Retrofitting underfloor heating into an existing home

Retrofitting is different from a new build. The floor already exists, the door heights are set and the heating pipes are already somewhere else. The best route depends on what the floor is made of, how much height you can lose, and how much disruption you can live with.

This guide sets out each route with real heights, a planning checklist and a priced example. For whether retrofit is worth doing at all, see is underfloor heating worth it?

Can you retrofit underfloor heating into an existing house?

Yes. Most existing floors can take underfloor heating, using a system matched to the floor type. Here are the usual routes.

Many people retrofit room by room, starting with a kitchen or bathroom when it is being refitted. The Energy Saving Trust notes that underfloor heating downstairs with radiators upstairs can cost less and cause less disruption.

How much floor height will retrofit underfloor heating add?

Overlay boards add about 15 to 25mm before the floor finish, electric mats add about 3 to 4mm in the tile adhesive, and joisted systems from above add little or nothing because the chipboard replaces the existing deck.

Add the floor finish and adhesive on top of these figures. Then check every place the new height meets something fixed:

• Door thresholds and the bottom of internal doors, which may need trimming
• Kitchen plinths and the gap under worktops for dishwashers and fridges
• The top or bottom stair riser, which must stay in line with the others
• WC pans, pedestals and skirting boards
• Meeting points with rooms that are not being heated

How do you fit underfloor heating over an existing concrete floor?

Lay routed insulation boards or low-profile panels directly on the existing floor, press the pipe into the grooves, and tile on top. This is the most common retrofit for kitchens and extensions with a sound concrete floor.

1. Check the floor is sound, clean, dry and flat. Fill hollows before you start.
2. Plan the loops and mark fixed units, so no pipe goes under them.
3. Lay the boards tight together. MaxHeatPanel boards are 1200 x 600mm and cover 0.72m² each, with grooves at 150mm centres.
4. Press in the pipe, 12mm in 18mm boards or 16mm in 25mm boards, and run the tails to the manifold or pump pack.
5. Pressure test before you cover anything. See pressure testing underfloor heating.
6. Tile direct onto the board using a flexible adhesive suited to heated floors.

Overlay floors have less mass above the pipe than screed, so they warm faster, but they can need a slightly higher water temperature. Polypipe recommends a design flow temperature of 45 to 50°C for its overlay range and quotes about 79W/m² at a 50°C mean water temperature for its 18mm panel. Check output against your room's heat loss. Read more in low-profile overlay underfloor heating.

Should I dig out the floor and use screed instead?

Dig out only if the floor is coming up anyway, is uninsulated, or you cannot lose any height. It gives the best result, with full insulation under the pipe and a screed that suits low flow temperatures, but it is a major job.

A traditional screed over underfloor heating pipe is typically 65 to 75mm of sand and cement on top of the insulation. A flowing liquid screed is thinner: one maker specifies a minimum of 25mm, nominally 30mm, of cover over the pipe. Add the insulation below, and digging out often means removing well over 100mm of old floor. Screed also needs drying time before the heating can be commissioned. See screed depths and drying times.

Building Regulations can apply. Approved Document L sets an improved U-value of 0.25W/m²K for renovated floors, where that is technically and functionally feasible and pays back within 15 years. New floors in extensions should meet 0.18W/m²K. Ask building control whether your work counts as renovation.

How do you retrofit underfloor heating into a timber floor?

From above, lift the floorboards and replace them with 22mm grooved chipboard, or fit aluminium spreader plates over the joists. From below, where you can reach the joists from a cellar or ceiling void, fit plates from underneath and leave the floorboards down.

Whichever method you use, insulate between the joists under the heating so heat goes up into the room, not into the ceiling below. See underfloor heating in joisted timber floors for build-ups.

How do you connect retrofit underfloor heating to the existing heating?

For one room, a single room mixing pump pack can run one underfloor loop straight off a radiator or boiler circuit, with no manifold. For several rooms, fit a manifold with a pump pack that matches your heat source.

• Gas or oil boiler, several rooms: manifold plus a blending valve pump pack, which mixes the water down to floor temperature.
• Heat pump: manifold plus a non-mixing pump set, because the water is already at floor temperature.
• One room off radiators: single room mixing pack, holding 35 to 60°C to the loop.

Put the manifold where loops from several rooms can reach it with short tails, often under the stairs or in a cupboard. Sleeve pipe in conduit where it passes through walls and doorways.

Controls without chasing walls. Wireless thermostats avoid cutting cable runs into finished plaster. The Heatmiser neoAir (£80) runs on batteries and talks to the UH8-RF wireless wiring centre (£134).

What about electric underfloor heating for a retrofit?

Electric mats are the simplest retrofit for one small room, because they add only 3 to 4mm and need no pipework. They cost more to run per kWh, so keep them to bathrooms, en-suites and small rooms used for short periods.

The electrics matter. BS 7671 Section 753 requires floor heating to be protected by an RCD rated at 30mA or less. In England, a new circuit, or work on circuits in a room with a bath or shower, is notifiable under Part P, so use a registered electrician. Over timber, an uncoupling membrane (from £57 for 5m²) helps protect the tiles.

Worked example: what does a retrofit kitchen cost?

A 12m² kitchen on a sound concrete floor, run off the existing boiler as one loop, tiled direct. Kit only, inc VAT, October 2026.

• 18 MaxHeatPanel 25mm boards (12 ÷ 0.72 = 16.7, plus cuts): £216
• 16mm 3-layer PERT-EVOH pipe, 100m coil: £61 (about 80m in the floor plus tails, one loop)
• Single Room UFH Mixing Pump Pack with Wilo 6m pump: £231
• Heatmiser DS1 dial thermostat: £24

Total about £532, or £44 per m². Add tile adhesive, tiles and labour, and allow for trimming doors and plinths. If the units take up floor space, the heated area and pipe go down. For larger floors and whole-house prices, see underfloor heating cost in the UK.

Products for this job

• Overlay boards for fitting over existing floors
• MaxHeatFix Low Profile Panel, 15mm, £89 per pack of 10 (3.6m²)
• MaxHeatPanel tile-direct routed XPS board, £11 to £12
• MaxHeatDeck cement-faced XPS board, 20mm, £15
• Fit-from-below aluminium double spreader plate
• Single Room UFH Mixing Pump Pack, £231
• Heatmiser neoAir wireless thermostat
• Heatmiser UH8-RF wireless wiring centre

Related guides: Low-profile overlay boards, Can you fit underfloor heating in every room? and Underfloor heating floor systems explained.

Frequently asked questions

Can you put underfloor heating on top of an existing floor?

Yes. Overlay boards and low-profile panels go on top of a sound, flat existing floor and add about 15 to 25mm before the finish. You can tile directly onto tile-direct boards such as MaxHeatPanel and MaxHeatFix.

Can you retrofit underfloor heating without lifting the floor?

Yes, two ways: lay overlay boards on top of a solid floor, or fit spreader plates from below a timber floor where you can reach the joists from a cellar or ceiling void.

What is the thinnest water underfloor heating for a retrofit?

Low-profile panels for 12mm pipe are the thinnest wet option. The MaxHeatFix panel is 15mm high and takes tiles directly on top.

Can you retrofit underfloor heating with a heat pump?

Yes, but check output. Overlay floors can need higher water temperatures than screed, so design for the lowest flow temperature the room's heat loss allows and connect through a non-mixing pump set.

Do I need building regulations approval to retrofit underfloor heating?

It depends on the work. Digging out and replacing a floor may count as renovating a thermal element under Approved Document L, and electric underfloor heating on a new circuit or in a bathroom is notifiable under Part P in England. Check with building control.

Can I keep my radiators and add underfloor heating?

Yes. Many retrofits add underfloor heating to one floor or one room and keep radiators elsewhere, all on the same boiler or heat pump.

Will retrofit underfloor heating be as good as in a new build?

It can be, if insulation and heat loss are right. Overlay floors warm faster than screed but can need a slightly higher water temperature. An uninsulated floor will lose heat downwards whichever system you use.

Sources: Polypipe: UFH for existing floors, Energy Saving Trust: underfloor heating, GOV.UK: Approved Document L, Volume 1, Mortar Industry Association: screeds with underfloor heating, Gyvlon: flowing screed data pack, IET: BS 7671 Section 753 floor heating, LBHF Building Control: notifiable electrical work