Retrofit feasibility study · London
Pre-1930 flats in London: conversions and mansion blocks
The questionHow many of London's older flats, the core of the private rented market, fall short of band C, and does it differ between converted houses and mansion blocks?
58.2% of certified pre-1930 flats in London are rated D or worse, and they hold 32.5% of London's privately rented certificates below band C. Conversions fare worse than mansion blocks, which are more constrained by heritage.
- Certified flats
- 438,305
- latest certificate per address
- Below band C
- 58.2%
- rated D to G
- Privately rented
- 49.2%
- of those with a known tenure
- In a conservation area
- 45.9%
- 5.1% listed
How the archetype was selected
Latest lodged RdSAP/SAP certificate per London address where property type is Flat or Maisonette and the construction age band is 'before 1900', 'Pre-1900', '1900-1929' or a single year before 1930. Split by the number of addresses in the GLA stock-model unit: 2-5 addresses (a proxy for houses converted to flats) and 6 or more (a proxy for mansion blocks and larger buildings). Neither the register nor the stock model records conversion directly.
Key figures
Data month: 2026-09 (lodgements to 2026-09-09). Shares are of the certified addresses in this archetype unless the row says otherwise.
| Band | Certificates | Share |
|---|---|---|
| A to B | 3,603 | 0.8% |
| C | 179,638 | 41.0% |
| D | 202,823 | 46.3% |
| E | 46,553 | 10.6% |
| F | 4,089 | 0.9% |
| G | 1,599 | 0.4% |
| Measure | Value |
|---|---|
| Certified flats | 438,305 latest certificates in 186,414 units |
| Below band C (D to G) | 255,064 (58.2%) |
| Below band E (F or G) | 5,688 (1.3%) |
| Certificate within 10-year validity at 22 October 2026 | 77.4% |
| Below C with a potential rating of C or better | 198,624 (77.9% of those below C) |
| Measure | Value |
|---|---|
| In a conservation area | 201,200 (45.9%) |
| Listed | 22,456 (5.1%): grade II 21,374, grade I or II* 1,082 |
| Measure | Value |
|---|---|
| Solid wall | 95.4% |
| Solid wall, no insulation | 92.1% |
| Single glazed | 27.3% |
| Secondary glazing | 4.3% |
| Mains gas main fuel | 89.9% |
| Measure | Value |
|---|---|
| Tenure recorded | 387,830 (88.5%) |
| Privately rented | 190,795 (49.2%) |
| Owner-occupied | 35.4% |
| Social rented | 15.4% |
| Measure | Value |
|---|---|
| F or G: below the E floor in law | 1,408 (0.7%) |
| Below C: the 2030 standard (confirmed policy, not yet law) | 108,220 (56.7%), which is 32.5% of all 333,113 privately rented London certificates below C |
| Measure | Value |
|---|---|
| Certified flats | 240,472 in 142,036 units |
| Below band C (D to G) | 62.8% |
| Below band E (F or G) | 1.2% |
| In a conservation area | 33.7% |
| Listed | 2.2% |
| Single glazed | 22.4% |
| Privately rented | 47.0% of those with a known tenure |
| Privately rented and below C | 60,680 (61.3%) |
| Measure | Value |
|---|---|
| Certified flats | 193,523 in 40,068 units |
| A to B | 2,664 (1.4%) |
| Below band C (D to G) | 52.3% |
| Below band E (F or G) | 1.4% |
| In a conservation area | 61.2% |
| Listed | 8.8% |
| Single glazed | 33.6% |
| Secondary glazing | 7.0% |
| Privately rented | 51.8% of those with a known tenure |
| Privately rented and below C | 46,352 (51.5%) |
| Borough | Certificates | Below C |
|---|---|---|
| Westminster | 40,320 | 49.3% |
| Kensington and Chelsea | 35,017 | 52.7% |
| Lambeth | 33,473 | 59.3% |
| Camden | 28,627 | 54.2% |
| Wandsworth | 25,799 | 58.8% |
| Recommendation | Code | Certificates | Share | Most common cost text, as lodged |
|---|---|---|---|---|
| 50 mm internal or external wall insulation | 7 | 394,290 | 90% | £4,000 - £14,000 (81.9% of rows) |
| Replace single glazed windows with low-E double glazed windows | 8 | 163,001 | 37.2% | £3,300 - £6,500 (83% of rows) |
| Low energy lighting for all fixed outlets | 35 | 160,218 | 36.6% | £15 (cost text varies by certificate) (11.9% of rows) |
| Draught proof single-glazed windows | 10 | 63,037 | 14.4% | £80 - £120 (82% of rows) |
| Floor insulation (suspended floor) | 57 | 62,948 | 14.4% | £800 - £1,200 (87.9% of rows) |
| Upgrade heating controls | 11-15 | 60,577 | 13.8% | £350 - £450 (75.2% of rows) |
| Replace boiler with new condensing boiler | 20 | 53,794 | 12.3% | £2,200 - £3,000 (84.2% of rows) |
| Floor insulation (solid floor) | 58 | 41,552 | 9.5% | £4,000 - £6,000 (87.9% of rows) |
Shares count certificates carrying the recommendation; across London, rows are loaded for 79-97% of the latest domestic certificates, depending on lodgement year, so these shares are lower bounds. Cost text is shown exactly as lodged, with the share of rows that carry it. It varies with the register version and is not a current price.
Findings
- 58.2% of the 438,305 certified pre-1930 flats are rated D or worse; 1.3% are F or G.
- 49.2% of those with a known tenure were lodged as privately rented, the highest share of the five domestic studies. The 108,220 privately rented flats below C are 32.5% of all privately rented London certificates below C, while pre-1930 flats are 17.3% of certified London homes.
- 92.1% have walls lodged as uninsulated solid brick, and 90.0% of certificates recommend internal or external wall insulation (most common cost text '£4,000 - £14,000').
- Flats in units of 2-5 addresses (likely conversions) are worse, with 62.8% below C against 52.3% in units of 6 or more. The larger buildings are more constrained: 61.2% are in a conservation area, 8.8% are listed and 33.6% are single glazed.
- 45.9% of these flats are in a conservation area and 5.1% are listed, so wall and window measures commonly need planning permission or listed building consent.
What it means for feasibility
Consents
Flats have no householder permitted development rights, so external wall insulation and replacement windows of a different appearance generally need planning permission. That is stricter in the 45.9% inside conservation areas, and the 5.1% that are listed need listed building consent. Internal wall insulation is the usual route for solid walls, but it takes floor area and sits within the flat, which can make it a leaseholder decision. Roof, floor and communal measures depend on the freeholder. Secondary glazing is often the consentable alternative to replacement windows. General context for England, not legal advice.
Sequencing
- Separate the decisions: internal wall insulation sits within the flat and can be a leaseholder's call, while roof, floor and communal measures depend on the freeholder.
- Check heritage early: 45.9% of these flats are in a conservation area and 5.1% are listed.
- Where windows are single glazed, consider secondary glazing, often the consentable alternative to replacement windows.
- Programme planning applications for external wall insulation or replacement windows of a different appearance, since flats have no householder permitted development rights.
Planning statements are general context for England, not legal advice. Article 4 directions vary by borough and were not checked for these figures.
Sources
- MHCLG Energy Performance of Buildings register, domestic certificates and recommendations, lodgements to 2026-09-09 (non-address fields OGL v3)
- GLA London Building Stock Model units and UPRN membership, OGL v3, 2026-09-12
- GLA London Datastore Conservation Areas, OGL v3, 2026-09-12
- Historic England NHLE listed-building polygons, OGL v3, 2026-09-12
Data month September 2026.
Building units (GLA London Building Stock Model): Contains public sector information licensed under the Open Government Licence v3.0 (Greater London Authority / UCL Bartlett Energy Institute and CASA). Contains OS data (c) Crown copyright and database right, published under the Ordnance Survey presumption to publish.
Energy performance certificates (MHCLG register): Energy Performance of Buildings Data England and Wales (c) Crown copyright.
Listed buildings (Historic England): © Historic England 2026. Contains Ordnance Survey data © Crown copyright and database right 2026. The Historic England GIS Data contained in this material was obtained on 12 September 2026. The most publicly available up to date Historic England GIS Data can be obtained from HistoricEngland.org.uk. Licensed under the Open Government Licence v.3.0.
Conservation areas (GLA London Datastore): Contains public sector information licensed under the Open Government Licence v3.0.
Method note
Every rating here is certified: the most recently lodged energy performance certificate for each address. Modelled ratings, the GLA’s 2017 estimates for buildings without a certificate, are never averaged in; where a population relies on them they are counted separately. Figures come from read-only queries run on 22 and 23 October 2026. Counts that could identify a building are merged or left out, and no addresses or owner names are published.
- The register's age bands are 1900-1929 and 1930-1949, so a 'pre-1919' archetype and a '1945-1980' archetype could not be isolated. Studies use pre-1930 and 1950-1982 instead, and the 1930-1949 semi study includes wartime and early post-war builds.
- Domestic archetype membership uses facts lodged on each address's latest certificate (property type, built form, age band), so domestic archetype counts are certified addresses by construction and unit-level certified coverage cannot be computed for them. Unit-level coverage is given for heritage-defined populations (conservation-area and listed domestic units) and for non-domestic units.
- 'Purpose-built', 'converted' and 'mansion block' are not recorded in the register or the stock model. The post-war flats and pre-1930 flats studies use the number of addresses in the GLA stock-model unit (2-5 vs 6 or more, from 2023 AddressBase UPRNs) as a proxy and label it as one.
- Certified means a lodged register certificate (RdSAP/SAP or CEPC). GLA 2017 aggregate rows and modelled (rvEPC) ratings are excluded from every distribution. Modelled ratings appear only as separate counts of units that rely on them.
- The unit of analysis for ratings is the address (UPRN): its most recently lodged certificate, with ties broken as the product does (lodged date, then record order). A stock-model 'unit' can be a whole block, so unit counts and address counts differ. 29.5% of London's latest domestic certificates are past 10 years from lodgement and may predate works since done.
- Numeric domestic recommendation codes are labelled with the register's own summary text, taken from certificates BuildGrade holds with both code and text. This assumes a code means the same across register versions. Code 7's current register text reads '50 mm internal or external wall insulation', and older certificates phrase it differently. Hot-water-cylinder codes 2 and 3 were left out of the top-measure lists.
- Recommendation rows are loaded for 79-97% of the latest domestic certificates and 97-99% of the latest non-domestic certificates, depending on lodgement year (some certificates carry no recommendations), so recommendation shares are lower bounds.
- Cost text is reported exactly as lodged and varies with register version (PV, for example, carries '£3,500 - £5,500', '£5,000 - £8,000' and '£9,000 - £14,000'), so the most common text and its share of rows is given. The texts are not current prices. Lighting cost text varies per certificate (most common share under 15%). Non-domestic recommendations carry a payback category (SHORT, MEDIUM, LONG, OTHER), not a £ cost.
- Heritage flags are spatial matches applied to the whole stock-model unit: listed means the centroid is within 5 m of an NHLE polygon or the footprint overlaps one by at least half; conservation area means the centroid is inside a GLA polygon. Every flat in a flagged block inherits the flag, so listed counts can be over or under. Listed matches: 10,294 domestic units by centroid and 5,551 by footprint.
- Tenure is as recorded at lodgement and may have changed since. Certificates with unknown or blank tenure are excluded from tenure shares (tenure-known share given per study). The MEES counts are screening counts, not compliance findings, and exemptions are not modelled.
- MEES status. Domestic: the E floor is law for private lets (since April 2020); band C by 2030 is confirmed policy, not yet law (government response 21 January 2026; needs legislation, and compliance is to be judged on reformed EPC metrics). Non-domestic: the E floor is law (since April 2023); the 2027 C milestone is not being taken forward, and B by 2030 was revised on 18 June 2026 to B from 2031 for buildings over 1,000 m², where cost-effective, which is not yet law. These are BuildGrade's recorded policy positions; check the current government position before relying on them.
- Wall descriptions marked 'as built ... (assumed)' are RdSAP defaults where the assessor could not verify the construction (for example 79.4% of 1930-1949 semis lodged as solid brick). Treat them as lodged descriptions, not survey findings.
- The potential rating on a certificate assumes every recommended measure is installed, including measures that may need planning permission or listed building consent, so 'potential C or better' is an upper bound on what is consentable.
- Consent statements (permitted development, conservation-area cladding, listed building consent, flats lacking householder rights) are general planning context for England, not legal advice, and are not computed from the database. Article 4 directions vary by borough and were not queried.
- London only: units outside the 33 London local authorities were not used. Other UK cities are in preparation; the Bristol and Cardiff pilots are not yet live.