Guide
How to tell if your house has solid or cavity walls
Measure the wall at a door or window: about 225mm (9 inches) usually means solid brick, while 260mm or more often means a cavity. On exposed brick, a pattern that shows the short ends of bricks every few rows points to solid walls. Most UK homes built before the 1920s have solid walls.
Check the thickness
Measure through an external door or window opening, including the plaster. Around 225mm suggests solid brick. Stone walls are thicker and almost always solid. Over 260mm on brick usually means a cavity.
Look at the brick pattern
Cavity walls are almost always laid in stretcher bond — only the long faces show. Solid walls are often laid in Flemish or English bond, where the short ends (headers) appear regularly.
Consider the age and type
Pre-1920s houses are usually solid. Many 1920s–30s houses have early cavities. Post-war system-built houses may have concrete or steel walls with no fillable cavity. Rendered or pebble-dashed walls hide the bond, so thickness is the best clue.
What it means for insulation
Cavity walls can often be filled, which is cheaper. Solid and system-built walls need external or internal insulation.
Questions
My walls are rendered — how can I tell?
Measure the thickness at a window, and check the age of the house.
Can a house have both?
Yes — extensions are often cavity-walled while the original house is solid.
Is an EPC reliable for this?
It's a useful clue but sometimes assumed rather than measured.