External wall insulation cost by property type
Costs are based on a full EWI system with mineral wool or EPS board insulation and a silicone thin-coat render finish. Brick slip or other premium finishes cost more.
| Property type | Approx. wall area | Typical total cost | Annual bill saving |
|---|---|---|---|
| Mid-terrace (2 exposed walls) | ~50-70 m2 | £5,000 - £9,000 | £200 - £350 |
| End-terrace (3 exposed walls) | ~70-100 m2 | £7,000 - £12,000 | £300 - £500 |
| Semi-detached | ~80-110 m2 | £8,000 - £14,000 | £350 - £550 |
| Detached house | ~130-200 m2 | £12,000 - £22,000 | £450 - £700 |
| Flat (external walls only) | ~30-60 m2 | £3,000 - £7,000 | £100 - £250 |
Bill savings are from Energy Saving Trust estimates for solid-wall gas-heated homes. London and South East installations are typically 15-25% higher than the national average.
What affects the cost of external wall insulation?
- Wall area: the primary cost driver — the total external wall surface area to be insulated and rendered.
- Insulation board type: EPS (expanded polystyrene) is the most affordable; mineral wool is fire-resistant and breathable (important for some masonry); phenolic board achieves higher performance in thinner profiles but costs more.
- Insulation thickness: typical thickness is 100-150mm. Greater thickness improves performance but costs more and extends the wall projection further.
- Render finish: thin-coat silicone render is standard and durable. Brick slips, timber cladding, or other premium finishes add £10-£30/m2.
- Scaffolding: required for all but single-storey work and typically adds £800-£2,500 to the project cost.
- Window and door reveals: every window and door requires careful detailing around the reveal, which adds labour time.
- Preparation: cracked or spalling masonry must be repaired before insulation is fixed; this can add £500-£2,000 on older solid-wall properties.
- Conservation area or listed building: additional planning and design requirements will increase costs.
- Region: London and South East are typically 15-25% more expensive than the national average.
Is external wall insulation suitable for your home?
External wall insulation is most appropriate for solid-wall homes — properties built before approximately 1920 using solid brick or stone construction without a cavity. It is also used on some post-war no-fines concrete, timber frame, or system-built properties where cavity wall insulation is not viable.
EWI adds thickness to the external wall (typically 100-160mm per face), which means window and door reveals become deeper and the appearance of the building changes. This is usually a positive aesthetic outcome when a quality render is specified, but it must be carefully designed to maintain weathertightness around all openings.
Properties in conservation areas or that are listed buildings will almost certainly require planning permission or listed building consent for EWI, and certain finishes may be restricted. Always check with your local planning authority before proceeding.
For properties that are not solid-wall — those built from the 1930s to 1990s with cavity walls — cavity wall insulation is a much cheaper and simpler alternative. External wall insulation is the right solution when the cavity is already filled, does not exist, or is not suitable for filling.
Planning permission and leasehold considerations
In conservation areas, EWI will almost certainly require planning permission as it changes the appearance of the building. Listed buildings require listed building consent. Leaseholders in flats must obtain landlord or management company consent before any external alterations. Failure to obtain the correct permissions can result in enforcement action and costly removal. Always check before instructing a contractor.
Grants for external wall insulation
External wall insulation is one of the most expensive home energy measures, but grant funding can significantly reduce costs. The ECO4 scheme (Energy Company Obligation) funds solid wall insulation for households receiving certain benefits or living in low EPC-rated homes. Where ECO4 applies, the work can sometimes be entirely free.
The Great British Insulation Scheme (GBIS) may also subsidise EWI costs for eligible households. Contact your energy supplier or check simpleenergyadvice.org.uk to see what you qualify for.
Some local councils also run their own grant schemes for solid wall insulation, often using funding from the Social Housing Decarbonisation Fund or the Home Upgrade Grant. Check with your local authority.
Even without grant funding, EWI adds long-term value to a property, improves its EPC rating (often by one or two bands), and makes it far warmer and more comfortable. The payback period is long at 15-25 years when self-funded, but the improvement in living conditions and thermal comfort is immediate.
Factor EWI into your solid-wall home purchase
If you are considering buying a pre-1920 solid-wall property, external wall insulation is a significant potential outlay. Our home-buying planner helps you model the cost of this and other energy improvements into your refurbishment reserve before you make an offer, so you can assess the true cost of bringing the property up to a comfortable EPC standard.