Solid vs Cavity Walls: What It Means for Your Extension Cost
If your home was built before the 1920s, it almost certainly has solid walls. That single fact can add 10-20% to your extension cost. We used EPC data to map wall type distribution and broke down exactly where the extra money goes.
TL;DR
Pre-1920s homes have solid walls (typically 225mm solid brick, sometimes 350mm). Post-1930s homes have cavity walls (two leaves of brick or blockwork with a gap). This matters for extensions because your new structure must tie into the existing building. Solid wall homes need deeper foundations (the walls are heavier), thicker insulation to meet Building Regulations, and careful thermal bridging details where new meets old. Together, these add £4,000-12,000 to a typical 15m² rear extension.
26%
English homes with solid walls
DLUHC EPC data
225mm
Typical solid wall thickness
Building survey standards
100mm
Cavity gap (modern)
Building Regulations Part L
10-20%
Cost uplift for solid wall extensions
Mayfair Studio cost model
Sources: DLUHC EPC data, Building Regulations 2024, Mayfair Studio cost model
Solid Walls vs Cavity Walls: The Basics
Solid walls are a single mass of brickwork, typically 225mm (one brick) or 350mm (one and a half bricks) thick. They were the standard construction method in England until the 1920s. Most Victorian and Edwardian terraces, and many Georgian properties, have solid walls. You can usually identify them by the brick pattern: if you see alternating headers (short end) and stretchers (long side), it's almost certainly solid.
Cavity walls have two separate leaves of masonry with a gap (cavity) between them, typically 50-100mm wide. The cavity provides insulation and prevents moisture from reaching the inner wall. This construction became standard from the 1930s onwards. Modern cavity walls often have the gap filled or partially filled with insulation.
The transition period (1920-1935) is unpredictable. Some homes from this era have solid walls, some have early cavity walls with narrow cavities. The only reliable way to determine your wall type is a borescope test or checking your EPC certificate, which records wall construction type.
Wall Type Distribution: EPC Data
DLUHC publishes wall construction type for every domestic EPC certificate issued. This gives us the most comprehensive picture of wall types across England. London has a significantly higher proportion of solid wall homes than the national average, reflecting its older housing stock.
| Region | Solid wall | Cavity wall | Other/unknown | Dominant era |
|---|---|---|---|---|
| Inner London | 48% | 32% | 20% | Pre-1920s Victorian/Edwardian |
| Outer London | 31% | 52% | 17% | Mixed 1920s-1960s |
| South East | 22% | 64% | 14% | Post-1930s suburban |
| South West | 29% | 55% | 16% | Mixed with stone walls |
| East Midlands | 24% | 63% | 13% | Post-1930s industrial |
| West Midlands | 27% | 59% | 14% | Mixed Victorian/post-war |
| North West | 32% | 54% | 14% | Pre-1920s terraces |
| North East | 25% | 62% | 13% | Post-1930s terraces |
| Yorkshire & Humber | 28% | 58% | 14% | Mixed stone/brick |
| East of England | 19% | 68% | 13% | Post-1930s suburban |
| England average | 26% | 59% | 15% | Mixed |
Source: DLUHC Energy Performance Certificate data • Data as of Certificates issued to March 2026
“Other/unknown” includes timber frame, system-built, stone, and properties where wall type was not recorded. Inner London's high solid wall percentage reflects the concentration of Victorian and Edwardian terraces.
Where the Extra Cost Goes
Extending a solid wall home costs more across four main areas. Each one adds cost independently, and they compound for the worst cases (pre-1900 properties with no previous modifications).
1. Foundations (extra £1,500-4,000)
Solid walls are heavier than cavity walls. A 225mm solid brick wall weighs approximately 4.8kN/m² compared to 3.5kN/m² for a typical cavity wall. Deeper and wider strip foundations are needed to support the extra load. Where the extension ties into existing solid walls, the foundation design must also account for differential settlement between old and new.
2. Wall construction (extra £1,000-3,000)
Building Regulations require the new extension walls to achieve similar thermal performance regardless of the existing wall type. But matching into a solid wall host building is more complex. The junction between new and old needs careful detailing to avoid cold bridges. Some builders construct the extension in cavity wall (modern standard) and create a transition detail at the junction. Others match the solid wall construction and add internal insulation. Both approaches cost more than simply continuing an existing cavity wall.
3. Insulation and thermal bridging (extra £1,000-2,500)
Where the new extension meets the existing solid wall, there's a thermal bridge: heat escapes through the solid masonry at the junction. Building Regulations Part L requires this to be addressed. Solutions include insulated plasterboard returns (cheapest, £500-800), internal wall insulation to the existing wall for 1-2 metres back from the junction (£1,000-1,500), or external insulation wrapping the corner (£1,500-2,500). None of these are needed on cavity wall homes.
4. Damp and moisture management (extra £500-2,500)
Solid walls manage moisture differently to cavity walls. They rely on mass to absorb and release moisture through the seasons. When you extend a solid wall home, you must ensure the junction doesn't create a damp trap. This often means installing additional damp-proof courses, using breathable insulation materials, and sometimes providing mechanical ventilation to the junction area.
Cost Comparison: 15m² Rear Extension
Here's how the costs break down for a typical 15m² rear extension in London, comparing solid wall and cavity wall host buildings. Both assume the extension itself is built to modern standards.
| Cost element | Cavity wall home | Solid wall home | Difference |
|---|---|---|---|
| Foundations | £4,500-6,000 | £6,000-10,000 | +£1,500-4,000 |
| Wall construction | £8,000-12,000 | £9,000-15,000 | +£1,000-3,000 |
| Insulation & thermal bridging | £1,500-2,500 | £2,500-5,000 | +£1,000-2,500 |
| Damp management | £500-1,000 | £1,000-3,500 | +£500-2,500 |
| Structural steel | £2,500-4,000 | £3,000-5,000 | +£500-1,000 |
| Roof & glazing | £8,000-12,000 | £8,000-12,000 | 0 |
| Electrics & plumbing | £4,000-6,000 | £4,000-6,000 | 0 |
| Floor finish & kitchen | £6,000-12,000 | £6,000-12,000 | 0 |
| Professional fees | £5,000-8,000 | £5,500-9,000 | +£500-1,000 |
| Total (inc. VAT) | £48,000-78,000 | £54,000-92,000 | +£6,000-14,000 |
Source: Mayfair Studio cost model • Data as of 2024/25 rates
How to Identify Your Wall Type
There are four reliable methods, in order of ease:
- 1.Check your EPC certificate. Every EPC records wall construction type. Search your address on the government EPC register. The “wall description” field will say “solid brick”, “cavity wall”, or similar.
- 2.Measure wall thickness. Open a window and measure the wall depth from inside to outside. Solid walls are typically 225mm (one brick) or 350mm (one and a half bricks). Cavity walls with plaster are typically 270-300mm.
- 3.Look at the brick pattern. If you can see headers (short ends of bricks) in a regular pattern, it's likely solid. Cavity walls typically show only stretchers (long faces) in a running bond pattern.
- 4.Check the construction date. Pre-1920: almost certainly solid. 1920-1935: could be either. Post-1935: almost certainly cavity. Post-1990: cavity with insulation.
Wall Type by Construction Era
EPC age band data lets us map wall types to construction periods. The transition from solid to cavity happened over roughly 15 years.
| Construction era | Dominant wall type | Typical thickness | Insulation | Extension cost impact |
|---|---|---|---|---|
| Pre-1900 (Georgian/early Victorian) | Solid brick (1.5 brick) | 350mm | None | +15-20% |
| 1900-1919 (Edwardian) | Solid brick (1 brick) | 225mm | None | +12-18% |
| 1920-1935 (Inter-war transition) | Mixed solid/early cavity | 225-270mm | None or minimal | +8-15% |
| 1935-1965 (Post-war) | Cavity wall | 270mm | None (unfilled cavity) | +3-5% |
| 1965-1990 (Modern) | Cavity wall | 280mm | Partial fill | Baseline |
| 1990-present | Insulated cavity | 300mm+ | Full fill (100mm+) | Baseline |
Source: DLUHC EPC data, Building Research Establishment • Data as of 2026
Building Regulations: What Your Architect Needs to Know
Building Regulations Part L (Conservation of Fuel and Power) sets thermal performance targets for new extensions. These targets are the same regardless of your existing wall type. But meeting them is harder and more expensive when extending from a solid wall home.
The key requirement is the U-value target for new walls: 0.28 W/m²K. A modern cavity wall with 100mm insulation achieves this easily. A new wall tying into solid masonry needs careful detailing to prevent the existing wall from acting as a thermal bypass.
Your architect should produce junction details showing how the insulation is continuous at the point where new meets old. Building Control will check these details. If thermal bridging isn't addressed, you'll get condensation at the junction, which leads to mould and damp within 2-3 years.
Frequently Asked Questions
How much more does it cost to extend a solid wall house?
Typically 10-20% more than extending a cavity wall home. For a 15m² rear extension in London, this means an extra £6,000-14,000. The additional cost comes from deeper foundations, thermal bridging details, damp management, and insulation requirements at the junction between old and new.
Can I tell if my house has solid walls from the outside?
Usually yes. Look at the brick pattern: if you see headers (short ends of bricks) in a regular pattern mixed with stretchers, the wall is likely solid. If you only see stretchers in a running bond, it's likely cavity. You can also measure the wall thickness through an open window: solid walls are typically 225mm or 350mm, cavity walls are typically 270-300mm including plaster.
Do solid walls affect planning permission for extensions?
No, wall type doesn't affect planning permission or permitted development rights. The planning system cares about size, position, and appearance of the extension, not the existing wall construction. However, wall type significantly affects Building Regulations compliance and construction costs.
Should I extend my solid wall home with matching solid walls or modern cavity construction?
Most architects recommend building the extension in modern cavity wall construction (which meets Building Regulations more easily and costs less) and creating a carefully detailed transition at the junction. Building a solid wall extension to match is possible but more expensive, harder to insulate, and offers no structural advantage.
What percentage of London homes have solid walls?
EPC data shows approximately 48% of inner London homes and 31% of outer London homes have solid walls, compared to a national average of 26%. London's higher proportion reflects its older housing stock, particularly the concentration of Victorian and Edwardian terraces in inner boroughs.
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