Converted flats vs purpose-built blocks: the 2026 EPC gap
If you are comparing two flats at the same price in the same city, the single biggest clue to their running cost is not the postcode or the service charge. It is the decade the building went up.
Homecost matched more than 800,000 flats in England and Wales to their most recent Energy Performance Certificate (EPC) and grouped them by the age of the building. The split is stark. Flats in pre-1930 buildings — which are almost all conversions of older houses — rate just 26.0% at EPC band C or above. Flats built since 2003, which are almost all purpose-built apartment blocks, rate 92.8%. That is a 66.8 percentage-point gap on the same property type, and it holds in every major city we checked.
The national picture
Every figure below is the latest certificate for each distinct flat, grouped by the building's recorded age band (EPC Open Data, fetched 2 August 2026).
| Build era | Flats with a current EPC | Rated C or above | Rated E, F or G |
|---|---|---|---|
| Pre-1930 (mostly converted houses) | 134,886 | 26.0% | 21.2% |
| 1930–2002 | 509,999 | 55.0% | 9.8% |
| 2003 onward (mostly purpose-built) | 157,342 | 92.8% | 0.5% |
Read down the middle column and the gradient is almost mechanical: 26% → 55% → 93%. The right-hand column is arguably the sharper story. More than one in five pre-1930 flats sits in the worst three EPC bands (E, F or G); among post-2003 flats it is one in two hundred. On the certificate model, a converted period flat is roughly forty times more likely to land in the bottom bands than a modern purpose-built one.
Across all flats with a certificate, regardless of age, about 60% reach band C or above — a headline that hides the split completely. The average is the midpoint of two very different populations.
A quick note on what we can and can't measure
EPC records do not state whether a flat is a conversion or purpose-built. We use build era as a proxy, and it is a strong one: purpose-built blocks were rare before the 1930s, so a pre-1930 flat is almost always a subdivision of a house originally built as a single dwelling; and flats built from 2003 onward are overwhelmingly purpose-built apartment schemes. The figures below are therefore build-era comparisons standing in for the conversion-versus-purpose-built question buyers actually ask.
Two further caveats apply throughout. An EPC rating is a standardised model, not a metered bill — actual energy use depends on the household, the tariff and the weather. And the register is a large sample, not a census: these are distinct flats with a current, linkable certificate, not every flat in the country.
The same story in every city
We ran the same split across sixteen cities and London boroughs with meaningful flat numbers in both eras. The direction never changes — only the size of the gap.
| City / borough | Pre-1930 flats | Pre-1930 C+ | Post-2003 flats | Post-2003 C+ | Gap |
|---|---|---|---|---|---|
| Nottingham | 238 | 10.5% | 755 | 95.1% | 84.6 pp |
| Leicester | 134 | 16.4% | 228 | 93.9% | 77.5 pp |
| Birmingham | 253 | 14.2% | 1,478 | 90.0% | 75.8 pp |
| Cardiff | 352 | 19.6% | 2,523 | 93.3% | 73.7 pp |
| Bristol | 1,536 | 22.2% | 1,773 | 93.7% | 71.5 pp |
| Hackney | 2,097 | 30.9% | 346 | 95.7% | 64.8 pp |
| Southwark | 2,663 | 25.5% | 495 | 90.1% | 64.6 pp |
| Islington | 2,154 | 31.0% | 397 | 95.2% | 64.2 pp |
| Camden | 2,458 | 29.1% | 134 | 93.3% | 64.2 pp |
| Lambeth | 4,924 | 27.7% | 415 | 87.7% | 60.0 pp |
| Leeds | 520 | 24.4% | 3,076 | 84.3% | 59.9 pp |
| Tower Hamlets | 971 | 34.2% | 871 | 93.5% | 59.3 pp |
| Wandsworth | 7,284 | 32.7% | 478 | 91.0% | 58.3 pp |
| Liverpool | 465 | 32.3% | 2,019 | 88.9% | 56.6 pp |
| Manchester | 193 | 34.7% | 1,489 | 88.9% | 54.2 pp |
| Newcastle upon Tyne | 4,570 | 41.8% | 573 | 92.1% | 50.3 pp |
The list is sorted widest-gap first. Nottingham is the extreme: barely one in ten of its period-conversion flats reaches band C, against 19 in 20 of its purpose-built stock. But even Newcastle, where old flats do best, shows a 50-point gap. Prime central London is no exception — Camden and Westminster's Georgian and Victorian conversions rate about as poorly as Nottingham's. Price and prestige do not buy a warmer certificate; build era does.
Why the era matters so much
The mechanism is building physics, not neglect. Homes built before roughly 1930 typically have solid (uninsulated) masonry walls, which lose far more heat than the cavity walls that became standard from the 1930s and the insulated construction required from the 2000s. When a period house is later carved into flats, that older fabric — solid walls, single-glazed legacy windows, individual older heating systems — usually stays. The Energy Saving Trust notes that solid walls let through significantly more heat than insulated cavity walls, and Historic England documents the practical and consent constraints on retrofitting older and listed buildings.
Purpose-built flats built from 2003 onward, by contrast, were constructed under successively tighter editions of the Building Regulations' energy standard (Approved Document Part L), with insulation, double glazing and modern or communal heating designed in from the start. That is the whole of the 66-point national gap in one sentence: the fabric was either built to keep heat in, or it wasn't.
The wider Homecost data set shows the same age gradient across all property types — terraced houses lag their local all-stock EPC wherever the terraces are older, and city-centre flat districts vary widely on EPC depending on how new the blocks are. Flats are simply the clearest case, because the same city holds both the oldest and the newest examples side by side.
What it means if you're comparing flats
A worse EPC band does not make a converted period flat a bad home — many buyers prize the ceiling heights, the streets and the character, and the rating can often be improved. But it does signal a higher likely running cost and, frequently, solid walls that are more expensive and more restricted to insulate. Two flats at the same asking price can carry very different energy profiles, and the EPC band — free to check on any listing — is the quickest tell. Ofgem's energy price cap sets the unit rates that turn any efficiency gap into pounds; how much a given band difference costs a particular household depends on how they live in the flat.
For a like-for-like view of the all-in monthly cost of any address — mortgage at the current Bank of England quoted rate, council tax, energy from the EPC and stamp duty for your situation — you can search a postcode on Homecost, such as central Nottingham, the city with the widest gap in our table. It is also worth reading our plain-English guide to what each EPC band actually means for buyers and how the greenest and least-green housing stock compares across UK cities.
This is general information about energy ratings, not financial or retrofit advice. Speak to a qualified adviser — a retrofit assessor, surveyor or energy specialist — before acting on any efficiency measure.
Based on more than 800,000 England-and-Wales flats with a current linkable EPC and a recorded build era, cross-referenced with HM Land Registry property records, fetched 2 August 2026. See more of our market analysis or browse all guides.