Heat Pump for Extension or Conservatory UK 2026

Look up your existing heat pump's rated output + compare to current heat demand:

• Heat pump nameplate rating: typically 5, 7, 9, 12 kW at standard conditions (7C outdoor, 35C flow).
• Current peak heat demand: from your installer's heat-loss calc, OR estimate from current property (typical UK 3-bed semi 5-7 kW at design conditions).
• Spare capacity: rated output minus current demand. Healthy install has 15-25% headroom; tightly-sized install has under 10%.

Example: 7 kW heat pump serving a 5.5 kW demand has ~21% headroom = ~1.5 kW available for an extension. A small extension (10-15m2) at 100-150 W/m2 design heat demand uses ~1.5 kW - just fits.

If your heat pump has spare capacity for the new space, extend the existing system:

• Add radiators or underfloor heating loop to the new room.
• Tie into the existing primary circuit via T-fittings + balancing valves.
• Update heat pump controller to recognise the new zone (most modern units support zone configuration).
• Re-commissioning: rebalance flow rates after install + verify all rooms still reach setpoint.

Typical install cost (small extension):

• 2-3 radiators + pipework: GBP 800-1,500.
• Or underfloor heating loop (10-15m2): GBP 1,200-2,500.
• Plumbing tie-in + commissioning: GBP 500-1,000.
• Total: GBP 1,500-3,500 typical.

SCOP impact: minimal if heat pump still has 10%+ headroom after adding extension. May see 0.1-0.2 SCOP reduction during cold-snap operation when the unit is closer to its capacity limit. Worth it for the marginal extra heated space.

If extension is 20m2+ OR heat pump is already near capacity, two options:

Option A: replace existing heat pump with larger model.

• Remove existing 5-7 kW unit; install 9-12 kW replacement.
• Reuses existing pipework, cylinder, radiators where compatible.
• Cost: GBP 6,000-10,000 net of BUS (full new install ex some shared infrastructure).
• Best for: extensions large enough to justify completely new sizing; properties planning long-term ownership.

Option B: add second small heat pump with hydraulic separation.

• Install a second 3-5 kW heat pump dedicated to the extension.
• Hydraulic separator (low-loss header) decouples the two heat pumps.
• Each unit runs independently + optimised for its zone.
• Cost: GBP 4,000-7,000 for second unit + install + hydraulic separation.
• Best for: extensions with different occupancy patterns (e.g. annex, granny flat); properties where retaining the original unit is preferred.

Choose Option A when: single-occupancy household with consistent heating preferences; extension shares thermal profile with rest of house; long-term ownership planned.

Choose Option B when: annex or independent space with different occupancy / hours; existing heat pump in good condition + recently installed; budget pressure (smaller second unit cheaper than full replacement).

Conservatories are notorious for heating challenges:

• Glazing accounts for 60-80% of surface area - even with double glazing (U-value ~1.4), total heat loss per m2 is 3-5x a brick wall.
• Roof typically polycarbonate or glass - poor U-value (~2.0-3.5).
• Floor often uninsulated if added as later extension.
• Air infiltration high at glazing-to-frame joints.

Result: a 15m2 conservatory might have 3-5 kW peak heat demand - similar to an entire heated bedroom in the main house.

Heat pump compatibility:

• Low flow temperatures (35C) struggle to push heat into the cold space; conservatory rarely reaches setpoint.
• Standard radiator output undersized for the massive heat loss.
• Underfloor heating works better if floor insulation upgraded - radiant heat compensates for cold glazing.

Best practice for conservatory:

1. Upgrade thermal envelope FIRST. Replace single-glazed with double or triple. Insulate floor + replace polycarbonate roof with insulated panels (if planning regs allow).
2. Specify oversized emitters. Larger radiators or UFH with low-temperature design.
3. Consider electric panel heating as supplementary. For occasional use spaces, dedicated electric heating may be more practical than extending the heat pump system.
4. Building Regs check. Many conservatory extensions exceed glazing % limits + require dispensation. Designer should confirm at planning stage.

Extensions over 50m2 (or that change the building's overall energy performance) need SAP compliance demonstrating:

• New construction meets current U-value standards (270mm loft, filled cavity walls, double glazing minimum).
• Adequate heat source for the combined property heat demand.
• Renewable energy contribution where applicable.

Your architect or designer handles the SAP calculation at planning stage. They'll confirm whether the existing heat pump can cover the combined demand OR whether a heat pump upgrade is required to meet Building Regs.

Conservatory extensions have separate rules - typically can be exempt from Building Regs if under 30m2 + separated from the house by external doors. Heating decision is then yours rather than regs-driven.