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Why is the sum of the rooms' square metres less than the house's area?

Because the house's area is measured around the outside of the external walls, while the rooms are measured inside them. Walls typically take up 15-20% — here's the arithmetic for a 120 m² house.

The arithmetic for a 120 m² house

A detached house measures 12.00 m × 10.00 m on the outside. The external wall is an ordinary 35 cm cavity wall, and the partition walls inside are light 10 cm walls. The house has a total of 34 running metres of partition wall.

Step 1: the house's area

12.00 × 10.00 = 120.00 m². That's the figure in the listing and in the official property register.

Step 2: what the external walls take

The 35 cm external wall cuts 35 cm off both ends in both directions:

• Internal length: 12.00 − 0.70 = 11.30 m
• Internal width: 10.00 − 0.70 = 9.30 m
• Area inside the external walls: 11.30 × 9.30 = 105.09 m²

The external walls have taken 120.00 − 105.09 = 14.91 m². That's 12% of the house — almost an entire room made up purely of brick and insulation.

Step 3: what the partition walls take

34 running metres of 10 cm partition wall: 34.0 × 0.10 = 3.40 m².

Step 4: the sum of the rooms

105.09 − 3.40 = 101.69 m²

The house is 120 m² in the listing and 101.69 m² spread across the rooms. The difference is 18.31 m², or 15.3%. No square metres are missing — they're standing upright.

How to check for yourself whether the difference is reasonable

You can work out the expected difference before you go looking for an error. The external walls' area is roughly the external wall's thickness × the house's outer perimeter, minus a bit at the corners where two walls overlap.

In the example: the perimeter is 2 × (12.00 + 10.00) = 44.00 m. 0.35 × 44.00 = 15.40 m². The corners overlap by 4 × 0.35 × 0.35 = 0.49 m², so 15.40 − 0.49 = 14.91 m² — exactly the same figure as in the external-wall arithmetic above.

Add the partition walls' thickness × their combined length, and you have the whole difference. If your house comes out roughly in line with that, everything is as it should be. If the difference is much bigger, the explanation is almost always a forgotten hallway, an unusually thick external wall, lots of small rooms, a stairwell, or premature rounding.

The five reasons the difference can be bigger than you expect

• The hallway is forgotten. The most common cause of all. People add up the living room, kitchen and bedrooms and forget the entrance, hallway, utility room, WC and storage. A hallway measuring 1.20 × 8.00 m is 9.6 m² that nobody thinks of as "a room".
• The external wall is thicker than assumed. A newer low-energy house can have a 40-45 cm external wall against a 1960s house's 30 cm. At the same outer dimensions, the newer house has noticeably less floor — it's the insulation taking up the space.
• There are lots of small rooms. Five bedrooms cost more running metres of partition wall than two large ones. Every 10 cm wall times 4 m of length is 0.4 m² of floor.
• There's a staircase. A stairwell opening in the floor structure is area that's counted on one storey but isn't floor on the one below. How the stairs count depends on how the home is officially registered, and that can differ from the floor you can actually walk on.
• There's rounding. Round seven rooms down to the nearest whole square metre, and you can lose up to 3.5 m² on rounding alone. Work with two decimal places throughout, and round only at the very end.

If the sum comes out BIGGER than the house's area, something's wrong

The order is always the same: the house's area is bigger than the sum of the rooms. Get the opposite, and there's an error, and it's almost always one of three:

• A shared wall has been counted in both rooms. If you measure each room out to the centre line of the wall between them, you count the same square metres twice. Two rooms sharing a wall must never count the same square metres — the wall belongs to neither of them.
• A room has been counted twice. Typically a through-room that's listed both as "entrance" and as part of "living room/entrance".
• A single room has been measured externally. An extension gets measured on the outside, the rest of the house on the inside — and then the sum stops adding up.

That's also why a shared wall has to be one wall on a floor plan, not two walls sitting back to back. Two half-walls stacked on top of each other make the house 10 cm wider than it really is, and the error propagates into every area calculated from the drawing.

So which figure should you use?

Both — for different things:

• The house's area (120 m²) is what you use to compare homes and to work out the price per square metre. It's the only figure that's comparable across different houses.
• The sum of the rooms (101.69 m²) is what you use for everything practical: flooring, underfloor heating, paint, cleaning, furniture and contractor quotes. Materials go on floors, not on gross areas.

When you work out price per square metre when buying a home, you need to use the same kind of area for every home you compare. Mix one house's overall area with another house's room sum, and you get a 15-20% difference that has nothing to do with the homes themselves.

Internal measurements are the only thing you can check yourself

There's a reason the sum of the rooms is the figure you can trust in practice: you can walk into each room with a tape measure and check it. You can't measure the house's external area without walking around it with a tape over the plinth, brickwork and projections — and even then you need to know the rules for what counts.

That's why HouseSense works in internal measurements. A room's area is measured from wall surface to wall surface, the way you'd measure it yourself, and shared walls are one common piece of wall between two rooms — never two. The sum of the rooms therefore comes out smaller than the official area, and that's the point: it shows the floor that actually exists. If you want to compare against the register's figure, add the external walls' area back on, and the two numbers meet.