Heating a garden room or home office: the options compared, and why a small air-to-air heat pump usually wins

The best garden room heating for a room you work in most days is a small air-to-air heat pump: a 2.5kW-class wall split that heats in winter, cools in summer, and delivers each kilowatt-hour of heat for about a quarter of the cost of any plug-in heater. On the Ofgem price cap for 1 July to 30 September 2026 (26.11p/kWh), a well-insulated 3m x 4m garden office costs around £2.09 a working day to keep warm with a panel heater, oil-filled radiator, infrared panel, or underfloor mat, and 42p to 52p with a heat pump. Over a 100-day winter that is roughly £209 against £42 to £52.

If you only use the room a few hours a week, do not spend £2,000 on it; a £40 oil-filled radiator on a timer is the right answer. For everyone else, here is how much heat the room needs, what each option costs, and the electrics, planning, noise, damp, and grant questions nobody puts in one place.

Why garden rooms are hard to heat

A garden room is a small box with three or four external walls, a floor over cold air, a thin roof, and a glazed front. It shares no warm surfaces with the house, has no thermal mass, so it cools within an hour of the heater going off, and almost never has gas. Every option below is electric except the log burner.

Insulation decides whether it can be heated at all. Home Energy Quotes puts a single-skin timber wall at a U-value "in the region of 2W/m²K or worse", while an insulated stud wall with 50 to 100mm of PIR gets towards 0.3, a five- to sixfold cut in heat loss. Buying new, ask for the insulation spec for floor, walls, and roof, plus double glazing. Converting a shed, insulating a garden room comes before any heater.

How much heat does a 3m x 4m garden room need?

Electric radiator suppliers size on about 100W of heat per square metre for an averagely insulated room, 80W/m² for a well-insulated new build, and 130W/m² for a poorly insulated one (Electrorad's guide; Best Electric Radiators uses the same 100W/m² and sizes up for "several outside walls, or a north-facing aspect"). A garden room has all its walls outside and a glazed front, so treat 100 to 130W/m² as realistic for a modern insulated room.

For 12m² that is 1.2kW to 1.6kW to hold the room warm on a cold day. A 2kW heater is the right size because it also warms the room quickly from cold; a 2.5kW-class split such as the Daikin Sensira 25 delivers 2.8kW and throttles down to 1.3kW once the room is warm. For other sizes, use our BTU calculator or read what size air conditioning do I need.

Garden room heating options compared

Every plug-in heater turns 1kWh of electricity into 1kWh of heat, so the wattage on the box sets the cost per hour. A heat pump moves heat from outside instead of making it, which is why a modern R32 split delivers 4 to 5kWh of heat per kWh of electricity (a SCOP, or seasonal coefficient of performance, of 4 to 5).

Assumptions: Ofgem cap, 1 July to 30 September 2026, 26.11p/kWh; "per hour" at full rated output; "per working day" a well-insulated 12m² room needing an average 1kW of heat over 8 hours including warm-up; "per winter" 100 working days, roughly November to mid-March; purchase prices UK retail bands, September 2026.

Rates correct as of September 2026 at 26.11p/kWh (Ofgem cap, 1 July to 30 September 2026). October's price cap moves electricity to 26.32p/kWh, which adds about £2 to a resistive winter and 50p to a heat pump winter.

The heat pump sum: electricity used equals heat wanted divided by SCOP, so 8kWh of heat at SCOP 4 is 2kWh, and 2 x 26.11p is 52p. A poorly insulated room needing an average 1.5kW adds half again: £3.13 a day and £313 a winter resistive, 78p and £78 at SCOP 4. For gas and storage heaters too, see cheapest way to heat a room.

Every plug-in heater in a garden room costs 26p per kilowatt-hour of heat. A small split costs 5p to 7p. Over a few winters, that gap pays for the split.

The options in turn

Panel, convector, and oil-filled heaters

This is what most garden rooms come with: Green Retreats includes "a thermostatically controlled oil filled radiator" with every building, and SMART Garden Offices fits a wall-mounted radiator as standard. They cost £40 to £115, plug into a socket, and warm an insulated room in 15 to 20 minutes.

Best for: a room used a few hours a week, or a stop-gap in your first winter. At 52p an hour the cost only hurts when the hours add up, and five days a week they add up to £200 or more.

Infrared panels

An infrared panel warms whatever it points at rather than the air. A 600W panel above a desk costs 16p an hour and keeps the person at the desk comfortable while the room stays cool; to bring the whole room to 20°C you need 1.2kW to 2kW of panels, which is fan-heater money for £150 to £400 a panel. The claims are picked apart in infrared heating panels vs air-to-air heat pumps.

Best for: one desk in a large or draughty room.

Electric underfloor heating

Mats come in 100W/m², 150W/m², and 200W/m²; ThermoSphere and Garden Room Hub both say 100W/m² is enough for a well-insulated garden room. A warm floor over a cold void is genuinely pleasant, but it runs at the same 26p per kWh of heat as a panel heater, costs around £820 to £1,385 supplied for a 4m x 3m room on Garden Room Hub's figures, takes 30 to 60 minutes to warm up, and must go in before the floor.

Best for: a room being built now, as a comfort layer rather than the main heat.

Log burner

Lovely and expensive. Checkatrade puts the stove at around £950, installation at around £2,000, and an external twin-wall flue at up to £2,550 more, so £3,000 to £5,000 before you light it (late 2024 figures). You need a HETAS-registered installer, Building Regulations Part J compliance, an Ecodesign stove, and a DEFRA-exempt appliance in a smoke control area. Kiln-dried logs cost about 11p per kWh of heat, and a stove cannot be put on a timer for 8am.

Best for: a garden lounge where the fire is the point. Not a garden office.

A small air-to-air heat pump (wall split)

A wall split is an air conditioner running in reverse: an indoor unit on the wall, an outdoor unit outside, and two small refrigerant pipes between them. Every wall split we review heats as well as cools, and the entry-level ranges are cheap. Unit-only prices for a 2.0 to 2.5kW pair, September 2026:

Supplied and fitted by an F-Gas certified installer, expect £1,500 to £3,000 for a single room; Energy Saving Trust quotes around £1,900. You get summer cooling in the same box, which matters more in a glazed garden room than anywhere else, a dry mode for damp days, a weekly timer and an app so it is warm when you sit down at 8am (the Sensira and MSZ-AY have Wi-Fi built in, the Comfora ships with the adapter, and the Seiya is Wi-Fi compatible), and 0% VAT on the installed system until 31 March 2027. The full case is in our air-to-air heat pumps guide; the cooling side of the same decision is in home office and garden room air conditioning.

Best for: any garden room or home office used three or more days a week. It is the only option that gets cheaper the more you use it.

A monobloc if an outdoor unit is out of the question

If the room sits hard against a boundary, a through-wall monobloc puts everything in one indoor unit breathing through two 202mm holes in the wall. The Olimpia Splendid Unico EVO heats at 1.5 to 2.4kW with a COP of 3.1 to 3.4, so it costs about a third more to run than a split but still a third of a panel heater, is sealed so needs no F-Gas engineer, lists at £1,489 to £1,849 before fitting, and is louder (26 to 42 dB(A)) because the compressor is in the room with you. If you can have an outdoor unit, buy a split. The alternatives are in air conditioning without an outdoor unit.

The electrics: armoured cable, Part P, and spare capacity

Whatever you fit, the room needs a proper supply: a steel-wire-armoured (SWA) cable run underground from the house consumer unit to a small consumer unit in the garden room, with a CAT 6 data cable alongside, as Green Retreats describes it. The electrician checks whether the house board has spare capacity or needs a sub-board. This is notifiable work: the Planning Portal says "any electrical work you carry out within your home, garden, garage, shed or other storage building must comply with the requirements of the building regulations", and a new consumer unit or circuit is notifiable under Part P, so use a registered competent person who can self-certify.

A heat pump draws less than the heater it replaces: a 2.5kW split delivering 2.8kW of heat pulls around 0.7kW, and the Sensira listing notes it "Requires standard 13A electrical supply". If the room already has a 2kW heater on its own circuit, the supply is adequate.

Planning: the garden room and the outdoor unit are two separate questions

The garden room. In England an outbuilding is usually permitted development if it is no more than 2.5m high within 2m of a boundary (4m with a dual-pitched roof, or 3m, further in), covers no more than half the garden with other additions, and is not forward of the principal elevation. Building Regulations do not normally apply under 15m² with no sleeping accommodation, or between 15 and 30m² if it is at least 1m from any boundary or substantially non-combustible.

The outdoor unit. Class G of the permitted development order covers an air source heat pump "on a dwellinghouse or a block of flats; or within the curtilage of a dwellinghouse or a block of flats, including on a building within that curtilage", so a unit on or beside a garden room can be permitted development. The same conditions apply as on the house: not "solely used for the purpose of cooling" (a reverse-cycle split qualifies, a cooling-only unit does not), no more than 1.5m³, and one unit per semi or terrace or two per detached house, counted across the whole property. It must also meet MCS 020-a: 37 dB(A) at 1m from the neighbour's nearest habitable-room window or door. Conservation areas, listed buildings, and flats have extra limits, and Scotland, Wales, and Northern Ireland differ. The house rules are in air conditioning planning permission; for a garden building, check with your local planning authority before you book the installer.

Noise: the neighbour's fence is closer than you think

A garden room usually sits at the bottom of the garden, so the outdoor unit is often nearer a neighbour's back window than the house is. Small outdoor units are quiet, around 46 dB(A) at 1m for the Sensira, but 37 dB(A) at the neighbour's window is the test, and the installer's MCS 020-a calculation decides whether the unit can go on the boundary side. Ask for it before agreeing a position. Indoors, 18 to 20 dB(A) on the lowest fan speed is quieter than a laptop.

Condensation and damp when the room is unheated

The winter problem in garden rooms is the nights and weekends, not the working day. Moist air from a working human, a kettle, and wet coats meets a cold glazed wall and condenses; GOV.UK's damp and mould guidance says the fixes are heating, ventilation, and insulation together. Left cold and shut for a fortnight at Christmas, a garden room comes back smelling damp with mould behind the desk.

Two habits prevent it. Keep some background heat on the coldest nights: a heat pump holding 8°C to 10°C costs about 31p a night at SCOP 4 for a room averaging 0.3kW, against £1.25 with a panel heater on its frost setting. And ventilate: open the trickle vents, or run the heat pump's dry mode for an hour after a damp day.

Will the £2,500 grant pay for a garden office heat pump?

Rarely, so budget without it. The Boiler Upgrade Scheme's £2,500 air-to-air grant requires the heat pump to "replace an existing fossil fuel or electric heating system" and to be "sized to meet the full space heating needs of the property". A split heating one outbuilding does neither. And as of September 2026 MCS is still completing the air-to-air scheme: the first products (Daikin multi-splits) were listed on the MCS Product Directory only on 1 September, and no installations had been completed under the grant as of July. Read the air-to-air heat pump grant if you are also considering one for the house.

Which should you buy?

• A few hours a week: a £40 to £60 thermostatic oil-filled radiator, or the convector the room came with.

• Three or more days a week: a 2.5kW-class wall split, fitted by an F-Gas certified installer, for £1,500 to £3,000. It saves £150 to £170 a winter against a panel heater in a well-insulated room and £230 or more in a leaky one, so it pays back in six to ten winters, with summer cooling thrown in.

• No room for an outdoor unit: a Unico EVO monobloc, accepting more noise and a lower COP.

• Building the room now: insulate to the 0.3W/m²K wall standard, size the SWA cable for a heat pump, and put the outdoor unit on the drawings.

Browse our reviewed wall-mounted split systems, the wider heating guides and buying guides, or find a local F-Gas certified installer to quote for the unit and the cable run together.