The average UK household spends around £1,700 per year on energy. The right energy efficiency upgrades can cut that by 40–60% — and increase your property's EPC rating, value, and comfort. But the order in which you do things matters enormously. Spend £8,000 on a heat pump before insulating your walls and you'll have an efficient heat source in a leaky building.
This guide gives you the retrofit hierarchy: fabric first, then heating, then renewables.
1. Why the Order Matters: The Retrofit Hierarchy
The retrofit hierarchy is the established principle in PAS 2035 (the UK standard for domestic retrofit):
- Fabric first — Reduce heat loss through walls, roof, floor, windows, and doors
- Heating system — Once heat loss is reduced, install an appropriately sized, efficient heating system
- Renewables — Generate clean energy once your demand has been reduced
Doing them in the wrong order is costly:
- Installing a heat pump in a draughty house means it runs at full capacity continuously and your bills remain high
- Installing solar PV before reducing consumption means you're generating energy you're also wasting
- Re-doing insulation after underfloor heating has been installed is significantly more expensive
2. Get a Free Assessment First
Before spending money, understand where you are.
Energy Performance Certificate (EPC)
An EPC rates your home from A (most efficient) to G (least efficient), with D being the UK average. It includes a list of recommended improvements with estimated cost and potential saving. Find your current EPC at epcregister.com (most properties have a lodged EPC). A new EPC costs £60–£120 from a Domestic Energy Assessor.
ECO4 / CERO Assessment
If your household is in receipt of qualifying benefits (Universal Credit, Pension Credit, certain others), you may be eligible for a free ECO4 assessment and funded measures. Contact your energy supplier or check gov.uk/energy-company-obligation.
TrustMark Retrofit Assessor (PAS 2035)
For a whole-house retrofit approach, a TrustMark Retrofit Assessor (qualified to PAS 2035) carries out a comprehensive assessment covering energy use, fabric, moisture, ventilation, heating, and renewables, and produces a Retrofit Assessment and a Medium-Term Improvement Plan. This is the gold standard for planning a multi-measure programme. Cost: £200–£500. Required for funded schemes (ECO4, Great British Insulation Scheme) on complex properties.
3. The Fabric-First Sequence
a. Loft Insulation
Why start here: The cheapest measure with one of the fastest paybacks. Heat rises — an uninsulated or under-insulated loft loses 25% of a home's heat.
Specification: 270mm mineral wool (or equivalent) is the current Building Regulations target. Many UK homes have 100mm or less.
Cost: £300–£500 DIY for materials; £300–£600 installed for a standard semi-detached house. Often free under ECO4 or Great British Insulation Scheme.
Payback: 2–3 years at current energy prices.
Common pitfalls:
- Ventilation: Blocking eaves ventilation with insulation causes moisture problems. Leave 50mm clear at the eaves and use breathable insulation membrane if needed.
- Cold water tank: If your cold water tank is in the loft, insulate around and over it but not beneath it — it needs to draw warmth from below to avoid freezing.
- Hatch: Insulate the loft hatch as well. An uninsulated hatch negates a lot of the benefit.
- Accessible vs inaccessible loft: The above costs are for accessible lofts. Cut loft or room-in-roof insulation is more complex and expensive (£2,000–£6,000+).
b. Cavity Wall Insulation (CWI)
Who has cavity walls: Most homes built after 1920 have cavity walls (two skins of masonry with a gap between). If your exterior walls are brick, check whether the bricks are laid in a uniform course (all 'stretchers') — this typically indicates a cavity.
Cost: £500–£1,500 installed (whole house). Often funded under ECO4 or the Great British Insulation Scheme.
How it works: Insulation (typically mineral wool, EPS bead, or polyurethane foam) is injected through small holes drilled in the outer leaf, which are then filled.
Who can do it: Only installers registered with CIGA (Cavity Insulation Guarantee Agency) or BBA-certified equivalent. CIGA registration is required for the insulation to be covered by a 25-year guarantee.
Potential problems:
- Bridging: Poorly installed insulation can bridge across to the inner leaf, creating cold bridges or moisture pathways. This is why registration matters.
- Moisture-exposed walls: Walls in areas of severe exposure (very exposed coasts, high rainfall areas) are not suitable for CWI without specialist assessment.
- Drill marks: Small filled holes will be visible on the exterior. On modern brick they are barely noticeable; on pointing-heavy or decorative brick they may require cosmetic attention.
External Wall Insulation (EWI) vs CWI: EWI applies insulation to the outside face of the wall. EWI can be applied to cavity walls where CWI has failed or is not suitable — but it is significantly more expensive.
c. Solid Wall Insulation
Who has solid walls: Homes built before approximately 1920, and some post-war properties. Solid walls have a single leaf of masonry (typically 230mm brick or 450–600mm stone).
Why it matters: Solid walls have a U-value of approximately 1.7–2.0 W/m2K compared to 0.5 W/m2K for an insulated cavity wall. Improving solid wall performance has a significant impact on heating bills — but the cost is high.
External wall insulation (EWI):
- Insulation boards fixed to exterior face, covered with render or cladding
- Cost: £8,000–£13,000 for a typical semi-detached house
- BCO notification required; planning permission needed in some conservation areas
- Increases wall thickness by 80–150mm at the eaves, requiring fascia/soffit extension
- Best performance: not limited by internal space; doesn't disrupt rooms
Internal wall insulation (IWI):
- Insulation boards or stud framework on interior face
- Cost: £3,000–£8,000
- Reduces internal floor area slightly (typically 70–100mm per wall)
- Disruptive: requires moving skirting boards, radiators, sockets, and architraves
- Breathability: In solid wall construction, only breathable materials should be used (wood fibre, hemp-lime) without a vapour barrier
Moisture management: Solid wall insulation changes the thermal gradient through the wall. A poorly designed system can cause interstitial condensation in the original masonry. Always use a specialist installer with solid wall experience.
d. Floor Insulation
Often overlooked, floor heat loss accounts for approximately 15% of total heat loss in an uninsulated home.
Suspended timber floor: Lift floorboards (or install via airbricks outside), then insert rigid insulation (PIR/PUR boards) between joists, supported on netting or timber battens. Cost: £500–£1,500 DIY for materials; £1,500–£3,000 installed.
Solid concrete floor: Insulation must go below the screed. This requires lifting the existing floor finish, laying an insulation slab (typically 100mm PIR), then re-screeding. Any underfloor heating pipes are embedded in the new screed. Cost: £2,000–£5,000+ depending on floor area and whether UFH is being installed simultaneously.
e. Windows and Doors
Secondary glazing vs replacement:
- Secondary glazing (internal secondary frame) adds a second pane without replacing the original window. Dramatically cheaper than replacement (£150–£400 per window DIY) and may be the only option for listed buildings. Can achieve U-values of 1.4–1.8 W/m2K with appropriate air gap.
- Replacement double-glazing achieves U-values of 0.8–1.4 W/m2K. Cost: £400–£800 per window installed.
U-value targets (Building Regulations Part L 2021):
- Windows (replacement): 1.4 W/m2K
- Doors (replacement): 1.4 W/m2K (glazed); 1.0 W/m2K (solid)
FENSA: All replacement windows and doors must be installed by a FENSA-registered (or equivalent Competent Person Scheme) contractor, or through a Building Regulations application. Non-FENSA installations without Building Regs sign-off create problems at sale.
4. Heating System
a. Boiler Upgrade
If your boiler is over 15 years old, upgrading to a modern condensing boiler will reduce fuel consumption by 15–30%.
Types:
- Combi: No hot water cylinder; heats on demand. Good for smaller homes.
- System: Requires a cylinder; good for homes with multiple bathrooms.
- Heat-only (regular/conventional): Requires both cylinder and header tank; typically found in older systems.
ErP rating: Modern condensing boilers have ErP (Energy-related Products) ratings; A-rated boilers achieve 90%+ seasonal efficiency.
Cost: £2,000–£4,500 installed (supply and install of boiler plus basic commissioning). Complex system changes cost more.
When to upgrade vs service:
- Service your current boiler annually — a well-maintained boiler runs efficiently
- Replace if: it's over 15 years old, efficiency has dropped significantly, repair costs exceed 50% of replacement cost, or you're planning to switch fuel type
b. Heat Pump
Types:
- Air Source Heat Pump (ASHP): Extracts heat from outside air. Suitable for most UK climates. Requires external unit (noise consideration: under 42dB at 1m from boundary is PD-compliant).
- Ground Source Heat Pump (GSHP): Extracts heat from ground via buried pipes or boreholes. More expensive to install (£15,000–£30,000) but more efficient; suitable for rural properties with adequate land.
Efficiency: Heat pumps produce 2.5–4kWh of heat for every 1kWh of electricity consumed (Coefficient of Performance, CoP, of 2.5–4). They work most efficiently at lower flow temperatures (40–55°C), which suits UFH and larger radiators.
Fabric-readiness: Before installing a heat pump, the building fabric should be in good condition. A heat pump in a poorly insulated building will run at high capacity, reducing efficiency and increasing bills. A SAP (Standard Assessment Procedure) calculation can determine whether the building is heat pump-ready.
Boiler Upgrade Scheme (BUS): £7,500 grant for ASHPs; £7,500 for GSHPs. Applied for by your MCS-certified installer before installation. Check eligibility at ofgem.gov.uk/BUS.
Planning: For listed buildings and in conservation areas, ASHP installation requires planning permission (PD rights removed). For other residential properties, PD applies subject to the 42dB noise limit and 1m setback from boundary.
c. Underfloor Heating (UFH)
Wet UFH: Hot water circuits embedded in screed. Requires boiler or (ideally) heat pump; low flow temperatures (35–45°C); very comfortable; runs cheapest per kWh. Screeding depth: 65–75mm.
Electric UFH: Resistive heating cables under tiles or boards. Cheap to install (£50–£150/m2) but expensive to run (direct electricity). Good for bathroom floors; not recommended as the primary heat source.
Retrofit considerations: Wet UFH in a retrofit requires lifting existing floors and re-screeding. Factor in screed drying time (65–75 days minimum for 65–75mm screed before floor finishes). Coordinate with floor insulation installation — both involve lifting the floor.
5. Renewables
Solar PV
Typical system: 4kWp (kilowatt peak) = approximately 10–12 panels. Generates roughly 3,500–4,000 kWh per year in southern England; 3,000–3,500 kWh in Scotland.
Cost: £5,000–£8,000 installed for a 4kWp system with a good inverter.
Smart Export Guarantee (SEG): You can export surplus electricity to the grid and receive payment under the SEG. Rates vary by supplier (3–15p/kWh). Register via your energy supplier.
Planning: Solar panels on the principal elevation of a listed building require planning permission. For other residential properties, PD applies subject to no overhang beyond the roof, and not exceeding the highest point of the roof.
Battery Storage
A home battery (typically 5–10kWh capacity) stores surplus solar generation for use in the evenings.
Cost: £3,000–£5,000 for 5kWh capacity, installed.
Value: Most valuable if you have a time-of-use electricity tariff (e.g. Octopus Go, Agile) that allows cheap off-peak charging. Also provides resilience during outages if specified as an off-grid-capable system.
6. Grants and Schemes in 2025
| Scheme | What it covers | Who qualifies | Maximum benefit |
|---|---|---|---|
| ECO4 | Insulation, heating (boilers, heat pumps) | Benefits recipients; some low-income households | Fully funded measures |
| Great British Insulation Scheme (GBIS) | Loft, cavity wall, solid wall insulation | EPC D–G; some means-tested; some open market | Fully funded or heavily subsidised measures |
| Boiler Upgrade Scheme (BUS) | ASHP (£7,500), GSHP (£7,500) | All owner-occupiers | £7,500 grant |
| Home Upgrade Grant (HUG2) | Insulation, heat pumps, solar (off-gas-grid homes) | EPC D or below; not connected to gas grid | Varies by local authority |
| Disabled Facilities Grant | Adaptations for disabled persons | Disabled occupants (means-tested) | Up to £30,000 |
| Warm Homes Plan | Broad energy efficiency programme (upcoming) | TBC — expected broad eligibility | TBC |
Check simpleenergyadvice.org.uk for a personalised overview of available schemes based on your postcode and circumstances.
7. EPC Rating Improvement Guide
| Improvement | Typical EPC rating uplift | Estimated installed cost |
|---|---|---|
| Loft insulation (top up to 270mm) | E to D or D to C | £300–£600 |
| Cavity wall insulation | F to E or E to D | £500–£1,500 |
| Solid wall insulation (internal) | F to E or E to D | £3,000–£8,000 |
| Solid wall insulation (external) | G to E or F to D | £8,000–£13,000 |
| New condensing boiler | E to D or D to C | £2,000–£4,500 |
| Air source heat pump | D to C or C to B | £8,000–£15,000 (after BUS grant) |
| Solar PV (4kWp) | D to C or C to B | £5,000–£8,000 |
| Double glazing (whole house) | F to E | £4,000–£10,000 |
| Secondary glazing | E to D | £1,500–£3,000 |
| Floor insulation | D to C (contributing) | £1,500–£5,000 |
Where to Start: Decision Table by EPC Rating
| Current EPC rating | Recommended first step | Estimated cost | Estimated annual saving |
|---|---|---|---|
| G | Loft insulation + cavity wall insulation (if applicable) | £800–£2,100 | £400–£700 |
| F | Loft insulation (if not done) + cavity/solid wall insulation | £1,000–£9,000 | £300–£600 |
| E | Solid wall or cavity wall insulation (if not done) + boiler upgrade if over 15 years old | £2,000–£8,000 | £200–£400 |
| D | Boiler upgrade or heat pump + floor insulation | £2,000–£15,000 | £150–£300 |
| C | Solar PV + battery storage (if tariff suitable) | £5,000–£13,000 | £300–£600 (inc. SEG export) |
| B | Solar PV optimisation + battery storage or EV charger | £3,000–£5,000 | £100–£200 additional |
Costs assume no grant funding. ECO4, GBIS, and BUS can significantly reduce out-of-pocket costs for qualifying works — check eligibility before committing. For a complete, up-to-date guide to every grant scheme available in 2025 — including grant amounts, eligibility tables, and how to apply — see our energy efficiency grants UK 2025 guide.
Build your personalised energy efficiency roadmap at Renovate Me — upload your EPC, set your budget, and get a step-by-step plan.
For a detailed guide to heat pump costs, the £7,500 BUS grant, MCS certification requirements, and a full suitability checklist, see our heat pump installation cost UK guide.