The Short Answer
Fill an empty cavity first. Ground source is an option where there's land, but on the bills alone it rarely pays. In DESNZ's measured figures, filling the cavity cut gas use by a median 16.6% in detached houses.
A detached house usually needs a bigger heat pump than a semi or terrace because it's bigger, not because it's less efficient. On our model a 130 sq m late-1970s detached house needs 8.5kW with unfilled cavity walls and 100mm in the loft, and 6kW with the cavity filled and the loft at 270mm. The DESNZ medians for those size bands are about £12,700 and £12,200, before the £7,500 Boiler Upgrade Scheme grant in England and Wales.
Estimate the Saving for a Detached House
Pre-set for a detached house on gas, using the median gas use of detached houses in England and Wales (£1,210 bill), with the install at the DESNZ median for the 8.5kW our model gives a 130 sq m late-1970s detached house. Put your own bill in if you know it.
1. What heats your home today?
Pick whichever your main heating uses.
Add up last year's bills, or use the typical figure for your home below.
3. What size is your home?
We use this to estimate the size of heat pump you'd need.
4. Do you have solar panels?
Free electricity from your roof cuts what the heat pump costs to run.
5. Your options
Both are on to start with. Leave them on unless you know they don't apply.
Your estimated saving
£499a year
Over 15 years that's around £7,485 off your heating bills.
Now: Gas boiler
£1,210 / year
With heat pump
£711 / year
You save
£499 / year
What about the upfront cost?
- Heat pump install (DESNZ median for this size)
- £12,700
- Less government grant
- -£7,500
- You pay
- £5,200
- A new gas boiler would cost
- ~£2,750
If your boiler needs replacing anyway, the heat pump costs about £2,450 more upfront (before any radiator upgrades), earned back in roughly 4.9 years from running cost savings. That assumes you have the gas supply disconnected, so you stop paying its standing charge (about £108 a year). Keep gas for cooking and it takes longer. It leaves out a heat pump's dearer servicing.
Like the look of £499 a year?
These are estimates. The only way to know your real saving is a free quote from an MCS-certified installer. Takes about 60 seconds, no obligation.
How we worked this out
Energy prices: Ofgem price cap for 1 October to 31 December 2026 (Direct Debit, GB average): gas 7.97p/kWh, electricity 26.32p/kWh. There is no VAT on domestic electricity from 1 October 2026 to 31 March 2027; the gas figure includes 5% VAT. Heat pump tariff blended at 20p/kWh (an estimate for Cosy Octopus, E.ON Next Pumped or EDF Heat Pump Tracker on a heat pump load profile, not a quoted rate). Heating oil 11.31p/kWh (111.52p/litre plus 5% VAT, over 10.35 kWh/litre; BoilerJuice UK average, 23 October 2026). LPG at the price you enter (we start from 75p a litre, our estimate: there's no published average).
Heat pump efficiency: a seasonal efficiency of 3.5. That sits between two measured figures: a median of 2.80 across 291 air source heat pumps in the government's Electrification of Heat trial (installed in 2020 and 2021, whole system), and an average of 4.01 across the 307 systems on heatpumpmonitor.org with a full year of data when we checked in October 2026, a self-selected group of air and ground source systems, mostly high temperature capable R290 units. The break-even with a 90% gas boiler on a standard tariff is a SCOP of about 2.97.
Typical bill ranges: for a 3-bed using 9,500 to 10,300 kWh of heat a year (DESNZ's median and mean gas use for 3 bedrooms in England and Wales, through a 90% boiler): gas £950 to £1,000 (with the standing charge, which the saving assumes you stop paying by having the gas supply disconnected), oil £1,250 to £1,350, LPG £1,200 to £1,300 on our LPG price, direct electric heating £2,500 to £2,700. At these prices a unit of heat from oil costs about 1.5 times as much as from gas, and from LPG about 1.4 times, which is why the saving from switching is bigger.
Solar: A 4kW system in central England generates around 3,800 kWh a year on our estimate (950kWh per kW). On the government's SAP 10.2 self-use formula, a typical 3-bed's heat pump uses about 350 kWh of that a year without a battery, and about 510 kWh with one (8 kWh usable). Most of its demand falls in winter, when panels make least, and the calculator works it out for your bill.
Install costs: DESNZ Boiler Upgrade Scheme statistics (August 2026), median cost of real installs by size: about £11,500 (4-6kW), £12,200 (6-8kW) and £13,900 (10-12kW). For the home size this page starts on, we've used the median for 8.5kW instead: about £12,700. New gas boiler about £2,750 (our estimate). The £7,500 grant is the Boiler Upgrade Scheme (gov.uk) for England and Wales; £9,000 for oil or LPG heating with no mains gas connection until March 2027, which this calculator applies when you choose oil or LPG (if you also have a mains gas connection, take that extra off).
Estimates only. Your actual saving depends on the installer, flow temperatures, your home's heat loss and your electricity tariff. Always get a heat loss survey from an MCS installer for an accurate quote.
Heat Pumps in Detached Houses: Bigger System, More Choice
A detached house has no party walls, so every wall faces the outside. It also has more choice: up to two air source units under permitted development in England, and a ground source system if you have the land.
Why Detached Houses Need More Heating
DESNZ's National Energy Efficiency Data-Framework puts the median detached house's gas use at 13,771 kWh a year, against 10,574 kWh for a semi and 10,012 kWh for all homes. Those are weather-corrected figures for mid-May 2024 to mid-May 2025, with cooking and hot water included. DESNZ says detached houses use the most "as they tend to be larger than other houses, but on a per square metre basis they have a similar gas consumption to other types of houses (with the exception of bungalows)".
Through a 90% boiler, 13,771 kWh of gas is about 12,400 kWh of heat a year, a little high because some goes on cooking. That's our estimate for a detached house, and the figure the calculator above starts from.
Insulation: Do the Maths Before You Size the System
We ran the site's example detached homes through the same heat loss model as our size calculator (standard RdSAP assumptions, a -5.1C design day in the Midlands):
| The house | Heat loss on the coldest day | Heat pump size | DESNZ median for that size band |
|---|---|---|---|
| 4-bed late-1970s detached, 130 sq m, cavity not filled, 100mm loft | 8.3kW | 8.5kW | £12,700 |
| 4-bed late-1970s detached, 130 sq m, cavity filled, 270mm loft | 5.8kW | 6kW | £12,200 |
| 5-bed early-1990s detached, 180 sq m, cavity filled, 270mm loft | 6.9kW | 7kW | £12,200 |
Cavity walls
For cavity walls filled mid-May 2023 to mid-May 2024, DESNZ's median first-year saving in detached houses was 16.6% (2,130 kWh) across 285 homes, measured against similar homes with no measure recorded. Read it as a typical saving rather than one special to detached houses: DESNZ cautions that its savings by property type vary a lot between years, likely because the samples are small, and show no systematic differences between types. At the median it pays back the Energy Saving Trust's £3,900 for a detached house in about 23 years on gas. The Energy Saving Trust says cavity fill may not suit a home that's exposed to extreme weather or flooding, in poor condition, at risk of rooms overheating, or with cavities narrower than 50mm.
Solid walls
The Energy Saving Trust says houses built before the 1920s are more likely to have solid walls, which can't be filled. Insulating them inside or out is expensive; our heat pumps in old houses guide goes through the costs and the cheaper jobs to do first.
The loft
We'd top it up if the loft has under about 100mm, but don't expect it to shrink the heat pump much. The Energy Saving Trust puts an empty loft taken to 270mm at £1,100 for a detached house in Great Britain, and a top-up from 120mm at £800. DESNZ's measured median saving for lofts in detached houses was 5.7% (765 kWh), from a small sample of 108 homes. At gas prices that's about £61 a year, so a top-up pays back slowly.
Only a room-by-room heat loss calculation settles the size for your house, and an MCS installer has to do one. Put your own home in here (it starts as a different house, so set the type, floor area, age, walls and loft to match yours):
What Size Heat Pump Do I Need?
Describe your home and we'll estimate its heat loss on the coldest design day, using the same default values an EPC assessor uses and the design temperatures MCS installers must use.
Estimated peak heat loss
6.3kW
At -5.1C outside, 20C inside
Heat pump size to look at
7kW
Smallest common size at or above your heat loss
Heat loss per sq m
70W
Lower is easier for a heat pump
What air source heat pumps of 6kW to 8kW actually cost
£12,164
median installed price, before the grant
£4,664
after the £7,500 Boiler Upgrade Scheme grant
Median of 8,054 real installations in that size band in 2025/26. Source: DESNZ Boiler Upgrade Scheme statistics, August 2026 (installations with a paid voucher, England and Wales). The grant applies in England and Wales.
Where the heat goes
What would shrink the heat pump
- Fill the cavity walls: -1.5kW
- Replace the windows with modern double glazing: -0.6kW
- Top the loft up to 270mm: -0.3kW
An estimate, not a design. MCS installers must size a heat pump to cover at least 100% of a room-by-room heat load calculation to BS EN 12831-1:2017 (MCS standard MIS 3005-D), and that survey replaces this number. Our defaults: U-values from BRE's RdSAP 10 specification (June 2026), window area from RdSAP 2012 Table S4, design outdoor temperatures from MIS 3005-D Table 2 column B, a whole-house 20C indoors, a rectangular plan with 2.5m storeys, no thermal bridging allowance, and air leakage from SAP 10.2 with a minimum of 0.5 air changes an hour. Solid floors use the design temperature rather than the annual average MCS specifies, so that part errs high. Flats and homes with extensions or room-in-roof conversions will be less accurate.
Ground Source: Where It Gets Interesting
A ground source heat pump collects heat from pipes buried in trenches or drilled into boreholes. The Energy Saving Trust says you'll need land near the house that machinery can reach from a road, with no trees in the way. A new-build 3-bed needs about 2 trenches of 30 to 40 metres, and a larger or older, less insulated home needs a bigger loop. Where space is short, boreholes are about 20cm wide and 75 to 200 metres deep, and cost more.
The cost gap is large. The Energy Saving Trust puts a trench system at typically around £15,000. But DESNZ's 2025/26 median for ground source installs of 8 to 10kW was £21,298 (from only 61 installs), against £12,700 for air source of the same size: about £8,600 more. Across all sizes the ground source median was £26,200, with the middle half of April to June 2026 installs at £18,136 to £42,110. The grant is the same £7,500.
We'd choose ground source on other grounds (no room or permission for an outdoor unit, or a quoted price well below the median) rather than on the bills alone; a big house on its own doesn't close the gap. At our assumed SCOP of 4.0 against 3.5 for air source, ground source heats our 12,400 kWh for about £820 a year against £930, a difference of about £120. At those medians, that would take about 74 years to recover the extra cost. Our ground source guide goes further.
Compare the two for your house here. The calculator starts on a 3-bed, so set the size to large or very large. It works on its own standard house sizes, so its figures won't match the example houses above exactly:
Ground Source Cost & Payback Calculator
A realistic estimate for your home, and the question that matters most: is the ground loop worth paying for over a cheaper air source heat pump?
Sets your rough annual heat demand
Boreholes cost more but need far less space
Used to work out your running-cost saving
Ofgem cap 26.32p; a heat pump tariff blends to around 20p
Install after £7,500 grant
£15,000
about £22,500 before grant
Running cost a year
£625
at SCOP 4
Saving vs your fuel
£216/yr
Fuel only. Have the gas supply disconnected and you also save its £108 a year standing charge.
The real question: ground source vs a cheaper air source pump
Extra to install vs air source
£10,300
Compared with an air source pump of the same size, so it's smaller than the gap between the overall typical prices
It saves vs air source
£89/yr
So the extra pays back in
Longer than the heat pump will last
For most homes, air source is the better-value call. Ground source runs a little cheaper, but not by enough to recover its much higher install cost before the kit wears out. Pick ground source for the silence, the space, or a property air source can't handle, not to save money.
Estimates only. Ground source prices are our estimate from DESNZ's April to June 2026 ground source median and quartiles (a small sample, not split by size or loop type), and vary a lot with ground conditions and access; heat demand is our estimate; air source prices are DESNZ 2025/26 medians for the matching size. The £7,500 Boiler Upgrade Scheme grant applies to ground and air source alike (£9,000 for oil or LPG with no mains gas connection, until March 2027). Running costs assume SCOP 4 (ground) and 3.5 (air), our estimates, at your chosen electricity rate. Always get MCS-certified quotes for real numbers.
The Outdoor Unit: Up to Two on a Detached House
In England a detached house can have up to two air source heat pumps as permitted development, so you won't need a planning application. Among other conditions:
- each outdoor unit is no bigger than 1.5 cubic metres
- the 37 dB noise limit is met 1 metre outside a neighbour's habitable-room window or door (on the MCS 020 a) calculation)
- no unit is on a pitched roof, or above the ground floor on a wall facing the road
- the house isn't listed
In a conservation area or World Heritage Site a unit can't go on a wall or roof facing the road at all. Wales has similar rules, and Scotland and Northern Ireland their own. Our heat pump noise guide shows how the noise check works.
Radiators: Some May Need Upgrades
Heat pumps run at lower flow temperatures than gas boilers: 35C to 55C, against the 70C to 80C a gas boiler supplies, so some radiators may be too small. Your installer works it out room by room, but our estimate for a larger detached or poorly insulated home is 7 to 12 radiators upgraded, £1,050 to £4,800. Our radiators guide has a checker for each room.
What It All Costs
Heat pump costs are the DESNZ median for each size band (Boiler Upgrade Scheme statistics, 2025/26 installs by capacity), and radiators are our estimate. Treat the radiators as a possible extra: DESNZ's costs are installers' own reported figures for the system, labour and VAT, and DESNZ doesn't say whether radiator work is in them. It also notes installers may differ on whether the figure includes the grant.
| The house | Air source | Before grant | After the £7,500 grant |
|---|---|---|---|
| 4-bed late-1970s detached, 130 sq m, cavity not filled, 100mm loft | 8.5kW | £12,700 | £5,200 |
| 4-bed late-1970s detached, 130 sq m, cavity filled, 270mm loft | 6kW | £12,200 | £4,700 |
| 5-bed early-1990s detached, 180 sq m, cavity filled, 270mm loft | 7kW | £12,200 | £4,700 |
The grant is £9,000 if you heat with oil or LPG and have no mains gas connection, until March 2027. In Great Britain, ECO4 can help with insulation for some households on benefits until 31 December 2026; our 1930s semi guide sets out who qualifies for each scheme.
Running Costs
At the 1 October to 31 December 2026 cap (26.32p a kWh for electricity, 7.97p for gas), on our 12,400 kWh of heat:
- 90% gas boiler: about £1,100 a year
- Air source, standard tariff: about £930 a year, at a SCOP of 3.5
- Air source, heat pump tariff: about £710 a year, at our estimate of a 20p blended rate
- Ground source, standard tariff: about £820 a year, at our assumed SCOP of 4.0
Those are fuel costs only; if you have the gas supply disconnected, you also stop paying about £108 a year of gas standing charge, which the savings calculator above counts. On oil or LPG the saving is bigger; our running costs guide compares every fuel, and our solar panels and heat pump guide covers adding solar.
Getting Started
- Check the walls and loft. If the cavity is empty, fill it: it's the measure with the bigger measured saving of the two, and it changes the size of heat pump you need.
- Decide whether ground source is worth pricing. If you have the land and access, get a ground source quote alongside an air source one.
- Get quotes when the boiler is due. If yours is more than 10 years old, start now, and ask each installer for the heat loss calculation and which radiators they'd change.
Ready to get quotes for your detached house?
Get free quotes from MCS-certified heat pump installers near you.
More: Heat pumps in semi-detached houses | Heat pumps in bungalows | How the £7,500 heat pump grant works
Frequently Asked Questions
Get quotes for your detached house
Ask each installer for the heat loss calculation behind the size they quote, and if you're considering ground source, for an air source quote to compare. Free quotes from MCS-certified installers.