with higher heat loss.
- Loose cable: 10 or 20W/m cable laid in an uncoupling membrane, for awkward shapes. The 20W/m cable can also be laid in a screed.
- Controls: a floor-sensing thermostat such as the Heatmiser neoStat-e or Slimline-E.
For a large extension that you will heat every day, wet UFH is usually much cheaper to run. See water vs electric underfloor heating. Electric UFH must be connected by a qualified electrician, with RCD protection.
How much insulation does a new extension floor need?
A new floor in an extension must meet the Building Regulations for energy efficiency. In England, Approved Document L (2021) sets a limiting U-value of 0.18W/m²K for a new floor in an existing dwelling. A U-value measures how much heat passes through, so lower is better. Your designer or building control will confirm the insulation thickness for your floor.
Underfloor heating has its own insulation guidance too. BS EN 1264 gives minimum insulation thermal resistance under the heating, which Danfoss data lists as:
| Floor is above | Minimum R value |
|---|---|
| A heated room | 0.75m²K/W |
| Ground, or an unheated space | 1.25m²K/W |
| Outside air | 2.00m²K/W |
In a new ground floor slab, the Building Regulations figure will normally need more insulation than these minimums. With underfloor heating, the insulation usually goes above the concrete slab and under the heated screed, so the heat goes up into the room rather than warming the slab. Fit edge insulation around the walls to stop heat leaking sideways and to let the screed expand.
A screed floor on good insulation is the usual build-up for a new extension. See screed for underfloor heating, and the insulation boards and castellated panels ranges.
Is underfloor heating enough for a room with lots of glass?
Not always. Glass loses heat much faster than an insulated wall, so extensions with bifold doors, roof lanterns and full-height glazing can lose more heat than the floor can give.
BS EN 1264 limits the floor surface to 29°C in occupied areas. Typical design data puts that at roughly 100W/m² at a normal room temperature. A 1m wide strip along an outside wall, called the peripheral or edge zone, can run up to 35°C, which gives more output where it is needed most.
To get more heat from the floor:
- Closer pipe centres: 100mm spacing instead of 150mm or 200mm, especially along the glazing.
- A separate edge loop: a tighter loop along the glazed wall.
- Higher electric output: a 200W/m² mat for electric.
- Good floor finish: tile and stone pass heat best. Thick rugs and high tog carpet cut output. See floor coverings.
Always start from a room-by-room heat loss calculation. Divide the room's heat loss by the heated floor area. If the answer is well above what the floor can give, plan extra heat, such as a radiator or trench heater, or improve the glazing. See pipe spacing and heat output for the figures.
Conservatories and building regulations
A conservatory can be exempt from parts of the Building Regulations, but heating it can affect that. The Planning Portal says an exempt conservatory should have independent heating with its own temperature and on/off controls, and should be separated from the house by external-quality walls, doors or windows. The rules on conservatories are outside the scope of this guide, so check with your local building control before you design the heating.
How should I zone an open-plan kitchen extension?
An open-plan kitchen, dining and living space often needs several loops, because one loop can't cover the area. It can be one zone or several.
- One zone, several loops: simple and fine if the whole space is used the same way. All its actuators open together on one thermostat. The Heatmiser UH8 can run up to 6 actuators per zone.
- Two or three zones: better if the space has distinct areas. A kitchen that gains heat from cooking and a glazed seating area that loses heat fast often want different settings.
Practical points for kitchens:
- Don't lay pipe or cable under fitted units, islands or appliances. Keep loops to the open floor.
- Put thermostats away from ovens, hobs, direct sun and draughty doors, or use a remote sensor.
- Consider a floor probe if the finish is wood or vinyl, to limit the floor temperature.
- Plan where the manifold goes early. A utility or under-stairs cupboard is ideal.
See zoning and wiring and thermostats explained.
Products for this job
- Single Room UFH Mixing Pump Pack with Wilo Pump
- ESBE Single Room Mixing Pump Pack with Grundfos UPM3 Flex
- 1 Port Dualmix Manifold Extension
- Dualmix Stainless Steel Manifold
- Electric Underfloor Heating Mat 200W/m²
- Edge Insulation Foam Roll
- All water underfloor heating
Frequently asked questions
Can I run underfloor heating in an extension off my radiators?
Yes, for one room, with a single room mixing pump pack. It needs the radiator circuit to be on when the floor calls for heat.
Can I add extension loops to my existing manifold?
Yes, if it has spare ports or can be extended, and the loops stay within length limits including the tails. Our design rules allow up to 12 ports per manifold.
Is underfloor heating enough to heat a conservatory?
Sometimes, but glazing loses heat fast. Check the heat loss against the floor output, use closer pipe centres or a higher output mat, and plan backup heat if needed.
What pipe spacing should I use in a glazed extension?
Closer spacing, often 100mm, along glazed walls, with wider spacing elsewhere if the heat loss allows. A heat loss calculation decides it.
Should the extension be a separate zone?
Yes. It will usually behave differently from the rest of the house, so give it its own thermostat.
When can I turn the heating on after screeding?
Not until the screed maker's curing time has passed, and then gradually. See screed for underfloor heating and commissioning.
Design your extension heating with MaxHeatUFH ProLoop. Upload a floor plan and get a Quick Design and a fully priced kit in about 3 minutes. Start your design
Sources: Danfoss UFH design guide (BS EN 1264), Part L 2021 U-values for extensions, Planning Portal: conservatories and building regulations, Warmup: BS EN 1264 surface temperature limits