The number one question we hear from landlords is: "What's this going to cost me?" It's entirely reasonable — and yet it's also the question that has the least satisfying single answer, because retrofit costs vary enormously depending on property type, current condition, the combination of measures needed, and how much grant funding can be unlocked.
What this guide will give you is a clear picture of typical costs per measure, realistic total project budgets by property archetype, an explanation of the main funding routes available, and a framework for thinking about the return on investment from your specific properties' perspective.
What Drives the Cost of a Retrofit?
Before looking at specific numbers, it helps to understand the variables. The cost of bringing a property to EPC C depends on:
- The starting point. A D-rated property needs less work than an E or F-rated one. A D that's just one measure away from C (say, adding loft insulation) could cost under £1,000. An E-rated Victorian mid-terrace with solid walls and a gas boiler is a very different proposition.
- Property construction type. Cavity wall properties are cheaper to insulate than solid wall properties. The cost differential is significant — typically £500–£1,500 for cavity wall fill versus £8,000–£25,000 for external or internal solid wall insulation.
- Heating system. If a boiler replacement or heat pump installation is required, this is typically the largest single cost item in any retrofit project.
- Which measures are installed together. Sequencing matters under PAS 2035: fabric-first improvements should precede heating system changes. Doing this correctly can avoid oversizing a heat pump or boiler, saving cost in the long run.
- Grant funding accessed. ECO4 and GBIS can cover measures entirely for eligible properties and tenants — transforming the economics for a significant proportion of the private rented stock.
Cost by Measure — What Each Improvement Typically Costs
The following figures are indicative costs for a standard 3-bedroom semi-detached property, installed by an accredited installer. Prices include VAT (currently 0% on most retrofit measures under the Energy Saving Materials Relief).
| Measure | Typical Cost | SAP Uplift |
|---|---|---|
| Loft insulation (top-up to 270mm) | £300 – £600 | 3–8 pts |
| Cavity wall insulation | £500 – £1,500 | 4–10 pts |
| Solid wall insulation (internal) | £8,000 – £15,000 | 8–15 pts |
| Solid wall insulation (external) | £12,000 – £25,000 | 8–15 pts |
| Floor insulation (suspended timber) | £800 – £2,000 | 2–5 pts |
| Double glazing (full house) | £4,000 – £8,000 | 2–5 pts |
| High-efficiency gas boiler replacement | £2,000 – £4,000 | 5–12 pts |
| Air source heat pump | £8,000 – £15,000 | 10–20 pts |
| Solar PV (3.5kWp system) | £5,000 – £8,000 | 5–15 pts |
| Smart heating controls | £150 – £400 | 1–3 pts |
| EPC assessment (RdSAP) | £80 – £150 | — |
Most D-rated properties need only one or two measures to reach C. The challenge is identifying the right combination — highest SAP uplift for the lowest cost — which is exactly what a proper retrofit assessment establishes.
Realistic Budgets by Property Type
Modern cavity wall house (1970s–1990s), currently EPC D
Typically needs loft insulation top-up, cavity wall fill (if not already done), and heating controls. Total cost: £800–£2,500. Often the simplest and cheapest upgrade path.
Victorian or Edwardian terrace, solid wall, currently EPC E
The most common challenge property type in Surrey and London. Needs either internal or external solid wall insulation, loft insulation, and often a boiler replacement or heating controls upgrade. Total cost without grants: £12,000–£30,000. With ECO4 or GBIS grant funding for insulation: potentially £3,000–£8,000 landlord contribution depending on eligibility.
1930s semi-detached, cavity wall, currently EPC D/E
Usually needs cavity wall insulation, loft insulation, and possibly a boiler upgrade. Total cost: £2,000–£6,000. Often highly grant-fundable.
Purpose-built flat (1960s–1980s), currently EPC D
Heating system upgrade and glazing replacement are typically the main levers. Shared wall construction means fabric improvements have less impact. Total cost: £3,000–£8,000.
Grant Funding — What's Available and Who Qualifies
This is the area where the economics of retrofit can shift dramatically. There are two main national schemes currently running, plus various local authority and Combined Authority schemes.
ECO4 (Energy Company Obligation, Phase 4)
ECO4 obligates energy suppliers to fund efficiency improvements in low-income and fuel-poor households. For private landlords, the key route is the ECO4 Flex mechanism, which allows local authorities to declare households eligible based on local criteria — this can include households on a range of means-tested benefits or households in properties with EPCs of D or below.
Under ECO4, the grant can cover insulation, heating upgrades, and sometimes first-time central heating installation. The landlord contribution is typically zero for fully eligible properties, though some schemes require a landlord contribution for higher-cost measures.
Great British Insulation Scheme (GBIS)
GBIS targets properties in the lower EPC bands (D, E, F, G) and households in or at risk of fuel poverty. It funds a single primary insulation measure — typically loft or cavity wall insulation. Unlike ECO4, GBIS has a general group track that doesn't require the tenant to be on benefits, making it more widely accessible to private landlords.
Boiler Upgrade Scheme (BUS)
The BUS provides a grant of £7,500 towards the installation of an air source heat pump (or £7,500 for ground source). This is available to landlords as property owners. It does not require tenant income eligibility. Applications are made through the installing contractor.
Grant stacking
In some cases it is possible to combine funding sources — for example, using GBIS for insulation and BUS for a heat pump replacement. A retrofit coordinator can identify the optimal combination for your specific property and tenant situation. This is often where the most significant financial savings are found.
Return on Investment — How to Think About the Numbers
Many landlords instinctively frame retrofit as a pure cost. It's worth reframing it through a few lenses:
- Compliance risk avoided. A £30,000 potential fine plus the inability to re-let the property makes the investment case straightforward for most properties.
- Asset value. Evidence is growing that EPC-rated properties command premium rents and sale prices, particularly in the London and commuter belt markets where Retrofield operates. A C-rated property in Guildford or Kingston is an increasingly different proposition to an E-rated one.
- Tenant retention. Lower energy bills for tenants reduce churn. This is a harder metric to quantify but real in practice — particularly relevant for longer-term tenants in houses.
- Mortgage eligibility. Several buy-to-let lenders now offer preferential rates for higher-EPC properties, and some are beginning to restrict lending on lower-rated stock.
The First Step Is Free
The most important thing to know is that you cannot accurately budget for retrofit works without a current, accurate energy assessment. A five-year-old EPC is not a reliable basis for planning — the RdSAP methodology has been updated, your property may have changed, and the improvement pathway depends on an accurate baseline.
A current RdSAP assessment costs £80–£150 and takes around an hour on-site. It gives you the definitive picture of your property's SAP score, the specific improvements needed to reach EPC C, the predicted uplift from each measure, and the starting point for identifying grant funding eligibility.
We offer free initial portfolio consultations for landlords across Surrey and London — a no-obligation conversation to help you understand the scale of the challenge and the funding landscape before committing to anything.