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What's the difference between a partition wall and an external wall?

An external wall faces the outdoors and has to keep out weather, cold and damp. A partition wall stands inside the home and divides rooms. Thickness is usually the quickest giveaway.

The external wall: the home's coat

An external wall separates the home's indoor climate from the weather outside. It does four jobs at once, and that is why it is thick:

• Carrying load. In most homes the external walls carry the floor structures and the roof. That is why you cannot simply cut a window bigger without adding a lintel.
• Insulating. The insulation layer takes up most of the thickness. A modern external wall typically has 190-300 mm of insulation; a house from the 1930s may have none at all.
• Keeping water out — while still letting construction moisture escape again. That is why a masonry external wall usually has a cavity and an outer leaf.
• Taking the wind load. Wind pushes against the side of the house, and the external walls are what brace it against being pushed out of shape.

Typical external wall thicknesses (always measure your own — these are orders of magnitude):

• Solid brick wall, older city building: 35-50 cm, sometimes thicker on the ground floor than higher up.
• Cavity wall in a 1960s-1980s house: typically 29-35 cm — an 11 cm outer leaf, an insulated cavity, and an inner leaf.
• Newer cavity wall or timber-frame wall built to modern insulation standards: 35-45 cm.
• Light timber external wall (a cabin or extension): 20-30 cm.

The easiest place to measure an external wall is in a window reveal: measure from the inner face to the outer face, ideally at the bottom of the window. If there is a deep windowsill you could set a cup on, you are standing in a house with thick external walls — and those centimetres are square metres you cannot furnish.

The partition wall: the room divider

A partition wall stands inside the home and owes the weather nothing. Its job is to divide rooms, and often to dampen sound. There are two main types:

• Light partition wall — a timber or thin-gauge steel frame with plasterboard on both sides, typically 7-12 cm in total. A standard wall with 70 mm studs and 12.5 mm board on each side comes to 9.5 cm. With double board on each side: 12 cm.
• Masonry or cast partition wall — a half-brick wall (typically 11-13 cm including render) or a full-brick wall (typically 23-25 cm including render), or a concrete wall in an apartment block. These are often, but not always, load-bearing.

A partition wall between two separate homes — a party wall between flats or terraced houses — is a different matter entirely. It is thick (often 25-40 cm, sometimes doubled with a cavity in between), and it exists for fire and sound reasons. It must never be touched without a professional, and any work on it needs sign-off from the authorities, whatever it turns out to carry.

Five signs you can see and measure yourself

1. Thickness. Under 13 cm: almost always an internal partition wall. Over 25 cm: probably an external wall or a party wall. Measure in a door or window reveal.
2. Is there a window in it? Windows sit in external walls. It sounds trivial, but it is also the single most reliable sign there is.
3. Is it cold? Put your hand on the wall on a winter's day. An uninsulated external wall is noticeably colder than a partition wall in the middle of the house. That says nothing about whether the wall carries load — only which way it faces.
4. Are there radiators under it? Radiators are traditionally placed under windows in external walls, because that is where the cold gets in.
5. Does it line up with the floor plan? Hold the home's outline up against reality. Every wall on the outline is an external wall. Everything inside it is a partition wall or a load-bearing internal wall.

The most important trap: "partition wall" doesn't mean "safe to knock down"

Many people use the word partition as a synonym for "a wall that isn't carrying anything". That's wrong, and it's an expensive mix-up.

A partition wall is defined by its location — inside the home — not by its role in the structure. Many detached houses have a load-bearing internal wall running through the middle, dividing the house lengthwise and carrying the roof structure or a floor above. It looks like an ordinary wall, it can be plastered and painted just like all the others, and it is still what is holding the roof up.

So it follows: establishing that a wall is not an external wall tells you nothing about whether it can be removed. Whether an internal wall is load-bearing can only be settled by a qualified building surveyor or a structural engineer who has seen the wall and the house. No app can answer that — not even HouseSense. An app can measure the wall. It cannot determine what the wall is carrying. And if a load-bearing wall is removed, the work needs sign-off from the authorities before it starts.

Why the difference matters for your square metres

The distinction between external and partition walls is also why your own measured area is typically smaller than the figure in the official public building register. The principle is the same in most countries: the register states the living area measured to the outside of the external walls, while you yourself measure internally, wall to wall. The difference is the full thickness of the external walls, all the way round.

Worked example: A house measures 9.60 m × 8.40 m on the outside = 80.6 m². The external walls are 35 cm thick, so internally that becomes (9.60 − 0.70) × (8.40 − 0.70) = 8.90 × 7.70 = 68.5 m². That is 12 m², or 15%, that disappears into the external walls — and on top of that come the internal partition walls, which also take up space. Two houses with the same registered area can differ by several square metres internally, simply because one has thicker external walls.

That is exactly the difference HouseSense makes visible: the app measures internally, the way you would with a tape measure yourself, and records every wall's thickness — both external walls and partition walls — as you scan or draw the home. You can then compare your own figure with the registered one and know why they differ. The official figure is still the official figure — your own is the one you could stretch a tape measure across.

In short

• External wall: faces the outdoors, 25-40 cm, insulated, almost always load-bearing, has the windows.
• Light partition wall: inside the home, 7-12 cm, board on a frame, rarely load-bearing — but that still needs confirming.
• Masonry internal wall: inside the home, 12-25 cm, can easily be load-bearing.
• Party wall between flats or terraced houses: 25-40 cm, fire and sound, never touched without a professional and sign-off from the authorities.
• And in every case: the question of what a wall carries is settled by a qualified building surveyor or a structural engineer — not by a thickness, and not by an app.