If you're choosing between two flats at a similar price — one in a converted Victorian or Edwardian house, one in a purpose-built block from the last twenty years — the Energy Performance Certificate is the fastest way to see how differently they're likely to run.
Homecost matched more than 750,000 flats in England and Wales to their most recent EPC and grouped them by the age of the building. Flats in buildings from before 1930 — almost always conversions of a house that was originally a single dwelling — rate C or above just 26.3% of the time. Flats in buildings from 2003 onward, almost all purpose-built apartment blocks, rate C or above 92.4% of the time. That's a 66-percentage-point gap between two flats that might look identical on a listings page (EPC Open Data, matched 8 September 2026).
The national picture
| Building age | Flats with a current EPC | Rated C or above | Rated E, F or G |
|---|---|---|---|
| Before 1930 (mostly conversions) | 150,645 | 26.3% | 22.7% |
| 1930–2002 | 474,508 | 55.4% | 10.4% |
| 2003 onward (mostly purpose-built) | 141,143 | 92.4% | 0.4% |
The bottom bands tell the sharper story. Nearly a quarter of pre-1930 flats sit in E, F or G — the three lowest ratings on the scale. Among post-2003 flats, it's fewer than 1 in 200. On these figures, a converted period flat is roughly 56 times more likely to land in the bottom three bands than a modern purpose-built one.
Across every flat with a certificate, regardless of age, 56.5% reach band C or above. That single average hides two very different populations — which is exactly why the building's age matters more than the headline number on any one listing.
Why the gap exists
EPC records don't say whether a flat is a conversion or purpose-built — Homecost uses the building's construction age as a proxy, and it's a reliable one. Purpose-built flats were rare before the 1930s, so a flat in a pre-1930 building is almost always a subdivision of a house built as a single home. Flats from 2003 onward are overwhelmingly purpose-built schemes, built under building regulations that require far higher insulation and airtightness standards than anything built a century earlier.
The practical difference is mostly about the walls. Most buildings from the 1930s onward have a cavity wall — two skins of brick with a gap that can usually be filled with insulation without altering how the building looks. Buildings from before 1930 typically have solid walls, which need internal or external solid wall insulation to improve — a bigger, more disruptive job, and in a flat it usually means agreement from the freeholder or the whole building, not just the person who owns that unit. If the building is listed or sits in a conservation area, external changes such as new windows or external wall insulation may also need separate consent from the local planning authority before any work can go ahead (gov.uk conservation area guidance; Planning Portal — altering a listed building).
None of that means a converted flat is a bad buy — many buyers choose one deliberately, for the ceiling height, the proportions or the location of older housing stock that a new block can't replicate. It does mean the EPC's recommendations page is worth reading in full, not just glancing at the letter on the front. It lists the specific measures assessed for that property — loft insulation, draught-proofing, heating controls — and roughly what each would cost and save. For a solid-wall conversion flat, some of the more disruptive measures (external wall insulation, for instance) may not be practical or may need consent, so the realistic list of what you could actually do is often shorter than the full recommendations page suggests.
What this means for running costs
The EPC rating is a modelled score, not a metered bill — it standardises for a typical household so different properties can be compared, and your actual bill will depend on how you use the property, your tariff and the weather. But the direction is consistent: lower-rated homes cost more to heat to a given standard. Homecost's separate analysis of EPC data for a typical 3-bed semi found a wide gap in modelled running costs between band A and band G — see Energy cost by EPC rating: what a 3-bed semi costs to run for the worked figures; the same direction of travel applies to flats, even though the exact numbers differ by property type.
If you're viewing a specific flat, the fastest way to see the local picture is to check its street on the Homecost postcode tool — try Nottingham's NG1 as an example — which shows the EPC band alongside the sale history and stamp duty for that address.
The wider pattern by city
The national split holds in every major city Homecost has checked, though the size of the gap varies. For the full city-by-city breakdown, see Converted flats vs purpose-built blocks: the 2026 EPC gap. If you want the basics of the A–G scale itself — who assesses it, how long a certificate lasts, and why it's a legal requirement to have one — start with EPC Ratings Explained.
Before you view
A few practical questions worth asking about any converted flat before you view or offer:
- What's the EPC rating, and when was it last assessed? Certificates last 10 years, so an older one may not reflect recent work.
- Is the building listed, or in a conservation area? This affects what improvements are realistically possible.
- Who owns the roof, windows and external walls — the freeholder, a management company, or is it split? Solid-wall insulation on a converted flat is rarely a job one leaseholder can do alone.
- What does the EPC's recommendations page actually list as feasible for this property, not just the theoretical maximum saving?
This is general information about how EPC ratings and building consents work, not financial, legal or property advice. Speak to a qualified surveyor, conveyancer or energy assessor before acting on anything above.
For more buyer guides like this one, browse the full Buyer Guides section.