Heat Pump for a Semi-Detached House

Heat Pumps in Semi-Detached Houses: A Solid Match

Semi-detached houses make up a huge chunk of UK housing stock, and they're well-suited to heat pumps. You've got one party wall (less heat loss than a detached house), usually decent outdoor space for the unit, and in many cases, the insulation work has already been done. Let's look at what you're actually dealing with.

Heat Loss: More Than a Terrace, Less Than Detached

A semi-detached sits in the middle of the heat loss spectrum. With one shared party wall, you lose less heat than a detached house, but more than a mid-terrace with two neighbours. A typical semi (depending on age, size, and insulation) loses somewhere between 10,000 and 16,000 kWh of heat per year.

That range drives the system size decision. Well-insulated: you're looking at a 7–8kW heat pump. Poorly insulated, larger property: you might need 9–10kW. Get a heat loss survey done before accepting a quote. It's not something you want guessed at.

Insulation: What Do You Actually Need?

The answer depends entirely on your home's current state. Here's how to think about it:

If cavity walls are already filled and loft is insulated

Good news: you may not need to do anything before installation beyond checking your radiator sizing. Many semis built post-1990 or updated in the last 20 years are already in reasonable shape. Ask your installer to confirm with a proper assessment rather than assuming.

If cavity walls are unfilled

Get them filled. It makes a significant difference, though it costs more than most people expect. The Energy Saving Trust puts it at around £2,200 for a typical Great Britain home (around £1,000 in Northern Ireland). That's one figure for a typical house rather than a per-house-type table, so treat it as the middle of your quote range, not a price list. Do this before the heat pump goes in.

If your loft insulation is thin or missing

Top it up to 270mm. The Energy Saving Trust puts a bare loft taken to 270mm at £750 in Great Britain, saving around £190 a year on a semi. If you already have 120mm, topping up costs about £600 and saves about £17 a year, which is a very different proposition. Check what's actually up there before you book anyone.

If walls are solid (older semis, some 1900–1920s stock)

You're in Victorian territory. Internal or external wall insulation becomes necessary, which adds cost and complexity. See our Victorian house guide for the full picture.

The Outdoor Unit: Side of the House Is Usually Ideal

Most semis have side access: a gate and passage down the side of the house to the back garden. This is a great location for the outdoor unit. It's out of sight from the front, sheltered from the worst weather, accessible for maintenance, and close to the house for short pipe runs. If you don't have side access, a rear garden ground mount works just as well.

Planning permission isn't usually required. Heat pumps on the side or rear of a semi are typically permitted development, as long as noise limits are met and you're not in a conservation area.

Radiators: Do a Proper Check

Heat pumps run at lower flow temperatures than gas boilers. If your radiators were sized for 70–80°C flow (as most are in a gas system), some may be undersized for the 45–55°C flow that a heat pump delivers efficiently. Your installer should calculate this room by room.

In a typical semi with good insulation, you might need 3 to 5 radiator upgrades. Budget £800 to £2,000, which is a quote range rather than a published figure. If you're planning a kitchen or bathroom renovation anyway, underfloor heating in those rooms is a great complement to a heat pump.

Cost Breakdown by Insulation Status

Running Costs

Here's the bit that changed in October 2026. A semi needs roughly 10,000 to 16,000 kWh of space heating plus about 2,000 kWh for hot water, so 12,000 to 18,000 kWh of heat a year. That's a typical range for the archetype, not a measurement of your house: only a room-by-room MCS heat loss calculation settles it. At the site-standard SCOP of 3.5 and the current cap:

• Heat pump: 3,430 to 5,140 kWh of electricity at 26.32p = £900 to £1,350 a year
• 90% gas boiler: 13,300 to 20,000 kWh of gas at 7.97p = £1,060 to £1,590, plus £108 of gas standing charge = £1,170 to £1,700 a year

The heat pump is £270 to £350 a year cheaper on a standard tariff. We used to say these were broadly level pegging, and on the July 2026 cap they were. The October 2026 cap put gas up 8.7% while electricity rose 0.8% and lost its VAT until 31 March 2027, which put the heat pump in front. Break-even against a 90% gas boiler is now a SCOP of 2.97.

A heat pump tariff improves it further. Cosy Octopus, the one designed for heat pumps, gives three cheap windows a day (04:00 to 07:00, 13:00 to 16:00, 22:00 to midnight) at 51% below the regional day rate, with a 16:00 to 19:00 peak at 50% above. It isn't a simple cheap-overnight deal and Octopus prices it regionally, so check the numbers for your area rather than assuming.

As an investment on its own the sum is still slow: £1,500 to £5,500 net to save £270 to £350 a year. The time it makes obvious sense is when the boiler is dying anyway, because then you're comparing the heat pump against the £2,750 you'd have spent on a new combi, not against zero.

Is Your Semi a Good Candidate?

Ask yourself:

• Do you have cavity walls that are already filled? (Check with your insurer or council, as many areas had mass cavity fill programmes in the 2000s)
• Is your loft insulation at 270mm or more?
• Do you have side or rear garden space for the outdoor unit?
• Is your boiler more than 10 years old?

If you answered yes to most of those, you're in good shape. The net cost after the Boiler Upgrade Scheme grant can be surprisingly affordable, and the long-term running cost savings are real.

More: How the £7,500 heat pump grant works | Heat pump running costs | Do you need new radiators for a heat pump?